Chemistry A Level Aqa Physical Model Answers

Front 1

Explain why chloroethanoic acid is a stronger acid than ethanoic acid? 2

Back 1

Electronegative chlorine withdraws electrons.

This stabilises the charge on COO- so chloroethanoic acid with dissociates more readily

Front 2

Explain why Ka values do not usually contain called for Ka for strong acids

Back 2

Strong acid fully dissociate

So Ka values would tend to infinity

Front 3

Suggest why initial rates are used rather than other rates to determine orders

Back 3

At t=0 the concentrations are known

Front 4

State how to work out initial rate from a graph

Back 4

Gradient of tangent

At t=0

Front 5

Explain why the the enthalpy for lattice dissociation potassium oxide is less endothermic than that of sodium oxide

Back 5

K ion is bigger than Na ion

Attraction between oppositely charged ions weaker

Front 6

Write equations for the Hydrogen oxygen fuel cell

Back 6

Positive electrode:

O2 + 2H2O+4e-→4OH-

Negative electrode:

H2 + 2OH- → 2H2O +2e-

Overall :

2H2 + O2 → 2H2O

Front 7

Show on a diagram how the emf of a Hydrogen oxygen fuel cell changes with time

Back 7

Front 8

State the main environmental advantage of the Hydrogen oxygen fuel cell

Back 8

Water is the only product

No pollutants

Front 9

Write equations to show vanadium V oxide as a catalyst in the contract process

Back 9

SO2+V2O5 → SO3 + V2O4

V2O4+ 1/2O2→ v2O5

Front 10

Describe how ions are detected on mass spec

Back 10

Ions hit the detector and accept an electron

This generates a current

Front 11

State and explain one similarity and one difference between spectra from an electron with higher energy

Back 11

Similarity

Some peaks at same m/z values

As same sample is being ionised and detected

Difference

Has extra peak at 1/2 m/z values

As knocked off two electrons

Front 12

Explain how VDWs forces arise

Back 12

the movement of electrons in the first molecule causes a temporary dipole

This induces a dipole in the second molecule

Causes an attraction between neighbouring molecules

Front 13

State the mz value of the ion that produces the biggest current and explain your answer

Back 13

128

most abundant ion

Front 14

Predict whether the atomic radius of 124 Te compared to 125Te

Back 14

Same

same no of protons and electrons

Front 15

Suggest how a student can check all the water has been removed

Back 15

Heat

To a constant mass

Front 16

Identify which isotope is deflected the most in the magnetic field, explain why

Back 16

70

lowest mz

Front 17

Explain why a decrease in temp decreases the rate

Back 17

Fewer particles have enough sufficient energy to react

Less successful collisions in given time

Front 18

Give two features of a reaction at equilibrium

Back 18

Concentrations remain constant

Forward rate = reverse rate

Front 19

Suggest why using powders increases rate

Back 19

increases successful collisions in given time

to increase the surface contact between particles

Front 20

with ref to IMFs explain why we can extract aldehydes with distillation but not coohs

Back 20

COOHs form hydrogen bonds

which are stronger than IMFs in aldehydes

Front 21

Explain how a mixture of lactic acid and sodium lactate can act as a buffer 3

Back 21

large lactate conc in buffer reacts with base

HA --> H+ + A-

added acid reacts with lactate ions

ratio ha/ a- stays almost constant

Front 22

Explain why the enthalpy of hydration for Cl- more negative than for Br- (3)

Back 22

chloride ion is smaller than bromide ion- higher CD

so force of attraction between chloride ions and water stronger

chloride ions attract the d+ of water

Front 23

Explain why entropy is zero when temp is zero

Back 23

Particles are stationary

No disorder

Front 24

Suggest why silicon dioxide is insoluble in water

Back 24

Macromolecular

Covalent bonding

water cannot break the covalent bonds

Front 25

Explain why after some time the current falls to zero

Back 25

the Cu2+ in each electrode will be at the same conc

Front 26

Write the equations for the lithium ion cobalt cell

Back 26

Li+ + CoO2 + e- --> Li+[CoO2]-

Li----> Li+ + e-

Overall Li + CoO2 ---> Li[CoO2]

Front 27

Explain why transition metal causes a solution to be coloured [2]

Back 27

Light is absorbed to excite an electron

Some wavelengths are absorbed as electrons jump to higher orbitals

The remaining colour wavelength is transmitted

Front 28

Deduce why the bonding in NO2 is covalent rather than ionic

Back 28

small difference in electronegativity

Front 29

Explain why initially the reaction between C2O42- and mno4- ions is initially very slow. [3]

Back 29

2 negative ions repel

Not many Mn2+ present initially to catalyse the reaction

The activation energy is too high

Front 30

Describe how colorimetry can determine the conc of sol [3]

Back 30

measure absorbance against solutions of known varying concentrations

Plot Graph absorbance against conc

Read off unknown conc from calibration curve

Front 31

Describe the steps you would take to determine the accurate temp of (delta H) that is not influenced by the surroundings [4]

Back 31

measure temp of water - colorimetry

add powder and stir once start clock

measure temp every minute after adding solid for 5 mins

plot temp against time.

plot line of best fit and extrapolate to time of mixing and determine temp change

