Chemistry A-Level
Atomic structure and isotopes
… The relative mass of an election is…?
… The relative atomic mass can be calculated from the relative abundances and m/z data using the equation:
relative atomic mass (Ar) = (combined mass of all isotopes) ÷ (combined abundances of all isotopes)
(combined masses = m/z × % relative abundance , for each pair of data points).
… What is meant by an isotope?
… A ‘Time-of-flight’ mass spectrometer accelerates ions in an electric field to measure the relative abundance of different isotopes within a sample .
The more massive ions accelerate at a slower rate and so reach the detector after the less massive ions (however, the gain in kinetic energy of each ion will be the same).
… The definition of relative atomic mass is:
“the ratio of the average …?… of one atom of an element to one twelfth of the mass of an atom of …?…-12.”
Polarity and intermolecular forces
… Some atoms attract bonding electrons more than other atoms. An atom’s ability to attract the electron pair in a c…?… bond is called …?…
… Covalent bonds may be polarised by differences in electronegativity, which causes a p…?… d…?…
… Why are the covalent bonds in diatomic gases of elements non-polar?
… In polar molecules, charge is arranged unevenly.
… If the polar bonds are arranged symmetrically so that the dipoles cancel each other out, then the molecule has no overall dipole and is non-polar.
… There is a gradual transition from ionic to covalent bonding.
… The higher the difference in electronegativity, the more …?… in character the bonding becomes.
… Intermolecular forces are much weaker than covalent, ionic or metallic bonds.
… The three types of intermolecular forces are …? The strongest type is…?
… How do induced dipole dipole forces occur in non-polar molecules or atoms? How do these forces change as the atom or molecule increases in size?
… What physical properties are affected by induced dipole-dipole forces?
… Induced dipole dipole forces can hold molecules in a lattice. For example, this occurs in iodine, I2.
… Polar molecules form permanent dipole-dipole interactions.
… Why is water is boiling point higher than expected in comparison to other similar molecules?
… How do you intermolecular forces explain the behaviour of simple covalent compounds, for example; melting and boiling points, solubility in water, and conductivity of electricity?
Enthalpy Changes and Calculations
… Enthalpy change, ∆H, is the heat energy transferred per mole in a reaction at constant pressure. The units of ∆H are…?
… What are the conditions for standard temperature pressure (stp)?
… What are the conditions for room temperature pressure (rtp)?
… The standard enthalpy change of…
- … Reaction ∆H(r) is the …?… change when the reaction occurs in the m…?… quantities shown in the chemical equation, under s…?… c…?…
- … Formation ∆H(f) is the e…?… change when one …?… of a compound is formed from its …?… in their standard states, under s…?… c…?…
- … Combustion ∆H(c) is the e…?… change when one …?… of a compound is completely …?… in …?… under s…?… c…?…
- … Neutralisation ∆H(neut) is the e…?… change when an …?… and an …?… exact together, under s…?… c…?… to form one …?… of …?…
… Describe the experimental method for determining ∆H of combustion for a fuel. What equipment would you use? What would you need to measure? Which equation would you need to use?
… Why is the Enthalpy change of combustion determined in a laboratory likely to be lower than the value shown in a databook? (Two reasons…)
Hess’s Law Calculations
… Hess’s Law says that the total enthalpy change (heat energy transferred) of a reaction, ∆H(r) is the same whichever route you take from reactants to products.
… Enthalpy changes of formation, ∆H(f), are useful for calculating enthalpy changes that cannot be found directly. For these calculations you will need to know ∆H(f) for all the reactants and products that are compounds (remember that ∆H(f) for elements is zero)
… Remember to multiply ∆H(f) by the number of moles of the product you are making.
… To work out ∆H(r) add up the ∆H values on the alternative routes in the direction of the reaction. If you go AGAINST an arrow direction, then change its ∆H sign.
… Enthalpy changes can also be worked out from given enthalpies of combustion or by using average BOND enthalpies.
… Breaking bonds always means putting energy IN (Endothermic +).
