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 created or destroyed, 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

What are the standard conditions?

Answer 8

100 kPa
298 K

Question 9

What does ‘in standard state’ mean?

Answer 9

The state and element / compound exists at in standard conditions

Question 10

Define enthalpy change of formation.

Answer 10

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

Question 11

Define enthalpy change of combustion

Answer 11

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

Question 12

Define enthalpy change of neutralisation

Answer 12

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

Question 13

What does the enthalpy change of reaction mean?

Answer 13

The energy change associated with a given reaction.

Question 14

What are the advantages of using a bomb calorimeter? (2)

Answer 14

• minimises heat loss
• pure oxygen used —> ensures complete combustion

Question 15

Why might experimental methods for enthalpy determination not be accurate?

Answer 15

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

Question 16

What does average bond enthalpy mean?

Answer 16

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

Question 17

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

Answer 17

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 18

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

Answer 18

ΔH = Σ(bond enthalpies of reaction) — Σ(bond enthalpies of products)