GCSE Chemistry C2 Notes

Ionic bonding
… Ionic bonding occurs between …?… and non-…?…

… The ions in an ionic compound form a g..?… structure or lattice. The e..?… forces of attraction between oppositely …?… ions act in all …?… so the ions pack closely together to form a r…?… pattern.

… Draw a dot and cross diagram to show how the ions bond together in potassium chloride (potassium is in group 1 and Chlorine is in group 7).

… Ionic substances are held together by very strong e…?… bonds.
This means that they have very high …?… and …?… points because a lot of …?… is needed to …?… the bonds.

… When can ionic compounds conduct electricity? Explain your answer (mention mobile ions).

… To find the formula of an ionic compound formed between a metal and a non metal, a quick method is to use the ‘drop and swap’ technique. For example, for aluminium oxide, the elements form the ions:
Al3+   O2-
So, the formula of aluminium oxide is
Al2O3
We have swapped the numbers over (ignoring their signs) and ‘dropped’ them from superscript to subscript.

… When you see chemical names ending in ‘ide’ then you know that there are ions present. For example, sodium chlorIDE.

… When you see chemical names ending in ‘ate’ then you know that Oxygen is present in the non-metal ion. For example, the formula for potassium nitrATE: KNO3  (subscript 3).

Ionic equations
… Write the full ionic equation from these two half equations that describe a displacement reaction:
Cl2 + 2e-  —->  2Cl-
2At-  —->  At2 + 2e-
(Hint: LH sides go together and RH sides go together, but don’t include the electrons).

… A full ionic equation of a reaction can be used to identify the ‘spectator ions’, which are ions that exist in the same form on both the reactant and product sides of a chemical reaction. For example, here is a precipitation reaction:
2 NaCl (aq) + CuSO4 (aq) → 2 NaSO4 (aq) + CuCl2 (s)
The full ionic equation is:
2 Na+ (aq) + 2 Cl- (aq) + Cu2+ (aq) + SO42- (aq) → 2 Na+ (aq) + SO42- (aq) + CuCl2 (s)
Note that the solid product is not written as its separate ions because it is not soluble.
Which ions appear on both sides? These are the spectator ions and can be removed to form the ‘net ionic equation’:
2 Cl- (aq) + Cu2+ (aq)   →  CuCl2 (s)


Covalent bonding

… A covalent bond is formed between non-…?… atoms when they s…?… electrons.

… What two types of structure can covalent substances form? Can you give an example of a substance with each type of structure?

… Draw a dot and cross diagram to show the bonding in methane, CH4 (one carbon atom bonded to four hydrogen atoms)

… Molecules are substances which have atoms which are co…?… bonded to each other.

… Simple molecules have …?… melting and boiling points because of the weak i..?… m..?… bonds, which can easily be broken with a small amount of energy (low temperatures).

… In an oxygen molecule, two pairs of electrons are shared to form a d…?… bond: O=O

… Giant covalent structures have …?… melting and boiling points because…?

… Diamond and graphite are examples of g…?… c…?… structures. Different structural forms of the same element are called ‘a…?…’.

… Graphite  is made up of …?… of carbon atoms which can slip over each other – for this reason it is an excellent lubricant.

… Graphite conducts …?… because each carbon atom has an unbonded (delocalised) …?… which can move freely within the lattice of carbon atoms.

… Can diamond conduct electricity? Explain your answer…

… Diamond is the hardest (difficult to scratch) naturally occurring substance found on Earth because…?
For this reason, industrial diamonds are used in cutting machinery.
However, diamond is NOT the hardest substance. Wurtzite boron nitride (synthetically made) and lonsdaleite (which comes from meteorites) are both harder!


Metals
… Describe the structure that forms from metallic bonding (key words to use: ions, sea, electrons, positive, lattice, delocalised).

… Metals form …?… metallic structures.

… Use the idea of metallic bonding to explain why metals can be bent and drawn into wires.

… Delocalised (freely moving) …?… in metals mean that they conduct …?… and …?… very well.