… Making bonds always means that energy is released (Exothermic -)
… Note that average bond enthalpies are not exact. The energy needed to break one mole of bonds in the gas phase is averaged over MANY difference compounds.
Isomerism
… An isomer (from the Greek, isomerès; isos = “equal”, méros = “part”) is a molecule with the same molecular formula as another molecule, but with a different chemical structure.
… Isomerism was first noticed in 1827 when Friedrich Woehler prepared silver cyanate and discovered that, although its elemental composition was identical to silver fulminate, its properties were quite different.
… Isomers exist in two basic families:
- structural isomers (e.g. but-1-ene and but-2-ene)
- stereo isomers (literally mirror images of each other)
… Isomers can undergo different reactions, especially in biology where orientation of the molecule becomes important with more complex reactants.
… Stereo isomers have the same structural formula but a different arrangement in space. These isomers can occur because the double bond between two carbon atoms cannot rotate.
… One of the isomers is called an E-isomer (same groups positioned across the double bond) and the other is called a Z-isomer (same groups either both above or both below the double bond). It can be useful to remember,”ze zame zide” 🙂
… Atoms with a larger atomic number are given a higher priority. Are is called the Cahn, Ingold and Prelog rules. It’s then possible to assign E/Z isomer names to C=C atoms with totally different groups attached to them.
… E/Z isomers can sometimes also be called cis (same side) or trans (opposite sides) isomers. However, this method doesn’t work if the carbon atoms have totally different groups bonded to them… But the E/Z system does work.
Electronic structure
… Electron shells are made up of sub-shells and orbitals
… Sub-shells are called s, p, d or f sub-shells.
… The number of orbitals in each sub-shell are…?
… The number of electrons that can occupy each orbital is…?
… What is the electron configuration of a silicon atom using sub-she’ll notation?
… Transition metals are often referred to as d block metals. These are elements which form some compounds in which there is an incomplete sub shell of d electrons.
Ionisation Energies Periodicity
… The first ionisation energy is the energy needed to remove one …?… of electrons from one mole of atoms in the …?… state. This is an endothermic process.
… The three main factors affecting ionisation energy are
- Nuclear c…?…
- A…?… r…?…
- S…?… (The number of electrons between the outer electrons and the nucleus)
… Ionisation energy …?… as you go down a group. This is because…?
… Ionisation energy …?… as you go across a period. This is because…?
… The unexpected DROP in the first ionisation energy between elements in groups 2 and 3 is due to…?
… The unexpected DROP in the first ionisation energy between elements in group 5 and 6 is due to…?
… Successive ionisation energies provide evidence for the structure of …?…
… There is a big jump in ionisation energy when a new s…?… is broken into.
… Within each shell, successive ionisation energies …?… . This is because…?
Physical properties periodicity
… Graphite has weak dipole-dipole forces between layers of carbon atoms
… Graphene is a single sheet of graphite carbon molecules. It is the best known electrical conductor – even better than graphite because…?
… Bond strength affects melting and boiling points across a period:
- For the metals (Li, Be, Na, Mg, Al), melting and boiling points …?… as you go across the period because the metallic bonds get …?… as the ionic radius …?… and the number of delocalised electrons …?…
- The elements with giant covalent lattice structures (for example, …?… and …?…) have strong …?… bonds linking all their atoms together, so a lot of energy is needed to break these bonds.
- The elements that form simple molecular structures have …?… melting and boiling points because…?
– the noble gases have the …?… melting and boiling points because…?
Electronegativity
… The element with the lowest electronegativity is …?…
… The element with the highest electronegativity is …?…
… In an ionic compound, the relative electronegativity of the anion compared to the cation determines the …?… or …?… nature of the bond.
The Mole
… Molar mass, M (in units of g/mol) is the same as the relative molecular mass, Mr. It is just telling us the mass of 1 mole of the substance.
… Number of moles = mass (g) / molar mass
… How many moles of chlorine molecules are present in 71g of chlorine gas? (Remember that chlorine has diatomic molecules)
… Avogadro’s constant is the number of particles in 1 mole.
