EDEXCEL Common AS Chemistry Questions

Try to encapsulate some of the most common AS questions styles you can get!

  • Created by: PhilMoze
  • Created on: 01-05-13 07:42

First Ionisation Energy

M(g) --> M(+1)(g) + e(-)

The enthalpy change when one mole of electrons is removed from one mole of neatural gasous atoms, to form one mole of singluar positve ions. 

Why does First Ionisation Energy Generally Increase Across Period 3? 

All of the elements occupy the same highest electron shell, (the third)(n=3). This means they are all (generally) the same distance from the nucleus. As the effective nucleaur charge increases across the period, the electrostatic attractions between the nucleus and electrons increase, which means that most energy is required to remove one from the atom.

Why do elements such as Al and S, drop in the trend? 

In the case of Al, the electron config is [Ne] 3s2, 3p1. This first electron in the p shells is at a slightly higher energy level from the other electrons, so it is shielded sightly by the s electrons. This means that it needs less energy to remove the electron from the atom. 

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First Electron Affinity

Cl(g) + e- --> Cl-(g)
(first) The enthalpy change when one mole of electrons is added to one mole of atoms, in the gasous state. 

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Boiling points of Compounds

NH3 and CH4

Do they hydrogen bond? Do they form normal dipoles IMF? How strong are their London Forces? 

In this case it's pretty simple as NH3 forms hydrogen bonds with itself because of the electron negative nitrogen bonded to the hydrogen. Whereas CH4 only forms weaks london forces between the molecules. Remember that NH3 will still have London forces, however the hydrogen bonds is the major factor.

2-methlypropane and Butane

In this case, they form no hydrogen bonds, no dipole interactions, so they rely of London forces. They both have the same number of electrons, however butane is more able to pack together because of it's straight chain. 2-methly-propane has a side chain which will not allow to pack as well as the butane. Therefore the London forces are stronger in butane, so it has a higher BP. 

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Experimental Error

Why might the lattice energy be higher than expected expermientaly compared to the theoritcal model?
In ionic compounds which have polarisation of the anion (happens in Silver compounds, eg AgI), by the cation, they will have a degree of covalent bonding. Covalent bonding isn't considered in the theortical model, so is lower.

Why might a standard enthalpy of combustion be lower than expected compared to book values?

Firstly, there will be heat lost to the surroundings, because as you burn the substance, heat is beating lost by the water. Some heat may be absorbed by the container, however this can be minimilsed.  
Another aspect is that it may produce water in the gas state. Water is liquid is standard conditions. In the gas state it will have a higher enthalpy, so if we measure the change when in gas state, we go a less exothermic reactions, as we haven't considered the energy that would be released when water condenses to a liquid. 

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Ion Colours

In the reaction between aqaous KMn2O7 and an Alkene, what colour change would be seen as the reaction progresses? And Why?

The manganate(VII) ion is a purple colour. In the reaction it will be reduced to a brown Manganese (IV) ion (in an oxide compound). So the reaction will turn from Purple to Brown. 

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There are groups: homogenous and hetrogenous. 

How do homogenous catalysts work? 

They provide an alternative mechanism/route of reaction, which has a lower activation energy. The catalyst is restored at the end of the mechanism. It follows in hess's law that the reactions will have the same overall enthalpy change, however it has gone by a different route. The lower activation energy means more successful collisions occur which increase the rate of reaction.

How do hetrogenous catalysts work?

They work by providing active sites on which molecules of different states reaction. They can decrease activation energy by have polar groups which polarises the bonds (enzymes use this method often). Overall it increase the rate of collision and activation energy, so increases rate of reaction.

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Mole Calculation Questions

I've just began remembering when I work with something like I2, I need to consider I have two iodine atoms. sigh

How do you convert between Cm3, Dm3, and M3? (useful stuff)

1cm3 = 0.001Dm3 

1dm3 = 0.001M3

Therefore 100cm3 = 0.1 Dm3! 

One mole of Gase always occupy the same volume (if at the same pressure).