… Metals have very strong bonds between the positive metal ions and the delocalised electrons. How does this affect the melting and boiling points of metals?

Extracting Metals

… Aluminium is extracted using electrolysis because the usual method of reacting an ore with carbon (to displace the metal) won’t work because Carbon is less …?… than Aluminium.

… During electrolysis, molten …?… is added to dissolve aluminium ore (aluminium oxide) and reduce the …?… point (to about 900 degrees C) so that the metal …?… can move about as a liquid. This reduces the amount of …?… required to melt the ore and therefore reduces the …?… of extracting the metal. (Without first dissolving the ore in this way, you would need to heat it to over 2000 degrees C!)

… Cost is an important factor in the extraction of metals. You can certainly use cost as a valid point in your answers, but make sure that you go into DETAIL. For example, “the cost of extracting Aluminium from its ore is much more than from recycling because more energy is needed”.

… Electrolysis can be used to electroplate conducting materials. The object to be plated is made the cathode so that metal ions from the electrolyte solution are deposited on the object as a thin layer of ‘plate’.
The anode is made from the plating metal and gradually dissolves into the solution as metal ions (this keeps the concentration of the solution the same).

For example, in copper electroplating:

Anode = copper:
Cu(s) —> Cu2+(aq) + 2e

Cathode = object to be plated
Cu2+(aq) + 2e —> Cu(s)

Electrolyte solution = copper sulpfate.


Polymers

… Thermosetting polymers form strong covalent ‘…?…-…?…’ bonds between their polymer chains.  This means the polymer does not soften or melt when it is …?… so it remains rigid.

… Thermosoftening  polymers have no ‘…?…-…?…’ bonds between the polymer chains. The inter-molecular bonds between the polymer chains are very …?… and so are easily …?… by heating.


Alloys

… Alloys are mixtures of …?… and are harder, stronger and less ductile/malleable than pure metals because…? (Think about the layers)

… What are the properties of ‘Shape Memory Alloys’?

… Brass is an a…?…. of the elements c…?… and z…?…

… Duralumin is an alloy of …?… and …?… widely used for aircraft structures because…?

… Bronze is an alloy of mainly c…?… and t…?… It is used to make coins, springs,


Ceramics
… The word “ceramic” comes from the Greek word for “of pottery”. Ceramic materials cover a very broad range, but mostly they are oxide, carbide or nitride compounds, either in ionic or covalent structures.

The general properties of ceramics are:

  • high melting temperature
  • high hardness
  • poor conductivity
  • high stiffness
  • chemical resistance
  • low ductility (brittle)

Many composites, such as fiberglass and carbon fiber, while containing ceramic materials, are not considered to be part of the ceramic family.

Composite materials
Composite materials are composed of at least two materials. These materials combine to give properties superior to those of the individual constituents.
Plywood is an example of a composite material as it is made by bonding layers of the same material (wood) but in two different directions. The wood layers are strong in tension along the length of the grain (fibres), but weaker across the grain, so bonding them at 90 degrees to each other creates a material that is strong in both directions.
Steel reinforced concrete is another example, where the steel mesh is pre-stressed while the concrete dries around it. Once dried, the steel mesh is released from tension and so compresses the concrete. This is useful because concrete is strong in compression and weak (brittle) in tension – so having a ‘background’ compression will counter any tensions created in the structure as it carries a load.

Product Life Cycle Analysis. 
… Any reasonable comments will be acceptable about the environmental impact during the stages of a product’s life including:

  • raw material extraction
  • materials processing
  • manufacture
  • distribution
  • use
  • repair and maintenance
  • disposal or recycling

Acids and Alkalis

… An alkali (a soluble …?…) is a source of …?… ions in solution.

… An acid is a source of …?… ions in solution.

… Bases are substances that will N…?… acids.

… Metal o…?…,  c…?… and h…?… are called ‘bases’. An …?… is a base which is soluble in water. For example…?

… Acids have a pH between …?… (a strong acid) and …?… (a weak acid).

… Alkalis have a pH between …?… (a weak alkali) and …?… (a strong alkali).

