Module 3 - 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 points

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)