year 12 chemistry easter revision session learning outcomes - abg: be aware of the key skills...

Year 12 Chemistry Easter Revision Session

Learning Outcomes - ABG:• Be aware of the key skills required in the AS

chemistry exams.• Pick up some useful exam tips.

Content (if time allows):• Definitions• ABG Question 1• Moles calculations• Ionisation energies• Shapes of molecules• Intermolecular forces

Definitions:• These count for a significant number of marks on AS

chemistry papers and learning them could easily move you up a grade in this subject.

• It’s not difficult to learn definitions but it does take time.

• Make flash cards for each one and test yourself everyday on 10 or 20 of these.

* Hand out of the key definitions from the ABG and CER papers.

Note that this list isn’t exhaustive – see your textbook and past papers for any others that might come up.

Definitions – example of why these are important:• The mock paper you sat in January had the following

definition questions:Relative isotopic mass – 2 marksDative covalent bond – 1 mark

• You also effectively needed to give the definitions of:Disproportionation reaction – 1 markSalt – 1 mark

• You also needed to know and recognise the definition of:Water of crystallisation – 1 mark

These 6 marks are 10% of the total marks on this paper!!This could easily cost you one or even two grades if you miss these marks!!

ABG definition game:• Challenge your partner to see who knows their

definitions best.• You’ll each be given a list of 5 definitions – in pairs test

each other and keep count of who gets the most right.

CER definition activity:• You’ll about to be shown some of the definitions that

typically appear on CER papers.• Your task is to either identify the definition name or to

write the definition out. Use your mini-whiteboards for this task.

CER definition activity:

Hydrocarbon

An organic compound of hydrogen and carbon only.


Homolytic fission

The breaking of a covalent bond with one of the bonded electrons going to

each atom, forming two radicals.


Electrophile

An atom (or group of atoms) that accepts a pair of electrons to form

a covalent bond.


Stereoisomers

Compounds with the same structural formula but with a different

arrangement of the atoms in space.


Standard enthalpy of formation

The enthalpy change that takes place when one mole of a compound in its standard

state is formed from its constituent elements in their standard state sunder

standard conditions.


Hess’ Law

If a reaction can take place by more than one route and the initial and final conditions are the same, the

total enthalpy change is the same for each route.


Le Chatelier’s principle

When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the

change.


E/Z isomerism

A type of stereoisomerism in which different groups attached to each

carbon of a C=C bond may be arranged differently in space because of restricted rotation of the C=C bond.


Structural isomers

Molecules with the same molecular formula but with different structural

arrangements of atoms.


Enthalpy

The heat content that is stored in a chemical system.

Definitions - exam tip:Look at the definitions which have been asked in the first question of all these past ABG papers:• June 13 – Relative isotopic mass• Jan 13 – Isotopes• June 12 – Relative atomic mass• Jan 12 – Relative isotopic mass• June 11 – First ionisation energy• Jan 11 – Relative atomic mass• June 10 – Relative atomic mass• Jan 10 – Relative isotopic mass• June 09 – Relative atomic mass• Jan 09 – Relative isotopic mass and Isotopes

If I were you, I would definitely make sure I know all my relative mass definitions!!

ABG Question 1:• As well as having predictable definitions in the first

question, they also have some predictable isotope questions.

• Look at these questions from the last 4 ABG papers:

e.g. June 13

ABG Question 1:

e.g. Jan 13

ABG Question 1:

e.g. June 12

ABG Question 1:

e.g. Jan 12

ABG Question 1:

• Now have a go yourselves at these type of questions – see handout .

• When you’ve finished compare your answers with the person next to you.

• You have up to 12 mins (remember that you have approx. 1 min per mark on the AS papers).

ABG Question 1:Answers – Specimen paper:

35 44 35

35 46 35

ABG Question 1:Answers – June 09:

12 12 12

12 13 12

ABG Question 1:Answers – Jan 10:

151Eu has 2 fewer neutrons / 153Eu has 2 more

neutrons / they have different numbers of neutrons

They have the same number of protons and electrons / they both have 63 protons and 63 electrons

ABG Question 1:Answers - June 10:

50 68 50

120Sn has two more neutrons / 120Sn has 70 neutrons /120Sn has a different number of neutrons

ABG Question 1:Answers - Jan 09:

Atoms with the same atomic number but different mass number / atoms with the same number of protons but different

numbers of neutrons

They have the same number of electrons / they have the same electronic structure /

configuration

Mass of the isotope / atom compared to 1/12th the mass of carbon-12.