… When an acid reacts with a base, a …?… reaction takes place to produce a …?… and …?… . The ionic equation for a neutralisation reaction is…?

… Acids will react with some metals to produce a …?… + …?… gas.

… Acids will only react with metals that are above hydrogen in the reactivity series.

… Acids react with some metal carbonates to produce …?… + …?… + …?…

… Metal + acid reactions are displacement reactions and involve the transfer of electrons, for example:
Mg + 2HCl —-> MgCl2  + H2
This is an example of a redox reaction:

  • the magnesium was turned into a 2+ ion and so has been …?…
  • the hydrogen went from a +1 ion into hydrogen gas and so has been …?…

Salts

… Method for preparing crystals of a SOLUBLE salt by reacting either:

  1. A metal (‘in excess’) with an acid 
    or
  2. An insoluble base (‘in excess’) with an acid. 

For both of these methods, you can …?… off the unreacted solid and then …?… off the water by heating the solution to form crystals of the salt (this process is called …?…). 

Note that it is good practice to heat the solution until the first crystals appear, then stop heating and leave to evaporate (crystallise) naturally. Here’s a useful video that demonstrates the method: https://youtu.be/FRaT0qOKZpU

  1. An alkali with an acid.
    Use an …?… or a …?… meter to find the ‘…?… point’ of the reaction (when it has completed and reached pH…?…), otherwise you may still have unreacted acid or alkali present.
    Remember to mention ‘add the reagent drop by drop near the end point’ and to ‘swirl the flask’ to help mix the reagents.
    Stop adding reagent when the indicator changes colour (when you reach pH7)
    Note v…?… of reagent added.
    Repeat without indicator (if used) using the same volume of reagents.
    Pour out solution into an e…?… dish.
    Heat and leave to c…?…

… When evaporating a solution to form crystals of a soluble salt (crystallisation), heat the solution until the first crystals start to form. Then set the evaporating dish aside so that the solution evaporates more slowly. This forms more regular crystals.

… The ionic equation for a neutralisation reaction is…?

… The balanced chemical equation for the neutralisation reaction between sodium hydroxide and sulfuric acid is…?

… To form crystals of an INSOLUBLE salt (a precipitate – a ‘cloudy’ solution) you can react two soluble salt solutions together. The compounds will often ‘swap partners’ to produce the insoluble precipitate.
For example:

Sodium carbonate (aq) + zinc sulphate (aq) —-> Zinc carbonate (s) + …?…

This is called a p…?… reaction. To collect a pure sample of the resulting solid you will first have to …?.. the product, and then w…?… the solid using distilled water and finally dry it.
Here is a useful video about this type of reaction: https://youtu.be/E1ODnGe9LnM

… Precipitation is an important way of removing pollutant metal i…?… from industrial waste water.

… Using the ‘drop and cross’ method to find the formula of a salt (an ionic compound) using the charges on the ions. For example, what is the formula of aluminium hydroxide?
Aluminium has a 3+ ion, and the hydroxide ion is a 1- ion
So the formula is (Al)1(OH)3 = Al(OH)3


Rates of reaction (Collision Theory)

Factors affecting the rate of a reaction:
… Increasing the concentration of reactants will …?… the rate of reaction. There are more p…?… and so more frequent c…?… and therefore a higher chance of a reaction happening each second.
The effect of concentration of the reactant on the rate of a reaction can be seen by the reaction between sodium thiosulphate and hydrochloric acid in a conical flask. Sodium thiosulphate reacts with dilute acid to produce sulphur dioxide, sulphur (a cloudy precipitate) and water. A white tile with a cross marked on it is placed under the conical flask. A stopwatch is used to time how long it takes for the cross to become no longer visible.

… Heating reacting particles will …?… the rate of reaction. Hotter particles move …?… and therefore collide with more e…?… and more f…?… , so there is more chance of a reaction.

… Increasing the surface area of a reactant will …?… the rate of reaction. A fine powder will have a much …?… surface area than a block of the same mass. As more surfaces are exposed, c…?… can happen more f…?… and therefore more reactions can occur per second.

