Unit 5 - Energetics, Redox, Inorganic Chemistry (ENERGETICS)
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Measuring enthalpy changes + Related Calculations and Experiments
Heat and temperature:
- Temperature: The average kinetic energy of the particles in a system. As particles
move faster, their average kinetic energy increases and so the temperature of the
system increases. Temperature is independent of the number of particles in the
system. It is measured using a thermometer.
- Heat: Measure of the total kinetic energy of all the particles in a given amount of
substance. It does depend on the number of particles in the substance. A bath has
more heat than a red hot nail because there are more particles in the bath.
- Heat flows from areas of low temperatures to areas of higher temperatures. So heat
would move from the hot red nail to the bath.
Measuring the enthalpy change for a reaction:
- Enthalpy change of a reaction is the heat given out or the heat taken in as the
reaction proceeds.
- There is no instrument that measures heat directly.
- To measure heat change, an apparatus must be arranged so that heat can be
transferred into a particular mass of a substance, usually water.
- Then, measure three things:
1) Mass of the substance that is being heated up / cooled down
2) Temperature change
3) Specific heat capacity of the substance being heated up / cooled down
Specific heat capacity (J K-1 g-1) is the quantity of heat needed to raise the temperature of 1g
of a substance by 1K. For water, the specific heat capacity is 4.2 J K-1 g-1.
Enthalpy change = q (kJ/mol)
Mass of substance= m (g)
Specific heat capacity (4.2 J K-1 g-1 for water)
Temperature change = ΔT(K)
, The simple calorimeter:
- Apparatus in figure 1 can be used to measure the enthalpy change when a fuel burns
- You burn the fuel to heat a substance of which the mass is known and then measure
the temperature change that takes place (final and initial temperature must be
measured)
- The process assumes zero heat loss.
- Simple calorimetry can be used to compare the standard enthalpy change of
combustion for a series of similar compounds because the errors will be very similar
for every experiment. However, experimental results can be improved by cutting
down on the heat loss.
- Reasons why experimental value is different to theoretical value:
1) Specific heat capacity of apparatus is not accounted for
2) Heat is lost due to dissipation into the surroundings
The flame calorimeter:
- Flame calorimeter is an improved version of the simple calorimeter used for
measuring standard enthalpy change of combustion of substances.
- It incorporates the following features to reduce heat loss:
1) Spiral chimney is made of copper
2) The flame is enclosed
3) The fuel burns in pure oxygen, rather than air.
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