Exam Questions

Question 1

Explain how the conditions of an exothermic reaction can be changed to allow the reaction to take place
(Thermodynamics)

Answer 1

Lower the temperature

So that G becomes negative

Because the TS term will be less positive and smaller than H value

Question 2

Why does the emf of a cell change when the electrodes are connected and a current flows?

Answer 2

Concentration of ions change and are no longer standard

Or

Emf is determined when no current flows

Question 3

How is the E cell value affected if the surface area of the electrodes double?

Answer 3

Unchanged

Question 4

Entropy of T2 is much larger than entropy of T1

Suggest why

Answer 4

Increase in disorder

Bigger at T2

Question 5

Suggest why the entropy is zero at 0K

Answer 5

Particles are in maximum state of order

Question 6

Suggest why the concentration of sodium hydroxide in a solution slowly decreases when left open in air

Answer 6

NaOH reacts with carbon dioxide

Question 7

Explain why the bond enthalpy of a Cl-Cl bond is greater than that of a Br-Br bond

Answer 7

Bonding pair is closer to nucleus

So attraction between nucleus and bonding pair is stronger

Question 8

Why is the lattice dissociation enthalpy of silver chloride greater than silver bromide?

Answer 8

Chloride ions are smaller than bromide ions

They are attracted more strongly to the silver ions

Question 9

Why is the theoretical value for lattice dissociation enthalpy of AgCl less than experiment value

Answer 9

AgCl has covalent character

Forces in the lattice are stronger than pure ionic attractions

Question 10

Suggest a suitable apparatus that could be used to measure out the sodium hydroxide solution that is being added to neutralize aluminum chloride

Explain why this apparatus is more suitable than a pipette

Answer 10

Burette

Delivers variable volumes

Question 11

Suggest why the enthalpy change in a ligand substitution reaction is approximately zero

(2 marks)

Answer 11

X-X bonds formed have similar enthalpy to X-X bonds broken

Same number of bonds broken and made

Question 12

Explain why a ligand substitution reaction will occur despite having an enthalpy change that is approximately zero

Answer 12

X number of particles form n>X number of particles

Therefore free energy change is negative

Question 13

Hydration enthalpy for Mg2+: -1920kJmol-1
Hydration enthalpy for Na+:
-406kJmol-1

Explain why there is a difference between the hydration enthalpies of the magnesium and sodium ions

(2 marks)

Answer 13

Mg ion is smaller and more charged

Mg ion attracts water more strongly

Question 14

Define mean bond enthalpy

(2 marks)

Answer 14

The enthalpy needed to break 1 mol of bonds

Averaged over a range of compounds

Question 15

Suggest one reason why you would expect the standard enthalpy of combustion of hydrogen to differ slightly from the mean bond enthalpy of water

(1 mark)

Answer 15

There must be a slight difference between the actual bond enthalpy in water and the mean bond enthalpy for the O-H bond

Question 16

What does a negative entropy change sign indicate about the product of the reaction relative to the reactants

(1 mark)

Answer 16

They are more ordered

Question 17

Write an equation for the reaction that occurs when magnesium is heated in steam

Describe what you would observe when this reaction occurs

(3 marks)

Answer 17

Mg + H2O > MgO + H2

White solid

White flame

Question 18

State and explain the effect if any, of increasing the pressure of oxygen on the emf of a hydrogen-oxygen fuel cell

(2 mark)

Answer 18

Increase emf

2H2 + O2 > 2H2O will move to the right

Question 19

Suggest why the use of a hydrogen-oxygen fuel cell might not be carbon-neutral

(1 mark)

Answer 19

CO2 IS RELEASED because fossil fuels are burnt to produce electricity TO GENERATE HYDROGEN