… The fourth way to increase the rate of reaction is to use a catalyst. This lowers the …?…. energy needed for the reaction to happen.

… Different reactions require different catalysts, so what might work for one reaction may not work for another.

… Many catalysts involve elements and compounds from the transition metals. These can be toxic, and so it is important that they do not pollute the environment.

… Two important areas of research into new catalysts are in nano-particles and enzymes (biological catalysts) which could increase the efficiency of chemical reactions in industry even more than standard transition metal catalysts.

… For a reaction where we measure the volume of gas produced:
    – how could we calculate the average rate of the reaction?
    – what would the graph of volume of gas produced vs. time look like?

… For a reaction where we measure the mass of reactants still left:
– how could we calculate the average rate of the reaction?
– what would the graph of mass of reactants vs. time look like?

… For a graph of product made (e.g. gas volume) against time, we could calculate the rate of a reaction at a particular time by drawing a t…?… line to the curve at the time we are interested in, and then calculate the g…?… of this tangent line.

… A reaction in which energy is transferred FROM the reacting substances TO their surroundings is called an …?… reaction.
Examples of this type of reaction include: c…?…, oxidation, and n…?….

… A reaction in which energy is taken in by the reacting substances FROM their surroundings is called an …?… reaction.
Examples of this type of reaction include: thermal d…?… and when some solids dissolve in water (as used in instant cold packs).

… We looked at how the transition metals platinum, palladium and rhodium are used in a catalytic converter in a car exhaust system. Toxic gases are produced due to the high temperatures in the engine when petrol is combusted. These toxic gases are made to react with each other in the catalytic converter by the catalysts, producing less harmful nitrogen and carbon dioxide gases. 


Nano Science!
… Nano science is about altering the properties of matter at the atomic scale. Nano particles have different properties to the bulk material they are made from.

… Nano means at the scale of a billionth of a metre (0.000000001m). How many nanometres are there in 1mm?

… Nano structures are typically between 1 and 100 atoms in size.

… Nano particles often have a HUGE surface area which can be useful – for example to make high efficiency catalysts and enzymes (to speed up chemical reactions and biological processes).

Other applications of nano science include:
Hydrophobic and hydrophilic coatings – e.g. Self cleaning and waterproof clothes/cars/buildings…

Materials which change colour when wet, or at a different temperature etc. (a butterfly uses nano structures on its wings)

Machines could be made at the nano scale to help repair the human body

Dry materials which stick to non-sticky surfaces and can support huge forces. Geckos use nano fibres in their feet to stick to walls.

Carbon allotropes  (allotropes are different forms of the same element) such as buckminsterfullerene (‘fullerenes’) have properties which make them useful in applications such as:

  • drug delivery systems
  • lubricants
  • catalysts,
  • smaller, more efficient and more durable electronic circuits
  • extremely strong, light and tough composite materials.

Electronic circuits that can be bent – wearable technology, bendable screens…

Microbots’ could be used for military, environmental or medicinal applications.

Nanoscience is a pretty amazing area which will become more and more important in this century. HOWEVER, safety and ethical issues must be carefully considered in the development of new nano materials and machines. Any new nano technology should be thoroughly …?… to make sure that it is …?… for us to use.

Here’s a link to a useful nanoscience video: http://youtu.be/70ba1DByUmM


Electrolysis essentials:

… Ionic compounds only conduct electricity when dissolved in …?… or when …?… because their charged ions can then move freely.

… Covalent compounds cannot conduct electricity in any state: solid, liquid, gas or in solution because they do not have any …?…

… Electrolysis is ‘splitting by electricity’. The electrodes are put into the electrolyte (e.g. the ionic solution or the molten ionic compound) and elements are deposited at them.