Under standard conditions (298K, 1atm pressure), 1 mole occupies 24Dm3. = 24000cm3 = 0.024 M3

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Lattice Energy

The enthalpy change of a substance when one mole of solid substance is formed from its gasous ions which are an indefinte distance apart. 

eg Mg2+ (g) + 2Cl-(g) --> MgCl2(s)

What factors effect Lattice Energy?
The size of the ions. Smaller ions have larger lattice energys. This the attraction between the smaller ions is stronger, due to the smaller ionic radius as less shielding occurs in bonding. 

The degree of polarisation. In compounds such as AgI(s), when it forms it's ionic lattice, the polarisation of the iodide ion for the Ag+ ion, will cause a degree of covalent bonding which is additional to the ionic attraction.

The arrangement of the ions. If there is a small anion, and a large cation, it would have a different ionic coordination number, compared to a compound with a two large ions. 

The charge of the ions. A Cl- ion, will feel a greater attraction for a Mg2+ ion, rather than a Na+ ion, (even though they are isoelectronic), because of the extra charge. MgCl2, has a much higher lattice energy than NaCl. 

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Bond Angles and Length

What will the bond angles be in CH4, NH3, NH4+, BH3 and CH3OH and why?
The methane molcules will have a bond angle of 109, because it forms 4 bonds, with no lond pairs. The electrons in the bonding are arranged so that they are at the minimum replusion. 

In ammonia, the nitrogen has a lone pair of electron. They produce a great replusion than the normal bonding electrons. This means that the other bonds are pushed together to a place of minium replusion. In this case the bond HNH, has the angle of 107. 

In the ammonium ion, it forms a tethreahdreal shape, with bond angles of HNH of 109. This is because the lone pair has formed a dative bond with a postive charge, so it acts a normal bonding pair.

In borane (BH3), the bond angles are at 120. This is a trigonal plannar. The outer shell of boron is often electron deffient, as in this case, and has no lone pairs. This means 3 bonding pairs are at minimum replusion at 120. (Borane acts are an electrophile in reactions as it can accept a pair of electrons, similar to Al in AlCl3). 

In the alcohol (methanol), the carbon will form a tetrahedral (109) shape between bonds. However the oxygen has two lone pairs so creates a COH angle of 104.5. 

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Types of Covalent Bond

In the C=C bond, how many electrons are involved, what types of bond form?

This bond is a double bond, and so contains 2 electrons. This will in most situations be from 2 from one carbon, two from the other,

Two electrons will form a Sigma bond, cause by two P orbitals overlapping head on.

(The electron Config of carbon is 1s2, 2s2, 2p2)

Two electrons will form a Pi bon, caused by two P orbitals overlapping above and below the plane of the nucleus. The P orbitals which overlap are at 90 degrees to those forms in the sigma bonding. 

Both these are covalent bonds, are a pair of electrons is being shared between two atoms. 

Remember no rotation can occur around the double bond, leading to geometric and optical isomers. 


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Sulfate - No colour 
Carbonate - No colour
Nitrate - No Colour

Cu2+ - Blue
Cu3+- colourless

Permangant compounds - purple
Manganese oxide - Brown solid

Dichromate - Orange
Chromate - yellow
Cr3+ - green 

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Li - Red
Na - yellow
K - Lilac

Mg - No colour
Ca - orange
Sr - Red 
Ba - Apple green 

ALKENES produce a DIRTIER flame than ALKANES (due to carbon ratio higher in alkenes).

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Free Radical Subs

The most interesting bit of alkane chemistry. All a bit dull really...

Cl2 --> 2Cl* (imagine the star is a dot for free radicals). UV LIGHT NEEDED. 

Why wouldn't this reaction happen in the lower atmopshere?! Not intense enough UV!
What type of bond breaking is this called?! homolytic!
What stage is this called?! initiation!

CH4 + Cl* --> HCl + CH3*
CH3* + Cl2 --> CH3CL + Cl*

What is this stage called? propogation!
Why do we know this happens?! No H* radicals form

Cl* + Cl* --> Cl2
CH3* + Cl* --> CH3CL

Why might ethane be a product?  two CH3* radicals can colide to form C2H6! Crazy.

