3.2.1: Enthalpy changes

Question 1

What does system mean in a chemical reaction?

Answer 1

The atoms and bonds involved in the

chemical reaction

Question 2

Explain the law of conservation

Answer 2

The amount of energy in an isolated system
remains the same. Energy cannot be destroyed
or created, It can only be transferred from one
form to another

Question 3

What energy change is breaking bonds associated with?

Answer 3

Energy is taken in to break bonds →

endothermic reaction

Question 4

What energy change is making bonds associated with?

Answer 4

Energy is released to make bonds →

exothermic reaction

Question 5

What is an endothermic reaction?

Answer 5

A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants

Question 6

What is an exothermic reaction?

Answer 6

A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants

Question 7

What does activation energy mean?

Answer 7

The minimum energy required for a reaction to take place

Question 8

Which way does the arrow for activation energy point on an enthalpy profile diagram?

Answer 8

Always points upwards

Question 9

What are the standard conditions?

Answer 9

100 kPa

298 K

Question 10

What does “in standard state” mean?

Answer 10

The state an element / compound exists at in standard conditions (100 kPa, 298 K)

Question 11

Define enthalpy change of formation

Answer 11

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions

Question 12

Give an example of an equation which represents standard enthalpy of formation

Answer 12

There are many e.g. H2 (g) + ½ O2 (g) → H2O (l)

Question 13

Define enthalpy change of combustion

Answer 13

The energy change that takes place when 1 mole of a substance is completely combusted

Question 14

Give an example of an equation which represents standard enthalpy of combustion

Answer 14

E.g. C (s) + O2 (g) → CO2 (g)

Question 15

Define enthalpy change of neutralisation

Answer 15

The energy change that takes place when 1 mole of water is formed from a neutralisation reaction

Question 16

What does enthalpy change of reaction mean?

Answer 16

The energy change associated with a given reaction

Question 17

How can you calculate enthalpy change from experimental data?

Answer 17

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K-1) and ΔT is the change in temperature

Question 18

What are the advantages of

using a bomb calorimeter? (2 marks)

Answer 18

● Minimises heat loss
● Pure oxygen used → ensures
complete combustion

Question 19

Why might experimental methods for enthalpy determination not be accurate?

Answer 19

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

Question 20

What does average bond enthalpy mean?

Answer 20

The mean energy required to break 1 mole of bonds in gaseous molecules

Question 21

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 21

Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.

Question 22

How to calculate enthalpy change of reaction using average bond enthalpies?

Answer 22

ΔH = Σ (bond enthalpies of reaction) -

Σ (bond enthalpies of products)