So 1 mole of oxygen has 6.02×10^23 oxygen MOLECULES.
How many ATOMS are there in 1 mole of oxygen gas?
… “Room temperature pressure” (r.t.p.) is
25°C = …?…K
and
101.3kPa = …?… Pa
… The ideal Gas Equation: pV = nRT
- What are the quantities and units of p, V, n and T.
- What is the value and units of the gas constant, R?
… How many cm³ are there in a m³? How could you work this out?
… How many dm³ are there in a m³? How could you work this out?
… It is an excellent idea to LIST your data before putting the numbers into the formula. Remember that you may need to convert your data into the standard units of m³, Pa, and K.
… Your answer should be rounded to the least number of significant figures used in the starting data (or one more is usually fine).
… Explain what relative atomic mass (Ar) and relative isotopic mass mean.
… How do you calculate the relative atomic mass from percentage abundance figures? For example, a natural sample of chlorine contains a mixture of Cl-35 (75%) and Cl-37 (25%).
… A mass spectrum of a Neon sample gives:
Mass/charge Relative isotopic abundance 20 114.0 21 0.2 20 11.2 How do you calculate the relative atomic mass from these mass spectroscopy results? ( The mass/charge ratio is usually the same as the relative isotopic mass.)
… To find the concentration of a solute as a percentage by mass, divide the concentration of the solute (in g/dm3) by the dentist of the total solution (in g/dm3):
% by mass of solute in the solution = (g/dm3 solute) / (g/dm3 solution)
Oxidation numbers and Redox reactions
… The Oxidation number tells you how many electrons an atom has.
… All uncombined elements have an oxidation number of…?
… The oxidation number of a single atom ion is the same as its charge.
… For molecular ions, the sum of the oxidation numbers is the same as the overall charge on the ion.
… Each element in a neutral compound made up of more than one element, will have its own oxidation number.
… Oxygen usually has an oxidation number of -2, except in peroxides where it is -1, (and in molecular oxygen where it is 0).
… Hydrogen always has an oxidation number of +1, except in metal hydrides where it is -1 (and in molecular hydrogen where it is 0).
… If electrons are transferred then it is a redox reaction: “OIL RIG”.
… If the oxidation number of an element increases, then the element has lost electrons and has been oxidised.
… If the oxidation number of an element has decreased, then the element has gained electrons and has been reduced.
Analytical techniques:
… In infrared spectroscopy a beam of infrared radiation is passed through a sample of a chemical. The infrared radiation is absorbed by the covalent bonds in the molecules increasing their vibrational energy.
… Bonds between different atoms absorb different frequencies of infrared radiation. Also, bonds in different places in the molecule absorb different frequencies too.
… An infrared spectrometer spectrum shows you what frequencies of radiation are absorbed by the molecules. This can be used to identify the functional groups in a molecule.
… Uses for infrared spectroscopy include breathalysers and instruments for monitoring concentrations of polluting gases in the atmosphere.
… A mass spectrometer is a machine which bombards molecules with electrons. This removes an electron from the molecule to form a molecular ion, M+.
… For most organic compounds the molecular peak is the one with the SECOND highest mass/charge ratio. This gives the compounds relative molecular mass. The small peak to the right of the molecular peak is called the M+1 peak, caused by the presence of the carbon isotope C-13.
… The molecular ion can be broken into smaller fragments, which show up on the mass spectrum, making a fragmentation pattern. You can use these peaks to identify the fragments and so piece together the structure of the complete molecule.
… Large computer databases of mass spectra can be used to identify a compound from its mass spectrum.
… It is only the m/z value that you’re interested in, so ignore the heights of the bars.
… To identify a compound from its mass or percentage composition, infrared spectrum and mass spectrum:
- Use the empirical formula and the mass peak to work out the molecular formula of the compound.
- Work out what functional groups are in the compound from its infrared spectrum.
3. Use the mass spectrum to work out the structure of the molecule by using the possible masses of fragments (e.g. methyl groups, O-H groups etc.)