Mole calculations:

• These appear in all examined aspects of the A-level chemistry course.

• You must be confident with the three main equations needed for mole calculations.

• Write these on your mini-whiteboard now.

Mole calculations:

• Moles = mass (g) / relative mass

• Moles = volume (dm3) x concentration (moldm-3) or

• Moles = volume (cm3) x concentration (moldm-3) / 1000

• Moles = gas volume (dm3) / 24 or

• Moles = gas volume (cm3) / 24,000

Mole calculations:

• We’ll work through a few examples from past paper questions together and then you can attempt some individually.

• See the following question from ABG June 2010. Use your mini-whiteboard to jot down your working and answers as we go through it together.

Moles = volume x conc.

= 24.6 x 0.1 / 1000 2.46 x 10-3

Moles = moles of H2SO4 x 2 (1:2 ratio)

= 2.46 x 10-3 x 2

4.92 x 10-3

4.92 x 10-3

Moles of NaOH in 250 cm3 = 4.92 x 10-3 x 10

= 0.0492

Mass in original sample = moles x relative mass

= 0.0492 x 40.0 = 1.968 g

% of NaOH = (1.968 / 2.00) x 10098.4 %

Mole calculations - exam tip:• You should always write out all your working as I did here.

• i.e. Write out what you’re trying to calculate,

then the equation,

then substitute in the numbers

and finally write out the answer.

• This means that should you get the final answer wrong the examiner can easily read through your working and see if you deserve any ‘error carried forward’ marks (ecf).

• If they can’t follow your working (or if there isn’t any) then it’s very unlikely you’ll get these.

Practise this!!!

Mole calculations:

• Here’s a moles calculation question from the CER June 2010 paper:

Moles = mass / relative

mass

= 1.00 x 106 / 64.1 15601

Moles = gas volume / 24

= 3.60 x 105 / 2415000

15000

Moles = gas volume / 24

= 3.60 x 105 / 24

% yield = (actual yield / theoretical yield) x

100

= (15000 / 15601) x 100 96.1

Mole calculations:

• Now try these two past paper moles calculations yourselves:

ABG Jan 11ABG Jan 10

• You have 9 minutes max.

Mole calculations:Answers - Jan 11:

Moles of HxA = vol x conc.

= (25/1000) x 0.05

1.25 x 10-3

Moles of NaOH = vol x conc.

= (12.5/1000) x 0.2

2.5 x 10-3


x = 2.5 x 10-3 / 1.25 x 10-3

2


Moles of NaOH = vol x conc.

= (25/1000) x 0.0882.2 x 10-3

Moles of H2SO4 = moles of NaOH / 2

(1:2ratio)

= 2.2 x 10-31.1 x 10-3

Mole calculations:

Conc. of H2SO4 = moles / vol

= 1.1 x 10-3 / (17.60/1000)

0.0625

Water of crystallisation

Mr (Na2SO4) = 2 x 23+ 32.1+ 4 x 16

= 142.1

322.1 – 142.1 = 180

x = 180 / Mr (H2O) = 180 / 1810

Mole calculations - exam tips:

• Remember to use the atomic mass values from the datasheet given to you in the exam.

• Remember to show all working (in a coherent way!).

• Check all calculations twice and then look at the answer to make sure it looks sensible.

• You can find plenty of moles questions to attempt yourselves at home in your textbook or in past paper questions.

Ionisation energies:• Write down the definition of first ionisation energy of

sodium.The energy required to remove one electron from each atom in one mole of gaseous sodium atoms to form gaseous Na+ ions.

• Now write this down as an equation, including state symbols.

Na (g) Na+ (g) + e-

• Now write down the equation showing the third ionisation energy of aluminium, including state symbols.

Al2+ (g) Al3+ (g) + e-

Ionisation energies:• Look at the graph of successive ionisation energies shown

below.• Identify the element is shown here.• Give reasons for your answer.

Oxygen:There is a large increase in IE after 6 e- have been removed. This shows that the 7th e- has been removed from an inner shell. This shell is closer to the nucleus so electrons experience more nuclear attraction.

Ionisation energies:• Now sketch an equivalent graph for sodium.

Ionisation energies:• Why do you see an increase in IE amongst electrons being

removed from the same shell?

