In the previous blog post, we have defined the Enthalpy Change of Combustion as well as other enthalpy changes.
For a quick recap, Standard Enthalpy Change of Combustion is defined as:
Energy released when 1 mole of a substance is completely burned in oxygen under standard conditions
e.g. C2H4 (g) + 3 O2 (g) → 2 CO2 (g) + 2 H2O (l) ΔHcθ(C2H4)
Experimental Determination of Enthalpy Change of Reaction
The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only be converted from one form into another. Based on this law, we can determine the enthalpy change of a reaction.
- System is defined as the reaction (part of the universe which is being studied).
- Surrounding is defined as the environment of the reaction (rest of the universe that is not part of the system).
The enthalpy change of a reaction is heat (at constant pressure). The heat given out (or taken in) by a reaction is transferred to the surrounding). The surrounding is often some water, a solution containing the reactants or products, or even the air.
Experimental Determination of Enthalpy Change of Combustion, ΔHcθ
The following experimental set-up can be used to determine the ΔHcθ for a fuel such as ethanol. The heat released during combustion (an exothermic reaction) is used to heat a known mass of water in a calorimeter.
The heat that is absorbed by the water causes the temperature of the water to rise. This amount of heat can be determined by measuring the temperature change.
Calculations involved will usually be based on the two formulae below:
1. Quantity of heat/energy released, q (J):
q = mc∆T
- m = mass of water, g
- c = specific heat capacity of water, 4.18 Jg-1K-1
- ∆T = increase in temperature, K / oC
2. Enthalpy change of Combustion, ∆Hc
O = – q / n
- q = quantity of heat released, J
- n = moles of substance combusted, mol
Let’s check out a GCE A-Level H2 Chemistry JC1 Promotional Examination question below and see how we can apply our knowledge to solve the question.
Use of the Data Booklet is relevant to this question.
A student carried out an experiment to determine the enthalpy change of combustion of ethanol. The set-up is the same as the one above.
The following results were obtained by the student:
- Initial temperature of water = 25oC
- Final temperature of water = 68oC
- Mass of ethanol burner before burning = 260.65 g
- Mass of ethanol burner after burning = 259.65 g
- Mass of can plus water = 160.00 g
- Mass of can = 60.00 g
Given that the enthalpy change of combustion of ethanol is −1370 kJmol–1, what is the efficiency of heat transferred to the water?
Info from Data Booklet => specific heat capacity of water, c = 4.18 Jg–1K–1
Heat absorbed by water = (160 – 60) x 4.18 x (68 – 25) = 17974 J
Mass of ethanol combusted = 260.65 g – 259.65 g = 1.0 g
Heat released by ethanol = 1370 x 1000 x 1.0 / [2(12.0) + 6(1.0) + 16.0] = 29783 J
The difference in the heat transferred is due to heat loss. Hence in this case, efficiency can be calculated by taking the ratio: (heat absorbed / heat released) .
% efficiency = (17974 J / 29783 J) x 100% = 60 %
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