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Combustion reactions require 3 things- fuel, oxygen and energy.
Combustion reactions are exothermic reactions in which the reactant combines with oxygen to produce an oxide. This is an oxidation reaction.
C3H8 (g) + 5O2(g) --> 3CO2 (g) + 4H2O (g) ΔH = -2220 kJ mol -1
COMPLETE AND INCOMPLETE COMBUSTION
The complete combustion of hydrocarbons occurs when there is sufficient oxygen for the fuel to burn.
The products of complete combustion are carbon dioxide and water.
Eg. CH4 (g) + 2O2(g) --> CO2 (g) + 2H2O (g)
When oxygen is not plentiful, incomplete combustion occurs.
The products of incomplete combustion are carbon monoxide or carbon and water.
Eg. 2CH4 (g) + 3O2 (g) --> 2CO (g) + 4H2O (g)
CH4 (g) + O2 (g) --> C (s) + 2H2O (g)
HEAT OF COMBUSTION
•The heat of combustion of a substance is the energy released when a specified amount (eg. 1 mol, 1g, 1 L) of the substance burns completely in oxygen.
•The heat of combustion is usually measured at conditions 298K (25 C) and 101.3kPa. Water will be a liquid under these conditions.
•Heats of combustion are measured using a calorimeter.
•Energy released= n x ΔHc
SPECIFIC HEAT CAPACITY
•The amount of energy required to raise the temperature of one gram of a substance by 1°C is called the specific heat capacity of that substance.
•The higher the specific heat capacity, the more effectively a material will store heat energy.
Water has a very high heat capacity, which is a consequence of the hydrogen bonding between its molecules
•Heat change gained or lost by a substance during a chemical reaction can be calculated by:
q = m x C x ΔT
where q is Energy (J); m is Mass (g); C is specific heat capacity (Jg-1oC-1); T is temperature (oC), Δ means change in.
Determining heat of combustion
•During combustion chemical energy is converted to thermal energy. The thermal energy released by a quantity of fuel can be used to heat a measured volume of water.
•The change in the water temperature can by used to determine the approximate amount of energy released as fuel.
0.355g of methanol (CH3OH) undergoes complete combustion in a spirit burner. The heat energy released is used to heat 100mL of water. The temperature of the water rose from 20.24 oC to 37.65 oC. Calculate the heat of combustion of methanol in kJ/mol.
Mass of water (m) = 100g
C (water) = 4.18 J/goC
ΔT= 37.65- 20.24 = 17.41 oC
Use q= mxCx ΔT = 100 x 4.18 x 17.41 = 7277 J = 7.277kJ
Find the mole of methanol, n(CH3OH) = 0.355/32.0 = 0.0111 mol.
Heat of combustion = -7.277/0.0111 = -656 kJ/mol.
Heat of combustion:
Calculate the amount of energy released when 3.60 kg of butane (C4H10) is burnt in an unlimited supply of oxygen.
Unlimited supply of oxygen- complete combustion.
n(C4H10) = m/M= 3.60 x 103 /58.0 = 62.1 mol.
Energy = n x ΔHc = 62.1 x -2886 = -1.79x105 kJ. (negative since exothermic)
The energy content is often expressed in units of kilojoules per gram or mega joules per tonne (106) if the quantity of fuel is great.
To work this out- ΔHc /M heat of combustion/molar mass
For ethanol- heat of combustion = -1367 kJ/mol
Molar mass= 46.0 g/mol
Heat of combustion per gram = -1367/46.0 = -29.7 kJ/g
Specific heat example:
•Calculate the energy required to heat 120mL of water for a cup of coffee to boiling point if the initial water temperatuer is 20.0°C.
Since the density of water is 1g mL, the mass of 120mL is 120g.
q = m x c x ΔT
Energy required to raise temperature of 120g of water by 1 degree = SHC x mass
4.184 x 120 = 502.0J
Since temperature rises by 80 degrees, the total energy required = J x ΔT
= 502.0 x 80 = 40160J = 40.2kJ
Steps to writing thermochemical reactions:
1.Write a combustion reaction.
2.Balance the equation.
3.Obtain the heat of combustion from a table of values.
4.Using your balanced equation look at the coefficient in front of your fuel - multiply ΔH by this value.
5.Write the value for ΔH at the end of your equation.
Revision poster - fuels and energy- available through TES teaching resources- VCE Chemistry Unit 3 and 4 fuels and energy revision