Download LI 1, 2 LI 3 Topic 9 Energy from Fuels National 5 A FUEL is a

Topic 9 Energy from Fuels
LI 1,
2
National 5
A FUEL is a substance which gives out ENERGY during combustion (when burned).
Combustion reactions are exothermic reactions. Exothermic reactions release heat
energy. Endothermic reactions are the opposite, they take in heat energy. Alkanes and
alcohols can be used as fuels.
Some fuels release more energy than others. Energy is measured in Joules (J)
but as this is a very small amount of energy it is more common to measure
energy in kilojoules (kJ). (1000J = 1kJ)
The amount of heat energy used or released in a chemical reaction is known as
the enthalpy change (symbol ∆H). Experiments can be carried out to measure
changes in enthalpy.
The following apparatus can be used to calculate the energy released when fuels
are burned.
The heat energy gained by the water = the energy released when the fuel is
burned
LI 3
This energy can be calculated using Eh = cm∆T where
Eh = the heat energy gained by the water in kilojoules (kJ)
c = specific heat capacity of water (4∙18 kJ kg -1 °C-1)
m = mass of water (kg)
∆T = change in temperature (°C)
Note:
 The value of c and its units are given in the data booklet.
 1kg of water has a volume of 1 litre. This means that 1ml of water = 1g so,
100cm3 of water = 100g = 0∙1kg.
1

If the mass is given in g divide by 1000 to get kg. e.g 100g = 100/1000 =
0∙1kg
Ways of reducing heat loss

A copper can is used rather than a beaker because copper conducts heat
better than glass. (Less heat is lost using copper)

A draught shield is used to provide heat insulation (prevent heat loss)