Nomenclature (Naming) of Organic Compounds (“systematic naming”)
… Count the carbon atoms in the longest continuous chain. This gives you the stem (e.g. “meth-“, “eth-“, “prop-“)
… The main functional group of the molecule usually tells you what homologous series the molecule is in (e.g. alcohols, aldehydes, ketones)
… Number the longest carbon chain so that the main functional group has the lowest possible number. If there is more than one longest carbon chain, pick the one with the most side-chains.
… Any side-chains or less important functional groups are added as a prefixes at the start of the name (alphabetical order after the number of the carbon atom each one is attached to).
… Use “di”, “tri” or “tetra” for multiple side chains or functional groups.
… For example, draw the displayed formula for 1-bromo-2-methyl-pentane
Shapes of molecules
… The molecular shape depends on the election pairs around the central atom.
… Lone pairs (LP) repel more than bonding pairs (BP).
… LP / LP angle > LP / BP angle > BP / BP angle
… The shape is described by the atoms, not the lone pairs. First draw out a dot and cross diagram of the molecule so you can count the election pairs, noting if there are any lone pairs. Then you’ll need to learn the following:
2 electron pairs around the central atom:
… Linear: 180°, e.g. BeCl2
3 electron pairs around the central atom:
… Trigonal planar: 120°, e.g. BF3 (no lone pairs) or ClF3 (which has two lone pairs above and below the flat triangle)
4 electron pairs around the central atom:
… Tetrahedral (no lone pairs): 109.5°, e.g. (NH4)+, CH4
… Trigonal Pyramidal (one lone pair): 107°, e.g. PF3, NH3
… Non-linear, or ‘bent’ (two lone pairs): 104.5°, e.g. H2O
5 electron pairs around the central atom
… Trigonal Bipyramidal (no lone pairs): 90° and 120°, e.g. PCl5
6 electron pairs around the central atom:
… Octahedral: all 90° (no lone pairs), e.g. SF6
Dynamic Equilibrium for (reversible reactions)
… At equilibrium the concentrations of reactants and products stay constant and the forward and backwards reaction rates are equal.
… Le Chatelier’s principle helps us to predict the effect of changing the reaction conditions on the equilibrium position (The amounts of products and reactants at equilibrium)
“If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.”
… Catalysts have no effect on the equilibrium position. They speed up the forward and backward reactions by the same amount so they do NOT increase yield.
Catalysts cause equilibrium to be established faster.
The Haber process
… As the production of ammonia is a reversible reaction we can favour the forward exothermic direction by:
- lowering the …?… (but not too much so that we maintain the overall …?… of reaction), and
- increasing the pressure to as high as possible given c…?… and safety factors.
… The Haber process requires an …?… catalyst to speed up the rate of reaction. This does NOT affect the y…?… but does mean that the equilibrium position will be achieved …?…. So if you remove some ammonia, then more will be produced quickly.
… Note that if ammonia could be removed immediately from the products formed, then the yield would also increase. This is because the forward reaction rate would increase when products are removed.
This doesn’t happen in practice because it is difficult to remove the ammonia immediately after it has been produced (it is separated by condensing the product mixture some time afterwards).
… Kc is the equilibrium constant.
For the reaction:
aA + bB <=> dD + eE
Then
Kc = ( [D]^d [E]^e) / ( [A]^a [B]^b )
If Kc is very large (could be in the 1000s!), then it means that the equilibrium position is mostly on the products side.
… In some questions, you may be given the starting amounts in moles for some of the reactants or products. These amounts will change as the equilibrium then starts to develop.
An “ICE” table (initial concentration, change in moles, equilibrium concentration) can help us to find out how many moles of reactants and products there are once the equilibrium has been established. Here’s a video to explain how it works:
https://youtu.be/tT-2xk9ZG_A
Note that the balancing numbers must be the x multipliers in the ‘change column’, so for example, if we start with 0.6M of both hydrogen and iodine gases for the reaction:
H2 + I2 <=> 2HI
I 0.6 0.6 0
C -x -x +2x
E 0.6-x 0.6-x 0+2x
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