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Topic 1 Energetics Revision Notes Free essay! Download now

Home > A Level > Chemistry > Topic 1 Energetics Revision Notes

Topic 1 Energetics Revision Notes

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Topic 1 Energetics Revision Notes

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Topic 1 Energetics
Revision Notes

1) Introduction

An enthalpy change is a change in heat energy measured at constant pressure.
Enthalpy changes refer to the chemicals not the surroundings.
The symbol for an enthalpy change is DH (D = change, H = heat energy)
The units for enthalpy change are kJ mol-1


2) Exothermic Reactions

In exothermic reactions, the chemicals lose energy so DH is negative. The surroundings gain energy so feel hotter.
In terms of a reaction with oxygen, oxidation is an exothermic process. Examples include:

Combustion burning fuels for heating and in engines e.g.
CH4 + 2O2 CO2 + 2H2O

Respiration oxidation of carbohydrates in living things
C6H12O6 + 6O2 6CO2 + 6H2O

3) Endothermic Reactions

In endothermic reactions, the chemicals gain energy so DH is positive. The surroundings lose energy so feel cooler.
Endothermic reactions require an input of heat energy or they will stop.
Examples of endothermic processes include:

Thermal decomposition of calcium carbonate
CaCO3 CaO + CO2

Photosynthesis
6CO2 + 6H2O C6H12O6 + 6O2

4) Measuring Enthalpy Changes

Measuring enthalpy changes by experiment is called calorimetry
Calorimetry works by using the energy released or absorbed in a reaction to change the temperature of a known mass of water
Calculating an enthalpy change from experimental results involves a two step process
Firstly q = -mcDT/1000, where m = mass of water in g, c= specific heat capacity of water (4.18 Jg-1K-1), DT = change in temperature)
Secondly, DH = q/n where n= number of moles of reactant
Enthalpy changes measured from calorimetry are smaller than the expected values because of heat loss to the apparatus and the environment
Other reasons for differences from standard values are non-standard conditions and evaporation of water
In the case of measuring enthalpies of combustion, differences from standard values can occur through incomplete combustion and evaporation of the fuel (if it is a liquid)


Example

The combustion of 0.15g of ethanol, C2H5OH, in a spirit burner increased the temperature of 75 cm3 of water by 12.5C. Calculate the enthalpy of combustion of ethanol in kJ mol-1.

q = -mcDT/1000
= -75 x 4.18 x 12.5/1000
= -3.919 kJ

n = 0.15/46
= 3.26 x 10-3 mol

DH = -3.919/3.26 x 10-3
= -1202 kJ mol-1


5) Enthalpy Changes

Standard conditions for measuring enthalpy changes are a pressure of 100 kPa and a temperature of 298K
Enthalpy change of combustion, DHc, is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions e.g.

C5H12(l) + 8O2(g) 5CO2(g) + 6H2O(l)

Some substances cannot be burnt and have zero enthalpy of combustion e.g. O2, CO2, H2O
Enthalpy ...

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