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The Alkene Common

Hydrogenation --> Such a pointless reaction, alkenes are so much more usefull. However, high temps, and a cat of Nickel are needed. This is an addition reaction.

Halogen reactions --> Room temperture occurance. Br2 is the bromine water test for alkenes are it decolourises! (Orange to colourless in alkenes). The Br-Br bond becomes an instantanous dipole, which allows it to act as an electrophile. 

Hydrogen Halide Reactions --> This is electrophilic addition. The H becomes added first, forming the most stable carbon-cation possible. And then the Br- Ion adds onto the carbon with the charge. 

Something to consider: In Br2 Water, because water is in excess, when a carbocation is formed, it more likely to meant the nucleuphilic molecule Water. The subsisution of water forms an alcohol. For example.

C+H2-CH2Br + H2O --> CH2(OH)-CH2Br + H+ 

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The Alkene to Diol reaction!

My favourite reaction in Unit 1!

This has to occur in neutral aqua solution. 

Mixing an alkene with at strong oxidation agent Potassium manganate(VII), creates a diol. The reaction being

H2C=CH2 + [O] + H20 --> HOCH2-CH2OH.

The solution would turn from purple to colourless, with a brown manganese oxide precipitate forming as the reaction occurs.

The [O] is used to show an Oxygen from the oxidising agent.

This is an oxidatation reaction, as the carbon gains an oxygen. 

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Describing Reactions

Always a bit tricky. 

What would you SEE, as Mg(NO3)2 is heated?!

The equation would be...
2Mg(NO3)2 --> 2MgO + 4NO2 + O2  

So, the white salt MELTS (a white solid would form ias MgO, however you wouldn't see this until it cools). 
NO2 is a BROWN gas.
O2 is invisible, sad :( Although we all know the test for oxygen YAY!

What would you SEE, as you add Bromoethane, water and Silver nitrate?!

Without know the equations you can guess that a bromide ion will reaction with the Ag+ ion, to form a cream precipitate! 

CH3CH2Br + H2O --> CH3CH2OH + Br- + H+
Br- (aq)+ Ag+ (aq)--> ArBr (s) 

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Thermal Stabilty

Touched on some thermal decomposition there...

Nitrate Decompostion.

In Group 1 --> Nitrite forms and oxygen. Except in Li+, (more polarising, smaller ionic radius etc), which forms NO2 and O2 and Li2O

In Group 2 --> Form NO2, O2 and the metal oxide. (more polarising, greater charge, more distoration of bonds, which weakens them)

Carbonate decomposition

Group 1 --> Only Li is polarising enough to allow thermal decomp. It follows the equation:

Li2(CO3) --> Li2O + CO2 

Group 2 --> All are polarising enough.Do test form limewater, and the salt melts (but MgO forms as a white solid) Follow the equation:

Mg(CO3) --> MgO + CO2 (bit dull). 

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Alcohol Oxidations

Oxidation of Primary Alcohol
- Occurs on heatings
- Reagents of H2SO4 and Potassium Dichromate (orange)
- As reaction occurs the Dichromate gets reduced to Chromate (green) ions
- If refluxed, product is Carboxylic acid, if not then mostly a alydehyde.

Oxidation of Secondary Alcohol 
- Occurs on heating
- Reagents of H2SO4 and Potassium Dichromate (orange)
- As reaction Occurs Dichromate gets reduced to chromate (green) ions!
- Can just be distilled straight off, as it only oxides once to a KETONE!

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Halogenoalkane Elim or Sub Reactions


Primary react with an Sn2 mechanism.
Secondary react with an Sn1 and Sn2 mechanism
Tertiary react with an Sn1 mechanism.

Two reactions could occur. The one which happens can be changed based on what degree of halogenoalkane used, and the conditions it's reacted under.

Alcoholic (ethanol solvent), solutions favour elimination.

Aqua solutions favour subsitution.

Tertiary haloalkanes favour elimination.

Primary  haloalkanes favour subs.

