5 Chemical energetics

Question 1

Entropy is a measure of the disorder of a system. Describe and explain what happens to the entropy of a gas when the temperature is increased. Ref: 9701/42/F/M/16

Answer 1

(higher temperature means that) particles have more energy; entropy (of the gas / system) increases because of an increase in the amount of disorder/randomness;

Question 2

How would the value of ∆Go , and hence the spontaneity (feasibility) of this reaction change as the temperature increases? Explain your answer. Ref: 9701/41/M/J/16

Answer 2

∆Go will become less positive/more negative as T increases,…because ∆So is positive (or –TSo is more negative)…therefore the reaction becomes more feasible/ spontaneous as T increases

Question 3

State and explain whether the following processes will lead to an increase or decrease in entropy. the reaction of magnesium with hydrochloric acid solid potassium chloride dissolving in water steam condensing to water Ref: 9701/41/O/N/16

Answer 3

(entropy) increases /is positive and H2 / gas is formed (entropy) increases /is positive and (KCl (aq)) solution has (free) moving / mobile ions /aqueous ions (entropy) decreases / is negative and decrease in gas

Question 4

Explain, with reference to ∆Go, why this reaction becomes more feasible at higher temperatures. Ref: 9701/41/O/N/16

Answer 4

T∆S is more positive than ∆H/T∆S increases / –T∆S more negative and ∆G is negative / decrease /less positive

Question 5

Explain what is meant by the term lattice energy. Ref: 9701/42/O/N/16

Answer 5

(energy change) when 1 mole of an (ionic) compound is formed or (energy change) when 1 mole of an ionic solid/lattice/crystal is formed (from) gas (phase) ions /gaseous ions (under standard conditions)

Question 6

Suggest why a temperature of 1000 C is usually used for this reaction, even though the reaction is spontaneous (feasible) at 25 C. Explain your answer. Ref: 9701/42/O/N/16

Answer 6

high Ea and to speed up the rate

Question 7

(i) Explain why ΔH4 is greater than ΔH3.

Ref: 9701/42/F/M/17

Answer 7

the second electron is removed from a (more) positively charged ion

Question 8

Chlorine is in Group 17. Suggest the trend in the first electron affinity of the elements in Group 17. Explain your answer

Ref: 9701/42/F/M/17

Answer 8

the electron affinity becomes less exothermic/negative down the Group 17

electron affinity depends (mainly) on the electron-nucleus distance which increases down Group 17

Question 9

Complete the table using ticks () to indicate whether the catalyst used in the reaction is heterogeneous or homogeneous.

Ref: 9701/42/F/M/17

Answer 9