… When electrolysing an aqueous SOLUTION:

  • you will either see a m…?… or h…?… gas produced at the cathode (negative electrode). How can you predict which you will see produced?
  • o…?… gas and w…?… is produced at the anode unless the solution contains halide ions in a high enough concentration (such as in brine).
    For example, in the electrolysis of NaCl solution: when chloride ions are in low concentration, the OH- ions will form oxygen gas and water at the anode.
    4OH-  ———–>  O2(g) + 2H2O + 4e
    In IGCSE this is written in a slightly different way:
    2H2O ————> O2(g) + 4H+ + 4e

Note that it is possible that chloride ions will also produce chlorine gas at the same time:
2Cl-   →   Cl2(g)  +    2e-

… Half equations can be written for the anode (where electrons are …?…) and at the cathode (where electrons are …?…)

… Electrolysis half-equations, for example, in the electrolysis of molten lead bromide:
Anode (electron ‘hoover’):
2Br-(l)  ———>  Br2(g)  +  2e-

Cathode (electron ‘shop’):
Pb2+(l)   +   2e-  ———>  Pb(l)

Take care when balancing half equations – remember that non-metal elements such as oxygen, chlorine, bromine tend to exist as diatomic molecules, e.g. Br2

… “OIL RIG”: When electrons are gained, …?… occurs. When electrons are lost, …?… occurs.

… A metal which is more reactive than iron can be used as a ‘sacrificial anode’ so that it corrodes instead of the iron. This is used in ships to protect their steel hull by placing blocks of zinc on the hull, in the water.
The zinc will be oxidised (oxidation is loss of electrons) instead of the iron:
Zn —-> Zn2+  +  2e-


Electrolysis of brine using a diaphragm cell

… Brine is concentrated sodium hydroxide solution, so chloride ions are in abundance. This means that chlorine gas is given off at the anode.
2Cl-   →   Cl2(g)  +    2e-

… Hydrogen gas is produced at the cathode (sodium is more reactive than hydrogen)
2H+  +  2e-   →  H2(g)

… Positive sodium ions and negative hydroxide ions are left behind. These combine to form sodium hydroxide solution which is tapped off.

… A membrane (or diaphragm) cell is used to let positive sodium ions through to the cathode side, but blocks OH- and Cl- anions from getting through to prevent contamination.
Here’s a useful diagram: https://goo.gl/images/bFU3pM

… Uses of chlorine:
to sterilise water supplies (chlorination)
making disinfectants
making plastics

… Uses of hydrogen:
pollution-free fuel
change oils into saturated fats for making margarine
Used in the Haber process to make ammonia.

… Sodium hydroxide is a very strong alkali (a soluble base) and has many uses:
making soap
Removing pollutants from water
Manufacture of paper
Processing food products.


Production of aluminium

… The usual method to extract a metal from its ore is to react the ore with carbon. However, carbon cannot displace aluminium from its oxide because it is …?… reactive.
Aluminium is extracted using electrolysis of a…?… o…?… (obtained from bauxite ore) in a solvent called C…?…. This lowers the melting temperature from 2000°C to 900°C and so decreases the energy required and heating costs.

… Impure copper can be p…?… by electrolysis in copper sulfate solution.
Copper atoms from the impure copper anode move into solution as 2+ ions.
They are then attracted to the cathode where they pick up 2 …?… to form solid copper metal.
What are the half equations at each electrode for this electrolysis reaction?

Electrolysis mass and volume calculations:

… 1 Faraday = 1 mole of electrons = 96500 Coulombs

…  Use “QFEM”:
“Q”: first work out the charge Q (in coulombs) that has flowed in the circuit using Q = I x t

“F”: then the number of Faradays  = Q / 96500 = the number of moles of electrons.

“E”: write the half equations for the electrolysis reaction and work out the number of moles of the element produced at that electrode.
This gives us the ratio of moles of electrons to the moles of the element.
For example, at the ANODE, if
2Cl-  =>  2e-  +    Cl2 (small subscript number)
Then 2 moles of electrons produce 1 mole of Chlorine MOLECULES.

“M”: calculate the mass or volume of element produced (use the mass – Mr – moles table)

It’s also useful to remember that 1 mole of any gas will occupy a volume of 24dm3.


