as chemistry 2006

AS Chemistry 2006

Revision session 2

Ionisation energies

Define the term ionisation energy (3 marks)

Write an equation to show the first ionisation enthalpy of Element “E” including state symbols (3 marks)

The enthalpy change when 1 electron is removed from 1 mole of atoms in the gaseous state.

E(g) E+(g) + e-

Complete the table for these isotopes of oxygen

Number of Protons

Number of neutrons

Number of electrons

16O

17O

18O

Ionisation energies 3

Given your answers to the table, would you expect the first ionisation enthalpies for each of the isotopes to be similar or different?

Explain your answer

Ionisation enthalpies 4

They would be basically the same- as they each have the same number of protons (and electrons) so each has the same nuclear charge holding the electrons- so equal amounts of energy would be needed to remove 1 mole of electrons in each case.

If you are uncertain-ask!!

Ionisation enthalpies 5

Ionisation enthalpies 6

Explain why the 1st IE is higher for (i) Ne compared to Ar (ii) Be compared to Li (iii) Be compared to B

Ionisation enthalpies still!

(i) Ar has an extra shell of electrons so there is a less strong pull from the positively charged protons, and there is also more shielding which reduces the pull still further therefore it is easier to remove the electron(s)

Therefore Ar has a lower IE (Ne is higher)

(ii) Be has an extra proton in its nucleus, but the extra electron has gone into the same shell (the 2nd). So the extra electron is no further away from a stronger pull

(iii) No idea?? Answer this first then- Write out the spd electron configurations for

the 2 elements.

Finally!

Be- 1s2 2s2

B- 1s2 2s2 2p1

The extra electron for B is in a new sub-shell- ie at higher energy- ie further away from the nucleus- thus easier to remove

Probably too hard for AS Level- but you never know- and you did have the knowledge- you just didn’t realise!!

4d

1s

2s

3s

4s

2p

3p

4p

3d

4f

Berylium, Be1s2 2s2

4d

1s

2s

3s

4s

2p

3p

4p

3d

4f

Boron, B1s2 2s22p1

An easier question

Draw the bonding in magnesium sulphide- showing the outer shell electrons only

Did you get it right?

Don’t forget the following traps- Metal + non-metal = IONIC bonding- so show

the IONS!! Show the difference between the two sets of

electrons;

Shapes of molecules

Draw the bonding in an ammonia molecule (outer shell electrons only)

Use this to state the bond angle, shape and explanations

Answers- 109° (is good enough for your

syllabus) Because it has 4 pairs of electrons which want to get as far apart as

possible/ minimise repulsions And has only 3 bonding pairs of

electrons Shape: Pyramidal (NOT

tetrahedral)

Metallic bonding

Draw a labelled picture to represent the bonding in metals and explain how it “is kept together!”

Please note I have not asked about dative bonding due to time!! But I expect your notes to cover it!

Metallic Bonding

Positive ions

Delocalised electrons

Group 2 Chemistry

Write balanced chemical equations (including state symbols) for

(i) the reaction between calcium and water (ii) the thermal decomposition of strontium

carbonate (don’t even think about o!!) State the trend in solubility of grp 2

hydroxides and carbonates, and the thermal stability of the grp 2 carbonates.

Group 2 Chemistry

Ca(s) + 2H2O(l) Ca(OH)2 (aq) + H2 (g)

SrCO3(s) SrO(s) + CO2 (g)

Trends- Decrease in solubility of the carbonates Increase in solubility of hydroxides Increase in thermal stability of carbonates

More on group 2

With reference to their respective electronic configurations explain why Magnesium and Calcium react in a similar way, but have different reactivities. (4 marks)

An answer They react in a similar way because they both

have 2 electrons in their outer shells and it is this that governs the chemical properties of an element- so we can expect them to react in a similar way.

They are different because outer most electrons in Ca are further away from nucleus therefore lost more easily = more reactive (Ca = 4 shells : Mg 3 shells), less energy needed to remove these 2 electrons

You could even..

…have drawn out the electronic configurations to show how clever you were….

Mg: 2,8,2 Ca: 2,8,8,2

Finally

By which property are elements arranged in the current periodic table?

PROTON NUMBER