As each e- is removed from a shell the remaining electrons experience less repulsion so the shell shrinks in size and is drawn in closer to the nucleus.

Therefore, the remaining e-experience a greater nuclear attraction so more energy is needed to remove them.

Ionisation energies:• Sketch a graph showing the first ionisation energies for the

first 20 elements.

Ionisation energies:• State and explain the trend seen in 1st IE across a period.• State and explain the trend seen in 1st IE down a group.

Before you start what 3 factors do you need to consider?

1. Nuclear charge2. Distance from

the nucleus3. Shielding

Ionisation energies:• State and explain the trend seen in 1st IE across a period.

IE increases across a period because attraction between the nucleus and outer electron increases:

Nuclear charge – this increases as the number of protons increases.Distance from the nucleus – this decreases slightly as atomic radius decreases across a period.Shielding – electrons are added to the same shell so shielding doesn’t change.

Ionisation energies:• State and explain the trend seen in 1st IE down a group.

IE decreases down a group because attraction between the nucleus and outer electron decreases:

Distance from the nucleus – this increases because there are more shells.Shielding – this increases because there are more inner shells.Nuclear charge – this increases as the number of protons increases but this factor is outweighed by the other two.

Shape of molecules:• You need to be able to draw and name the shapes formed

by common covalent molecules.• You then need to be able to suggest bond angles based on

these shapes.• Finally you may need to explain why molecules adopt

these shapes.

*Complete the handout on ‘Shapes of Molecules’.

When you’ve finished then write an explanation of why ammonia has the bond angle you’ve suggested.

Molecule

Diagram

Shape

Bond angle

No. of bonding

pairs/groups

No. of lone

pairs

Methane

Water

Boron

trifluoride

Carbon dioxide

Ammonia

Tetrahedral

Non-linear

Trigonal planar

Linear

Pyramidal

109.5o

104.5o

120o

180o

107o

4

2

3

2

3

0

2

0

0

1

Shape of molecules:When you’ve finished then write an explanation of why ammonia has the bond angle you’ve suggested.

• Ammonia has 3 bonding pairs and 1 lone pair.• Electron pairs repel.• Lone pairs repel more than bonding pairs.

See ABG Jan 12 Q5 for a very similar question to this.

Shape of molecules – exam tips:• If you can’t remember the number of bonding pairs/lone

pairs/shape of a molecule/bond angle etc. then draw out a dot-cross diagram of the molecule.

• It will take a few extra seconds but will then give you a much greater chance of getting the marks for this question.

• Don’t just guess (unless you’re really short of time)!!

Intermolecular forces:• You need to be aware of 3 types of intermolecular forces.

Write these on your mini-whiteboard now.

Intermolecular forces:• Now match up the three types of intermolecular force with

the two correct molecules (mini-whiteboards):

• Van der Waal’s forces

• Permanent dipole-dipole

• Hydrogen bonds

Br2 H2O HCl

CHCl3 CCl4 NH3

Br2 and CCl4

HCl and CHCl3

H2O and NH3

Intermolecular forces:Van der Waal’s forces

• How do VdW forces form?• Caused by random movement of electrons in shells.• This produces instantaneous dipoles.• These induce dipoles in neighbouring molecules.• These small instantaneous and induced dipoles result in

weak attractions between the molecules.• Why is chlorine a gas at room temperature whereas iodine

is a solid?• As you go down group 7 the number of electrons

increases and so does the strength of the VdW forces.• This results in an increase in boiling point.

Br2 and CCl4

Intermolecular forces:Permanent dipole-dipole

• What type of molecule has this type of intermolecular force?• A polar molecule – an unsymmetrical molecule

containing polar bonds.• How does electronegativity vary as you down a group and

across a period?• It decreases as you go down a group and increases as

you go across a period.

HCl and CHCl3

Intermolecular forces:Hydrogen bonding

• Draw a hydrogen bond between two water molecules.

H2O and NH3

Year 12 Chemistry Easter Revision Session

Learning Outcomes - ABG:• Be aware of the key skills required in the AS

chemistry exams.• Pick up some useful exam tips.

Content (if time allows):• Definitions• ABG Question 1• Moles calculations• Ionisation energies• Shapes of molecules• Intermolecular forces

year 12 chemistry easter revision session learning outcomes - abg: be aware of the key skills...

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