The Hydrogen Fuel Cell

… The hydrogen fuel cell first splits up hydrogen into H+ ions and electrons using a catalyst at the anode:
H2  →  2H+   +   2e−
The electrons then flow around the external circuit transferring electrical energy (for example to drive a motor in an electric car). 
The H+ ions flow through the electrolyte.

The H+ ions and electrons then combine with oxygen gas at the cathode to form water:
O2   +   4H+   +   4e−  →  2H2O

Here’s a useful video to show how a fuel cell works:
https://youtu.be/6UwSazq8GTU


Chemical Cells

… The voltage (potential difference) produced by a cell depends mostly on the type of e…?… (usually metals) and the electrolyte used. A simple cell can be made by connecting two different metals which have difference r…?… in contact with an electrolyte.

… Batteries consist of two or more cells connected together in s…?… to provide a greater potential difference.

… In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. When this happens it means that the cell can no longer push electrons around the circuit and we say that the cell has been used up (or ‘the battery has gone flat’).

… Alkaline batteries are non-rechargeable. Rechargeable cells and batteries can be recharged because the chemical reactions are r…?… when an external electrical current is supplied. This is done by pushing electrons back into the anode using an external power supply.

… An electrochemical cell converts chemical energy into electrical energy using a REDOX reaction. Oxidation (loss of …?…) occurs at the anode which is the more reactive metal. Reduction (gain of …?…) occurs at the cathode which is the less reactive metal.
For example:

Mass and moles calculations

… Isotopes are the same elements (same atomic number) but with different numbers of …?… (different mass number).

… Chadwick provided the evidence to show the existence of neutrons. This was necessary because isotopes are atoms of the same 
element (same number of protons), but with different numbers of neutrons.

… Ar is the “relative a..?… mass” used for individual atoms.

… Mr is the “relative f..?… mass” for a compound or molecule. Just add up all the individual Ar values in the compound.

… What is the relative formula mass of magnesium bromide (MgBr2)?
Ar of magnesium = 24, Ar of  bromine = 80

… A “mole” is a very large number of particles (6 hundred thousand million million million)!

… The Ar of an atom (or Mr of a compound) tells us the MASS of 1 mole of a substance. So for example, carbon has a relative atomic mass (Ar) of 12. This means that 1 mole of carbon has a mass of …?…g

… To calculate the number of moles or mass (in grams) of an element or compound, use a Mass-Mr-Moles table:
Mass
Mr
Moles
Going Down the column means Division (Mass ÷ Mr = Moles)
Going Up means mUltiply (Moles × Mr = Mass)

… How many moles are there in 168g of Calcium Oxide (CaO)?
Ar of Calcium = 40, Ar of O = 16

… The percentage by mass of an element in a compound
= ( Relative mass of the element / Mr of the compound ) x 100%

… What is the percentage by mass of hydrogen (Ar = 1) in water, H2O?   (Ar of oxygen = 16)

… Calculating masses of products or reactants.
First you will need a balanced chemical equation for these questions.

Use the Mass – Mr – Moles table using column headings for the key substances in the question (use words rather than the formulae).  It’s useful to have the reactant in the first column and the product in the second column.

the moles ratios will be given by the balancing numbers from the balanced equation.

work in a clockwise or anti-clockwise spiral, remembering that going Down means Divide, and going Up means mUltiply.

If you are given tonnes, then follow exactly the same method. (Technically you won’t be finding moles, but millions of moles – but it still works out!)

… For example, what is the maximum possible mass of calcium oxide that could be made by burning 4.0g calcium in air?

… In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used.
The reactant that is completely used up is called the l…?… reactant because it limits the amount of products.

… If one of the reactants is limited (and so the other is in excess), you will need to have THREE columns in your Mass-Mr-Moles table.
Compare the balancing numbers in the chemical equation against the moles of each reactant to see which one is the limiting reactant and which one is in excess.
Use the ratios involving the limiting reactant’s number of moles (i.e. ignore the reactant that is in excess).

