Topic 8 Energetics 1

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 negativeenthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants

Question 7

Describe the diagram for an endothermic reaction

Answer 7

Reactants lower than products

Question 8

Describe the diagram for an exothermic reaction

Answer 8

Reactants higher than products

Question 9

What does activation energy mean?

Answer 9

The minimum energy required for a reaction to take place

Question 10

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

Answer 10

Always points upwards

Question 11

What are the standard conditions?

Answer 11

100 kPa
298 K

Question 12

What does “in standard state” mean?

Answer 12

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

Question 13

Define enthalpy change of formation

Answer 13

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 14

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

Answer 14

H 2 (g) + ½ O 2 (g) → H 2O (l)

Question 15

Define enthalpy change of combustion

Answer 15

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

Question 16

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

Answer 16

C (s) + O 2 (g) → CO 2 (g)

Question 17

Define enthalpy change of neutralisation

Answer 17

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

Question 18

What does enthalpy change of reaction mean?

Answer 18

The energy change associated with a given reaction

Question 19

How can you calculate enthalpy change from experimental data?

Answer 19

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 20

Draw a simple calorimeter

Answer 20

[TOP]
thermometer in beaker with known amount of water
gauze
tripod
spirit burner
heatproof mat

Question 21

Why might experimental methods for enthalpy determination not be accurate?

Answer 21

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

Question 22

What does average bond enthalpy mean?

Answer 22

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

Question 23

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

Answer 23

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 24

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

Answer 24

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

Question 25

What is Hess’s Law?

Answer 25

States that the enthalpy change for a reaction is the same regardless of the route taken