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

Endothermic reaction

Energy is taken in to break bonds

Question 4

What energy change is bond making associated with?

Answer 4

Exothermic

Energy is released to make bonds

Question 5

What is an endothermic reaction?

Answer 5

A reaction with an overall positive enthalpy change

Enthalpy of products > enthalpy of reactants

Question 6

What is an exothermic reaction?

Answer 6

A reaction with an overall negative enthalpy change

Enthalpy of products < enthalpy of reactants

Question 7

Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction

Answer 7

diagram

Question 8

What does activation energy mean?

Answer 8

The minimum energy required for a reaction to take place

Question 9

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

Answer 9

always points upwards

Question 10

What are the standard conditions?

Answer 10

100 kPa

298 K

Question 11

What does “in standard state” mean?

Answer 11

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

Question 12

Define enthalpy change of formation

Answer 12

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 13

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

Answer 13

H2 (g) + 1/2 O2 (g) ———> H2O (l)

Question 14

Define enthalpy change of combustion

Answer 14

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

Question 15

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

Answer 15

C (s) + O2 (g) ———–> CO2 (g)

Question 16

Define enthalpy change of neutralisation

Answer 16

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

Question 17

What does enthalpy change of a reaction mean?

Answer 17

The energy change associated with a given reaction

Question 18

How can you calculate enthalpy change from experimental data

Answer 18

Use the equation Q = mc Delta T

Where m is the mass of the substances being heated (usually water)
C is the specific heat capacity of that substance (water SHC = 4.18J-1K-1)
Delta T is the change in temperature

Question 19

Draw a simple calorimeter

Answer 19

calorimeter

Question 20

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

Answer 20

Minimises heat loss

Pure oxygen used –> ensures complete combustion

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

Delta H = Bond enthalpies of reaction - bond enthalpies of products