Elimination reactions form WATER, and an halogen ion. 
Eg. (CH3)3CBr + OH- --> CH2=C(CH3)2 + H2O + Br-  

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Speed of Sub Haloalkane reactions

Iodine forms the weakest bond with a carbon, so it reacts the quickest when compared to similar haloalkane compounds.

Iodo > Bromo > Chloro (fluroine-carbon is a very strong bond and doesn't sub easily at all if at all).

Haloalkanes react quickest by the Sn1 mechanism, so tertiary haloalkanes react quicker than primary haloalkanes of the same halogen. 

Tertiary > Secondary > Primary

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Silver Nitrate + Water + Haloalkane reactions

Throw all these in together and you get...

CH3CH2CHBrCH3 + H2O --> CH3CH2CHOHCH3 + H+ + Br - (hydrobromic acid is mostly dissociated in water)

Br-(aq) + Ag+(aq) --> AgBr(s) (cream precipitate)

Iodine you get yellow
Chloride you get white! 

This first reaction is a nucleuphilic subsitition.Water can act as a nucleuphilc because it has a donateable lone pair of electrons. (Well, two really but we only need one).

The section is a precipitate reactions. The Br- ions are formed from the first reaction. 

Speed of reactions are similar to the hydroxide subs as easiler.

Teritary > Secondary > Primary
Iodo > Bromo > Chloro

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Group 7 Properties

Chlorine - A gas at room temperture. Pale Green. Toxic. And it's pale green as a solution also. Quite reactive as it has quite a smaller atomic radius which allows it to attract electrons nicely. In Hexane it is also pale green. 

Bromine - Bit of a mix at room temp. In a dynamic eqilibrium in a closed system, with brown gas and a brown liquid interchanging. In hexane it is orange. In water solution it is also orange.

Iodine - Purple solid at room temp. In aqua it's brown. In hexane it's violet. 

If you mix Bromine in water it will be an orange liquid. Now add a hexane layer on top and shake. The water layer will become colourless as the bromine dissolves in the hexane. The hexane layer will become orange. 

Add NaI and Br2 in aqua solution and the solution will turn brown as the I2 molecules form. Add hexane and shake and the bottom layer will become colourless (Halide ions themselves have no colour, as the I- and Na+ will be (aq) ions). The hexane layer will become violet. 

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I lie. Here are some lovely Group 2 Reactions which are pretty dull :( 

Metal oxides are BASE
MgO + 2HCl --> MgCl2 + H2O
MgO + H2SO4 --> MgSO4 + H2O

Metals decomposition
2Mg(NO3)2 --> 2MgO + 4NO2 + O2  
MgCO3 --> MgO + CO2 (how boring) Test with limewater. To test speed of Decomp, collect gas over water.

Oxide with water
MgO + H2O --> Mg(OH)2 (magnesium hydroxide) 

Metal with Water
Mg + 2H2O --> Mg(OH)2 + H2 (more vigerous down the group)  (group 2 hydroxides become more soluble down the group and more vigerous reactions) 

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MORE FUN! (Group 7 Basics)

These are more interesting...
JUST A QUICK NOTE: Iodine is I2, iodide is I-. Test for Iodine is the colour in hexane dissolving from the water). Test for iodide is the silver nitrate. Iodine will NOT react with Silver nitrate.

KBr + H2SO4 --> HBr + KHSO4 (sulfuric acid acting an acid)
2HBr + H2SO4 --> Br2 + SO2 +  2H2O (redox reaction, the sulfuric acid acts as the oxidising agent). 

Chloride ions can't reduce the H2SO4.
Bromide ions can reduce the H2SO4 to the SO2
Iodide ions can reduce the H2SO4 to Sulfur and Hydrogen Sulfide.

Halide IONS are reducing agents as they do the following reaction
2I- --> I2 + 2e- (Iodine is the strongest, Chlorine doesn't really reduce, Bromine can do it slightly)

Halogen molecules (Cl2 and Br2) are oxidising agents as they do:
Cl2 + 2e- --> 2Cl- (Chlorine is the strongest, bromine can do it, but iodine can't)

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