A tripod is not used so all the heat is transferred to the copper can (none
of the heat is lost in heating up the gauze of the tripod)
Example 1
A pupil set up the following apparatus.
She burned the fuel butanol-2-ol until the temperature increased by 40°C.
Calculate the energy released by the reaction.
(Remember, 200cm3 of water = 0.2kg)
Eh = cm∆T
= 4∙18 x 0∙2 x 40
The energy released = 33∙44 kJ
Example 2
Calculate the energy released when methanol is burned completely in oxygen
causing 100g of water to increase in temperature from 18°C to 25°C.
Eh = cm∆T
= 4∙18 x 0∙1 x (25-18)
The energy released = 2∙926 kJ
2
Example 3
A student investigated the amount of energy released when an alkane burns
using the apparatus shown.
The student recorded the following data.
Mass of alkane burned
Volume of water
Initial temperature of water
Final temperature of water
Specific heat capacity of water
(i)
1g
200 cm3
15 °C
65 °C
4·18 kJ kg–1 °C–1
Calculate the energy released, in kJ.
Eh = cm∆T
= 4∙18×0∙2×50
The energy released = 41∙8 kJ
(ii) Suggest one improvement to the student’s investigation.
Any one from:
 heat insulation e.g. use a draught shield
 repeat to get average
 move burner nearer to can
 remove tripod and clamp can
 stir water
 thermometer not touching copper can
 use clay triangle on tripod
3
Problem solving
The table below gives information about the amount of energy released when
one mole of some alkanes are burned.
Name of Alkane
methane
ethane
propane
butane
(i)
Energy released when one mole is burned (kJ)
891
1560
2220
2877
Describe the relationship between the energy released
and the number of carbon atoms in the alkane molecule.
As the number of carbons increases the energy released increases
(ii)
Predict the amount of heat released, in kJ, when one mole of
pentane is burned.
Any value from 3520 to 3550
LI 4
Calculations based on balanced equations
Any reaction can be represented using a balanced formulae equation.
Given a balanced equation the quantities of reactants and products can be
calculated.
The balanced equation for a chemical reaction can be used to calculate:
1. the mass of product formed (given the mass of reactants used)
2. the mass of reactants used (given the mass of product formed)
4
LI 5
Example 1
The following equation shows the complete combustion of methane in oxygen.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
What mass of carbon dioxide will be formed from 4g of methane?
Step 1
Convert the mass given in the question to moles.
Number of moles of methane in 4g
=
=
= 0∙25 moles
Step 2
Write down the number of moles of each substance under the formula in the
balanced equation. The number of moles is equal to the balancing number. If no
balancing number is present the number of moles is 1
CH4(g) + 2O2(g)
(1 mole)
CO2(g)
→
(2 moles)
(1 mole)
+
2H2O(l)
(2 moles)
Circle the 2 substances mentioned in the question
This is the mole relationship and shows that:
1 mole of methane and 2 moles of oxygen react to produce 1 mole of carbon
dioxide and 2 moles of water.
5
Step 3
Write down the mole relationship between the 2 substances circled in the
equation
i.e. methane and carbon dioxide.
1 mole of methane (CH4)  1 mole of carbon dioxide (CO2)
We have already worked out in step 1 that there was 0∙25 moles of CH4
Therefore,
0∙25 moles of CH4  0∙25 mole of CO2
Step 4
Now that the number of moles of carbon dioxide is known, convert this to mass:
Mass of CO2 in 0∙25 moles
= moles x GFM
= 0∙25 x 44
Mass of carbon dioxide formed = 11g
6
Example 2
The following balanced equation shows the reaction of sodium carbonate with
hydrochloric acid.
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
What mass of sodium chloride is produced when 10∙6g of sodium carbonate is
reacted with excess hydrochloric acid?
Step 1
Convert the mass given in the question to moles.
moles of sodium carbonate (Na2CO3)
=
=
= 0∙1 moles
Step 2
Write down the number of moles of each substance under the formula in the
balanced equation. Circle the 2 substances mentioned in the question. The
number of moles is equal to the balancing number. If no balancing number is
present the number of moles is 1.
Na2CO3(s) + 2HCl(aq)
(1 mole)
(2 moles)
→ 2NaCl(aq)
+ CO2(g) +
(2 moles)
(1 mole)
7
H2O(l)
(1 mole)
Step 3
Write down the mole relationship between the 2 substances circled in the
equation
i.e. sodium carbonate and sodium chloride.
1 mole of Na2CO3 → 2 moles of NaCl
We have already worked out in step 1 that there was 0∙1 moles of Na2CO3
Therefore,
0∙1 mole of Na2CO3 → 0∙2 mole of NaCl
Step 4
Now that the number of moles of sodium chloride is known, convert this to
mass:
Mass of NaCl in 0∙2 moles
= moles x GFM
= 0∙2 x 58∙5
Mass of sodium chloride is produced = 11∙7g
8
Example 3
The following balanced equation shows the reaction of a fuel used by drag cars
called nitromethane (CH3NO2) with oxygen.
4CH3NO2 + 3O2 → 4CO2 + 6H2O + 2N2
180kg of fuel is required for the car to travel 1 mile. What mass of oxygen is
needed to burn this mass of fuel?
Step 1
Convert the mass given in the question to moles. (Remember 180kg = 180 x 1000
= 180,000g
moles of nitromethane (CH4NO2)
=
=
= 2950.8 moles
Step 2
Write down the number of moles of each substance under the formula in the
balanced equation. Circle the 2 substances mentioned in the question. The
number of moles is equal to the balancing number. If no balancing number is
present the number of moles is 1.
4CH3NO2 + 3O2
(4 moles)
(3 moles)
→ 4CO2
+ 6H2O(l)
(4 moles)
9
(6 moles)
+
2N2
(2 moles)
Step 3
Write down the mole relationship between the 2 substances circled in the
equation
i.e. nitromethane and oxygen.
4 moles of CH3NO2 → 3 moles of O2
We have already worked out in step 1 that there was 2950.8 moles of CH3NO2
Therefore,
2950.8 moles of CH3NO2→ ¾ x 2950.8 moles of O2
2950.8 moles of CH3NO2→ 2213.1 moles of O2
Step 4
Now that the number of moles of oxygen is known, convert this to mass:
Mass of O2in 2213.1 moles
= moles x GFM
= 2213.1 x 32
Mass of oxygen required is = 70,819∙2g (or 70∙82 kg)
Note
Always remember to use the correct units in the answer. Marks are
deducted if the units are missing or wrong.
These questions are worth 2 marks. Make sure that you write down every
stage of your working. Don't just write down numbers. Show what they mean.
If you make a mistake in your working but can show the examiner you know how
to calculate the answer you will still get some marks.
10
Pupil Self Evaluation
Energy from Fuels
Number
Learning Intention
1.
Fuels are substances which release energy when burned.
Alkanes and alcohols can be used as fuels.
Success criteria
2.
Combustion reactions are exothermic reactions.
The opposite of this is an endothermic reaction.
3.
Different fuels provide different quantities of energy and
this can be measured experimentally and calculated using
I can
 State that fuels are substances which release
energy when burned
 State that alkanes and alcohols can be used as
fuels
I can:
 state that exothermic reactions give out heat.
 state that combustion reactions are exothermic.
 state that endothermic reactions take in heat and
are opposite to exothermic reactions.
I can calculate the quantities of energy produced in a
reaction using Eh = cmΔT.
When a substance is combusted the reaction can be
represented using a balanced formulae equation.
The quantities of reactants and products in these
reactions can be calculated.
I can represent a reaction using a balanced formulae
equation.
I can calculate the quantities of reactants and products
from a balanced equation
Eh = cmΔT.
4.
5.
11