Thursday, 26 February 2015

4.5 Heat of Combustion

Heat of combustion for different alcohols
■ Heat of combustion

The heat released when a 1 mol of compound undergoes complete combustion with oxygen under standard conditions.

Example of combustion reaction:
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l), ∆H = -1376kJ mol-1
Combustion reaction is an exothermic reaction because heat that is released causes the reaction temperature to increase. Thus, the heat of combustion is always negative.
■ Determination of the heat of combustion:

Heat released by fuel
Liquid fuel burned in air.
Determined the mass of fuel that has been burned.
Determined the number of moles of fuel were burned.

Heat released by fuel
Determined the volume of water used.
Determined the increase in water temperature.
The heat absorbed by the water is calculated by using the formula mcθ.
■ This video contains information regarding heat of combustion.
Laboratory Activity 4.5.1 : Heat of Combustion of Different Alcohol
Worked-example 4.5(a)
2.30g ethanol is burned completely in excess oxygen and the heat released can raise the temperature of 800cm3 of water by 20°C. Calculate the heat of combustion of ethanol.
[Specific heat capacity = 4.2Jg-1 °C-1, density of a solution of 1g cm-3, Atomic relative mass: H,1; C, 12; O,16]

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
Number of moles of ethanol
= 2.3(2×12)+(6×1)+(1×12)
= 2.346
= 0.05mol
Heat change = mc θ
= 800X 4.2 X 10
= 67200J
= 67.2kJ
0.05 mol of ethanol burn to release 67.2 kJ
Thus, 1 mol of ethanol burn to release
= 67.20.05
= 1344kJ
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) , ∆H = -1344kJ mol-1

Worked-example 4.5(b)
The following thermochemical equation shows the reaction between glucose and oxygen:
C6H12O6 + 6O2(g) → 6CO2(g) + 6H2O(l) , ∆H = -2400kJ mol-1
If 36.0 g of glucose is burned completely in excess oxygen and the heat released is used to heat up 2 kg of water. Calculate the water temperature changes.
[Specific heat capacity = 4.2Jg-1 °C-1, density of a solution of 1g cm-3]

Number of moles of glucose
= 36(6×12)+(12×1)+(6×16)
= 36180
= 0.2mol
Heat change = 0.2 X 2400
= 480kJ
Heat change = mc θ
480000 = 2000 X 4.2θ
θ = 4800008400
θ = 57.14°C

Comparison of the heat of combustion of various types of alcohol
■ Various types of alcohols

Alcohols is the more efficient fuel for human use.

The higher the heat of combustion of an alcohol (measured in part by the temperature change of water caused by the alcohol), the better it is as a fuel.

The following table shows the heat of combustion for various types of alcohols.
Name Molecular formula No of atom carbon per alcohol molecule Relative molecular mass Heat of combustion, ∆H (kJ mol-1)
Methanol CH3OH 1 32 -715
Ethanol C2H5OH 3 46 -1376
Propan-1-ol C3H7OH 3 60 -2017
Butan-1-ol C4H9OH 4 74 -2679
If the number of carbon atom per molecule of alcohol is higher, the heat of combustion also increases. This is because more atom carbon will cause more release of heat during the formation of bonds.
The difference in heat of combustion between alcohol member is almost the same because each alcohol member difference is in one group of −CH2.

The difference between heat of combustion for different fuels
■ Sources of fuels

Most fuels used by humans today are fossil fuels such as coal, natural gas, and petroleum breakdown.
■ Fuel value

The amount of heat produced by the complete combustion of a given amount of fuel.

Also refer to the heat of combustion of fuel.

Measure unit: kJ mol-1 or kJ g-1

Fuel values of various type of fuel
Fuel Fuel value in kJ g-1
Hydrogen 141.9
Methane 50.0
Ethane 47.8
Propane 46.3
Butane 45.7
Pentane 45.3
Gasoline 47.0
Diesel 45.0
Worked-example 4.5(c)
Compare the effectiveness of combustion for the following fuels.
Name Molecular formula Relative molecular mass Heat of combustion, ∆H (kJ mol-1)
Methanol CH3OH 32 -715
Ethanol C2H5OH 46 -1376
Propan-1-ol C3H7OH 60 -2017
Butan-1-ol C4H9OH 74 -2679
The effectiveness of the combustion of fuel can be compared by expressed the heat of combustion in kJ g-1.
Name Heat of combustion, ∆H (kJ g-1)
Methanol -22.34
Ethanol -29.91
Propan-1-ol -33.61
Butan-1-ol -36.20
The heat that is released from the combustion of 1 g of butan-1-ol is more than other ethanol.
Thus, butan-1-ol is a much more effective fuel compared to other ethanol.

⇲ For exercise(objective and subjective), download for free on Android OS.

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