Heat of Combustion

Question 1

What is a combustion reaction?

Answer 1

A reaction in which a substance is burned

Question 2

Define heat of combustion.

Answer 2

The heat of combustion of a substance is the heat change when one mole of the substance is completely burned in excess oxygen

Question 3

Why is the phrase ‘completely burned in excess oxygen’ important in the definition for the heat of combustion?

Answer 3

This is because some elements have more than one oxide

Question 4

How are heats of combustion accurately measured?

Answer 4

They are accurately measured using an instrument called a bomb calorimeter

Question 5

What is a calorimeter?

Answer 5

A calorimeter is any container used for the determination of heat changes

Question 6

Describe a bomb calorimeter.

Answer 6

A bomb calorimeter consists of a small steel container (the bomb) with a screw-on cap and various other items

Question 7

What are the steps involved in using a bomb calorimeter to measure the heat of combustion of a compound?
(6)

Answer 7

Known mass of substance placed in the crucible inside the bomb

Bomb is filled with oxygen under pressure (O in excess = substance completely burned)

The bomb is placed in a known quantity of water contained in the calorimeter

Substance is ignited electrically using an ignition coil of wire

Rise in temp of water is measured - heat produced measured using formula…

The heat produced when one Mole of the substance is completely burned in excess oxygen is then calculated

Question 8

What is placed in the crucible inside the bomb?

Answer 8

A known mass of the substance whose heat of combustion is being measured

Question 9

Where is the known mass of the substance whose heat of combustion is being measured?

Answer 9

In the crucible inside the bomb

Question 10

What is the bomb filled with?

Answer 10

Oxygen under pressure

Question 11

Why is the bomb filled with oxygen under pressure?

Answer 11

To ensure the oxygen is in excess to make sure that the substance will be completely burned in oxygen

Question 12

What is filled with oxygen under pressure?

Answer 12

The bomb is filled with oxygen under pressure

Question 13

What is the bomb placed in?

Answer 13

The bomb is placed in a known quantity of water contained in the calorimeter

Question 14

What is placed in a known quantity of water contained in the calorimeter?

Answer 14

The bomb

Question 15

How is the substance ignited?

Answer 15

Using an ignition coil of wire

Question 16

What is the ignition coil of wire used for?

Answer 16

To ignite the substance

Question 17

How is the heat produced measured?

Answer 17

Using the formula:

Heat produced = mass x specific heat capacity x rise in temperature

Question 18

What is measured using the following temperature?

Heat produced = mass x specific heat capacity x rise in temperature

Answer 18

The heat produced is measured using the formula

Question 19

What is finally calculated after using the bomb calorimeter?

Answer 19

The heat produced when one mole of the substance is completely burned in excess oxygen

Question 20

What is the secondary function of the bomb calorimeter?

Answer 20

To compare the efficiencies of various fuels

Question 21

Other than in the number of moles, how may chemists wish to measure the heat given out by a fuel?

Answer 21

In terms of the mass of a fuel rather than the number of moles

Question 22

Define kilogram calorific value.

Answer 22

Kilogram calorific value of a fuel is the heat energy produced when 1 kg of the fuel is completely burned in oxygen

Question 23

What can kilogram calorific values be calculated from?

Answer 23

Heats of combustion for fuels that have a definite chemical formula such as methane and butane

Question 24

How is the calculation for kilogram calorific values carried out?
(2)

Answer 24

The heat of combustion is divided by the relative molecular mass of the fuel (which has a definite chemical formula)

Multiply by 1000 to get the kilogram calorific value

Question 25

When calculating the kilogram calorific value, why do you need to divide your answer (heat of combustion divided by RMM) by 1000?

Answer 25

As the heat of combustion divided by the RMM is only the heat given out when one gram of the compound is burned (you want Kg not g)

Question 26

What are kilogram calorific values used for?

Answer 26

To compare the efficiencies of various fuels

Question 27

According to kilogram calorific values, methane is a good fuel explain this.

Answer 27

Methane has a very high kilogram calorific value

Question 28

Other than its high kilogram calorific value, explain why methane is a good fuel.

Answer 28

It burns with a clean flame and can be easily distributed by being piped to homes

Question 29

How are the kilogram calorific values of variable compositions measured?

Answer 29

They have to be measured using a bomb calorimeter