Energy from fuels

Question 1

Why are many different homologous series used as fuels.

Answer 1

All of the chemicals in these families (alkanes,alkenes,cycloalkanes,alcohols and carboxylic acids) have stored energy which can be released when they are burned. This is why so many of them are used as fuels.

Question 2

What is the definition of a fuel?

Answer 2

A fuel is a substance that can be burned to release heat energy.

Question 3

What is a combustion reaction?

Answer 3

A fuel reacts with oxygen from the air to produce energy, this is known as a combustion reaction. Combustion is an example of an oxidation reaction and is always exothermic.

Question 4

When looking at energy, what are the two types of chemical reaction?

Answer 4

Exothermic- a reaction that releases energy in the form of heat (the surroundings get hotter).
Endothermic- a reaction that absorbs heat from its surroundings (the surroundings get colder).

Question 5

Where can the energy released during combustion be used for?

Answer 5

The energy released during the combustion reaction of a fuel, can be used to operate machinery and cars, for example.

Question 6

When does complete combustion occur?

Answer 6

Complete combustion occurs in a plentiful supply of oxygen.

Question 7

How many joules make up one kilojoule?

Answer 7

1000 joules= 1 kilojoule

Question 8

Describe the specific heat capacity of water.

Answer 8

When 1kg of water absorbs 4.18 kJ of energy from the flame, the temperature goes up by 1oC.
This measurement is called the specific heat capacity of water.

Question 9

What is the symbol for the specific heat capacity of water?

Answer 9

The symbol for the specific heat capacity of water is the letter “C”.

Question 10

Where can the value for the specific heat capacity of water be found?

Answer 10

This value for the specific heat capacity of water is quoted in your data booklet:
c = 4.18 kJ kg-1 oC-1.

Question 11

What is the mass of 1cm3 of water?

Answer 11

1cm3 of water has a mass of 1g

Question 12

What is the volume of 1kg of water?

Answer 12

1cm3 of water has a mass = 1g

Therefore 1kg of water has a volume =1000cm3 (1 litre)

Question 13

What equation can be used to calculate the energy released by a fuel on combustion?

Answer 13

To calculate the energy released by a fuel on combustion we can use the following equation:
Eh = m c ΔT

Question 14

In the equation, Eh = m c ΔT what does Eh represent?

Answer 14

Eh=energy required to heat water (KJ)

Question 15

In the equation Eh = m c ΔT what does the letter “C” represent?

Answer 15

c = specific heat capacity of water= 4.18 kJ kg-1 oC-1.

Question 16

In the equation Eh = m c ΔT what does “m” represent?

Answer 16

m= mass of water heated (Kg)

Question 17

In the equation Eh = m c ΔT what does “ΔT” represent?

Answer 17

ΔT= temperature change of the water (0C)

Question 18

Why is energy released by burning fuels in a laboratory usually lower than that in the data booklet?

Answer 18

When carrying out the experiment the energy is usually considerably lower than that shown in your data booklet. This is due to:
Heat loss to the surroundings (this occurs even with the draught shield)
Incomplete combustion of the fuel
Evaporation of the alcohol.

Question 19

Suggest additional practical improvements you could make to the experiment of energy released by burning fuels in a laboratory?

Answer 19

Insulate the sides of the beaker.
Use a loose fitting lid over the beaker.
Put out the flame using the lid of the spirit burner and weigh with
the lid on, to reduce the evaporation of alcohol.
Regular stirring of the water (to distribute the heat evenly).

Question 20

What are some variables you must keep the same if you were to set up the experiment of the energy released by burning fuels in a laboratory with different alcohols?

Answer 20

The volume of water.
The distance between the flame and the beaker.
The material of the beaker.

Question 21

What is a bomb calorimeter and why is it used?

Answer 21

In industry, a bomb calorimeter is used to overcome the problems we encounter when carrying out experiments to calculate the energy released by a fuel in a laboratory. The results produced by a bomb calorimeter are more accurate. For example, the fuel is burned in pure oxygen, not air, reducing incomplete combustion.