Calculating empirical formulae
… The empirical formula of a compound is the simplest ratio of the elements in the compound. For example, HO is the empirical formula for hydrogen peroxide. However, the molecular formula is H2O2.

… Use the “Mass – Mr – Moles” table method. The headings are the elements (in words) that make up the compound you are investigating. You don’t need a chemical equation for this calculation.
(Mr could sometimes also be the Ar of a substance, it just sounds better!).

… Work down the table finding the moles of each element present.

… Remember to divide the mole ratio by the smallest number in the ratio to get whole numbers.

… *** Note! For empirical formula questions, ALWAYS treat the headings of the Mass-Mr-Moles table as individual ATOMS, not molecules. For example, if the question says that 227g of calcium react with 216g of fluorine gas, then our table would look like this:

                 Ca             F
Mass     227g         216g
Mr          40              19
Moles

Here we have used the relative ATOMIC mass of fluorine (Ar) = 19… not Mr = 38 (which would be for the molecule of fluorine gas, F2).

… Suppose 3.2 g of sulfur reacts with oxygen to produce 6.4 g of sulfur oxide. What is the empirical formula of the oxide? (Ar of sulfur is 32 and the Ar of oxygen is 16)

… When given a percentage by mass of an element within the compound, treat the percentages as masses – it will still work.
For example, what is the empirical formula of vanadium oxide that contains 56% of vanadium?

… To work out the relative atomic mass from isotope percentage abundances, first imagine that you have 100 atoms. For example:
Chlorine exists in two isotopes on Earth:
Cl-35 with abundance 75%
Cl-37 with abundance 25%

So if we had 100 atoms, then on average we would have the following masses:
35 x 75 atoms = 2625
37 x 25 atoms = 925
Total mass of these 100 atoms = 2625 + 925 = 3550

So the average mass of 1 atom = 3550/100 = 35.5, which is the relative atomic mass of Chlorine (as seen on the periodic table)

… ‘Water of crystallisation’ is the name given to molecules of water which bond into the ionic lattice. This forms a crystalline structure which we call ‘hydrated’. A good example of a hydrated ionic compound is the blue crystals of copper sulfate:
CuSO4.5H2O
This means that for every 1 mole of copper sulfate, there are 5 moles of water that bond in the crystal.

… If you heat a hydrated ionic compound, then this will drive off the water of crystallisation usually leaving a white powder – an anhydrous compound, i.e. without water.
CuSO4.xH2O  —->  CuSO4      +    H20

… We can use the Mass – Mr – Moles table to find the number of moles of water of crystallisation in the same way as for finding the empirical formula of a compound.
The two column headings will be the PRODUCTS of heating the hydrated crystals: the ionic compound and water, for example:

               CuSO4             H20
Mass
Mr
Moles

First calculate the mass of anhydrous copper sulfate and of the water of crystallisation using the mass data given. Then work out the moles in the usual way to find ‘x’ (Down is Divide).
(This method is very similar to finding the empirical formula of a compound)

Volumes of gases

… 1 mole of gas occupies 24dm³ at room temperature pressure (rtp). This means that the moles of a gas are proportional to its volume and we can use this to work out volumes in reactions.

For example, if 50cm³ of ethyne gas is burned, what volume of oxygen is required and what are the volumes of the gas products? 

2C2H2(g) + 5O2(g) = 4CO2(g) + 2H2O(g)
(Note that the numbers after the letters should be subscript!)

2 parts = 50cm³, which means that 1 part = 25cm³

So 5 × 25cm³ = 50cm³ of oxygen gas is required.
The action produces:
4 × 25cm³ = 100cm³ of carbon dioxide gas
and
2 × 25cm³ = 50cm³ of water vapour.


Percentage Yield

… Why do we often collect less product from a chemical reaction than predicted from its equation (less than 100% yield)?

…  % yield = (actual mass of product / theoretical maximum mass of product) x 100%

… You may need to use a Mass-Mr-Moles table to work out the theoretical maximum mass of a product.


Atom economy

Industrial processes need as high an atom economy as possible, because this:
Reduces the production of unwanted products
Makes the process more sustainable
Atom economy = mass of desired product from equation / total mass of products from equation
Note that the total mass of products should be the same as the total mass of reactants.

