Chapter 9

Question 1

Enthalpy

Answer 1

Measure of heat energy stroked in a chemical system

Question 2

Enthalpy change

Answer 2

Difference in enthalpy between reactants and products in a reaction

Question 3

What is an exothermic reaction

Answer 3

Exothermic reactions give out heat energy

Has a negative enthalpy change because energy needed to make bonds is larger than energy released to break bonds

Energy transferred from system to surroundings

Question 4

What is an endothermic reaction

Answer 4

Reaction that absorbs heat

Has a positive enthalpy change

Surroundings to system

Question 5

Activation energy

Ea

Answer 5

Minimum amount of energy needed to break reactant bonds and start a chemical reaction

Question 6

Standard conditions definition

Answer 6

Standard set conditions for experimental measures that allow comparisons of different sets of data to be made

Question 7

Standard pressure

Answer 7

100 kPa

Question 8

Standard temperature

Answer 8

298 K

Question 9

Standard concentration

Answer 9

1 mol dm-3

Question 10

Standard state

Answer 10

Physical state of substance

Question 11

Standard enthalpy change of reaction

Answer 11

Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation, under sc

Question 12

Standard Enthalpy change of formation

Answer 12

Enthalpy change when 1 mole of a compound is formed from its elements

Question 13

Standard enthalpy change of combustion

Answer 13

Enthalpy change when 1 mole of a substance completely reacts with oxygen, under sc

Question 14

Standard Enthalpy change of neutralisation

Answer 14

Enthalpy change when an acid and an alkaline react together to form 1 mole of water

Question 15

Enthalpy change of combustion experiment

Answer 15

Pour 150cm3 water into beaker
Record initial temp
Add methanol to spirit burner
Weigh spirit burner
Burn methanol whilst stirring water with thermometer
After 3 mins - record max temperature of water
Re weigh spirit burner

Question 16

Why the experiment for combustion enthalpy change is not very accurate

Answer 16

• heat lost to surroundings other than water
• incomplete combustion
• evaporation of methanol from wick- must reweigh quick
• non standard conditions

Values will be less exothermic

Question 17

Experiment for neutralision and reaction

Answer 17

Combine known substances 
In insulated container 
Lid on 
Measure temp change 
Calculate heat given out using 
Q=mct

Question 18

Average bond enthalpy

Answer 18

The energy required to break one mole of a specified type of bond in a gaseous molecule

Calculated from actual bond enthalpies in different chemical environments

Question 19

Limitations of using average bind enthalpies

Answer 19

• actual bond enthalpies are different
• species need to be gaseous molecules
• non standard enthalpy

Question 20

Hess law

Answer 20

If a reaction can take place by 2 routes and the start and finish are the same the total enthalpy change is the same for each