3) Enthalpy

Question 1

How is the enthalpy change, ΔH calculated?

Answer 1

ΔH = H(products) - H(reactions)

Question 2

What type of reaction is a negative ΔH?

Answer 2

Exothermic.

Question 3

What type of reaction is a positive ΔH?

Answer 3

Endothermic.

Question 4

Use ‘Enthalpy’ card to test knowledge of the enthalpy profile diagram of an exothermic reaction.

Answer 4

Rate knowledge 1-5.

Question 5

Use ‘Enthalpy’ card to test knowledge of the enthalpy profile diagram of an endothermic reaction.

Answer 5

Rate knowledge 1-5.

Question 6

Define activation energy.

Answer 6

The minimum energy required for a reaction to take place.

Question 7

Use ‘Enthalpy’ card to test knowledge of the activation energy on an enthalpy profile diagram of an exothermic reaction.

Answer 7

Rate knowledge 1-5.

Question 8

Use ‘Enthalpy’ card to test knowledge of the activation energy on an enthalpy profile diagram of an endothermic reaction.

Answer 8

Rate knowledge 1-5.

Question 9

What are standard conditions?

Answer 9

• 100 kPa.

- 298K.

Question 10

What is the standard concentration for solutions?

Answer 10

1 mol dm^-3.

Question 11

Define the term enthalpy change of formation.

Answer 11

The enthalpy change when 1 mole of substance is formed

from its constituent elements.

Question 12

Define the term enthalpy change of combustion.

Answer 12

The enthalpy change for the complete combustion

of 1 mol of a substance.

Question 13

Define the term enthalpy change of neutralisation.

Answer 13

The formation of 1 mol of water from neutralisation.

Question 14

What is average bond enthalpy?

Answer 14

The breaking of 1 mol of bonds in gaseous molecules.

Question 15

Is bond breaking exothermic or endothermic?

Answer 15

• Energy is required to break bonds.

- Bond breaking is endothermic.

Question 16

Is bond making exothermic or endothermic?

Answer 16

• Energy is released when bonds form.

- Bond making is exothermic.

Question 17

Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic.

Answer 17

• Bond breaking absorbs energy.
• Bond making releases energy.
• More energy released than absorbed.

Question 18

Suggest three reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values.

Answer 18

• Heat released to the surroundings.
• Incomplete combustion.
• Non-standard conditions.

Question 19

Why do bond enthalpies have positive values?

Answer 19

Bond breaking is endothermic and so energy has to be put in to break a bond.

Question 20

In ∆Hfθ, what are the conditions indicated by the symbol θ?

Answer 20

• Standard conditions.
• 100 kPa.
• 298K.

Question 21

State one important manufacturing process in which hydrogen is used

Answer 21

The production of ammonia.

Question 22

What is meant by the term exothermic?

Answer 22

A reaction that releases heat.