For example, hydrogen can be made from methane and steam:
CH4 + 2H2O → CO2 + 4H2

Mr of H2 = 1 + 1 = 2
Mr of CO2 = 12 + 16 + 16 = 44
Atom economy for making hydrogen = (4×2)/(44 + 4×2) = 8/52 × 100 = 15.4%
(Note that we DO need to use the balancing numbers from the equation)


Analysing Substances – instrumental methods

… Elements and compounds can be detected and identified using instrumental methods, which are more accurate, sensitive
and rapid compared to chemical tests.

Flame emission spectroscopy

… Flame emission spectroscopy can be use to identify …?… ions in solution

… The sample is put into a …?… and the light that is given out is analysed by a spectroscope which can identify different wavelengths the light.

… The resulting l…?… sp…?… can be analysed (by a computer) to identify the ions and their relative concentrations.

Chromatography

… Paper chromatography can be used to separate and help identify mixtures, for example artificial colours used in foods. Some food colourings are unsafe for human consumption.

… In paper chromatography, a spot of the pigment to be analysed is put onto paper and a solvent is allowed to move through the paper (via capillary action). The constituent colours in the pigment move different distances depending on their solubility (the most soluble pigments are carried the furthest).

… The Rf value of each pigment = greatest distance moved by pigment / distance moved by solvent. Rf values are specific to each pigment in a particular solvent.

… By using Rf values, paper chromatography can be used to compare the constituent pigments of an unknown colour to a database of safe pigments.

… The ‘baseline’ height on the paper should be drawn in pencil so that it doesn’t dissolve in the solvent.

… What are three main advantages of using the instrumental analysis techniques of Gas Chromatography and Mass Spectrometry compared to traditional chemical methods?

… Compounds in a mixture can be …?… using Gas Chromatography.

… In Gas Chromatography, the individual compounds of the vaporised mixture travel at different …?… through a long c…?… and come out at different ‘r…?…’ times. These times can be compared with the results from known compounds to help …?… the compounds in the mixture.

… The gas chromatography column has its own type of detector at its output. This only detects the presence of a substance without necessarily being able to identify it – so a graph of retention time vs concentration can be plotted.

… The output from a …?… …?… (GC) column can be linked into a …?… …?… (MS) which can be used to find the r…?… m..?… m..?… of a compound from its molecular …?… peak. This is the peak furthest to the …?… on a mass spectrum graph.

Chromatography – in more detail

… In all types of chromatography, substances are separated as they are carried in a mobile phase (think hitchhikers getting lifts in cars) which passes through a stationary phase (think cafés along the route).

… Different substances travel at different speeds (some hitchhikers like to stop off in cafés more than others!), so some move further than others in a given time.

… In paper chromatography, the stationary phase is paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous organic (carbon-based) solvent.

… For each chemical in the sample, there is a dynamic equilibrium (a balance) between the stationary phase and the mobile phase. The overall separation after a certain time of travelling depends upon how strongly attracted the chemicals are to the mobile and the stationary phases.

… Thin layer chromatography (TLC) is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert (unreactive) substance (eg silica) supported on a flat, unreactive surface (e.g. glass)

… TLC tends to produce more useful chromatograms than paper chromatography as they show greater separation of the components in the mixture – and are therefore easier to analyse.

… The movement of a substance during chromatography, relative to the movement of the solvent, is measured by calculating its retardation factor (Rf).
Rf value = distance travelled by substance / distance travelled by solvent

… Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances (standard reference data) under the same conditions.

… In gas chromatography (GC), the mobile phase is an inert gas (eg helium).

… The stationary phase is a very thin layer of an inert liquid on an inert solid support – such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).

… The amount of time that a substance takes to pass through the column is called its retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.

… GC is very sensitive so can detect which chemicals and how much of them there are from a tiny sample. A gas chromatogram is a graph of substance concentration (vertical axis) vs retention time (horizontal axis).

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