Front 32

Explain why there are differences in hydration enthalpies of the Mg2+ and Na+

Back 32

Mg2+ is smaller and more highly charged than sodium ion

magnesium ion attracts water more strongly

Front 33

Define electron affinity for chlorine

Back 33

the enthalpy change when one mol of chlorine atoms forms one mol chloride 1- ions

all atoms and ions in gaseous states

Front 34

Define mean bond enthalpy

Back 34

the enthalpy to one mole of a H-O bonds

averaged over a range of compounds

Front 35

Suggest why the electron affinity for flourine has a negative value

Back 35

there is an attraction between the nucleus and electron

energy is released

Front 36

Explain why the theoretical enthalpy of lattice dissociation for AgF is different from its experimental value

Back 36

AgF isnt perfectly ionic/ has some covalent character

theoretical lattice enthalpy assumes only ionic interaction

Front 37

State structure, bonding and physical properties of SiO2

Back 37

Macromolecular

covalent bonding

hard, brittle

insoluble

doesn't conduct electricity

Front 38

suggest why hydration of an ion is exothermic

Back 38

water is polar

cl- attracts h in water

Front 39

use half equations to show how an electrical current is generated

Back 39

hydrogen electrode produces electrons

oxygen electrode accepts electrons

Front 40

Describe how to prepare a standard solution

Back 40

weigh a weighing boat containing solid on a 2dp balance

transfer to beaker and reweigh weighing boat

record difference in mass

add deionised water

stir

until all solid has dissolved

transfer to volumetric flask

with washings

make 250 cm3 mark with water

invert

Front 41

Explain why the complexes are different colours

Back 41

in each of P and the oxidation state of Cr is +3

the electron configuration is the same\

ligands are different

different energies of d electrons

different wavelengths of light are absorbed,

different wavelengths of light are transmitted

Front 42

Explain thermodynamically why this reaction occurs- chelate effect

Back 42

4 particles becomes 7

entropy increases

delta h is approx zero

delta g is negative

Front 43

suggest how the risk associated with cisplatin can be minimised

Back 43

use in small amounts

Front 44

suggest two properties of ions that influence the value of lattice enthalpy

Back 44

ionic radius

ionic chage density

Front 45

why does entropy increase as particles gain kinetic energy

Back 45

as temp increases particles vibrate

disorder increases

Front 46

Outline an experiment to demonstrate that MgO has ionic bonding

Back 46

Melt it

molten oxide conducts electricity

Front 47

why is SiO2 insoluble

Back 47

macromolecular

covalent

water cannot break the covalent bonds/ enthalpy of hydration is less than bond enthalpy

Front 48

State different features that cause a change in the value for delta E [3]

Back 48

metal

ligand

shape

oxidation state

coordination number

Front 49

What is the effect of the addition of a catalyst on rate [3]

Back 49

increases rate

lowers activation energy by providing an alternative route

allows more molecules able to react

Front 50

Justify the use of a compromise temp

Back 50

High temp gives a low yield

but low temp gives a low rate so compromise

Front 51

Suggest how metals can be extracted from their sulfide ore. expain how pollution problems arise

Back 51

extract from metal oxide

pollution sulfur dioxide

causes acid rain

Front 52

explain in terms of electronegativity why the boiling point of H2S2 is lower than H2O2

Back 52

S is less electronegative than O.

no hydrogen bonds

Front 53

Name the type of cracking that produces a high percentage of alkenes

Back 53

thermal

high temps and pressure

Front 54

Describe how you would measure the melting point of aspirin [2]

Back 54

Place sample of aspirin into capillary tube

heat in melting point apparatus slowly near melting point

Front 55

Compare the production of ester from acylcchloride and alcohol vs carboxylic acid and alcohol

Back 55

1

Faster

bigger yield

purer product

Disadvantage

HCl toxic fumes

Front 56

Outline briefly why column chromatography is able to separate a mixture [3]

Back 56

the solvent is moving

the aumina beads remain stationary

separation as different compounds have different balance between retention to the stationary phase and different solubility in the moving phase

Front 57

Suggest why the databook enthalpy is different to experimental enthalpy [2]

Back 57

heat loss or absorbed by apparatus

incomplete combustion

may not be in standard states

MBE vary with different compounds

Front 58

Outline the method of recrystallisation

Back 58

Dissolve in MINimum volume

of hot solvent

filter hot

allow to cool

filter

dry residue under suction

Front 59

with reference to electrons, explain why aqueous copper 2 ions are blue

Back 59

absorbs certain frequencies of light\

d electrons excited

the colour observed is the light transmitted

Front 60

State the meaning of each of the 3 terms in DE=hv

Back 60

Delta E = is the energy gained by the excited electrons

h=plancks constant

v= frequency of light absorbed by Cu2+

Front 61

explain why the shape of CuCl4 differs from the hexaaqua copper ion

Back 61

Cl- too big- to fit more than 4 round Cu

Front 62

Explain how and why iron ions catalyse the reaction between iodide ions and S2O8 2- ions . write equations [5]

Back 62

2Fe2+ +S2O8 2- ---> 2SO4 2- +2Fe3+

2Fe3+ + 2I- ---> I2 + 2Fe2+

How: as iron ions have variable oxidation states

and provide a route with a lower activation energy

why: two negative ions repel

Front 63

Describe how electron impact ionisation is done 4

Back 63

High energy electrons

From electron gun

Fired at sample

Knocks off one electron

Front 64

Describe how electrospray ionisation is done (4)

Back 64

Sample dissolved in volatile solvent

Injected through a hypodermic needle giving a fine mist

Tip of needle has high voltage

Each gains a proton as it leaves the needle

Front 65

Explain why Fe h20 and Fe cl4 are different shaped

Back 65

Cl- is a bigger ligand

Can only for 4 around