Introduction

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Solution Calorimetry

Calorimeters are devices used to estimate the energy changes associated with chemical reactions.

They have several design features which increase the accuracy of the measurement:

1. Insulation prevents heat loss and leads to a more accurate temperature change of the water
2. The stirrer ensures that the heat is evenly distributed around the water to give the most accurate temperature
3. Electric heaters are included to calibrate the equipment

• In solution calorimeters the reaction takes place in an aqueous solution. The energy change occurring during the reaction is calculated using:
  Energy change (J) = Calibration factor (J/°C) x temperature change for the reaction (°C)

If the temperature of the water INCREASES then the reaction is EXOTHERMIC (meaning there will be a negative delta H value)

If the temperature of the water DECREASES then the reaction is ENDOTHERMIC (meaning there will be a positive delta H value)

• Solution calorimeters do still have their limitations:

• Can only measure reactions that occur in solution, meaning they cannot measure energy changes involved with combustion reactions
• The calorimeter itself absorbs some of the heat energy meaning there is not a 100% transfer of heat energy to the water, resulting in a lower change in temperature than expected

Calibration of calorimeters

• Although calorimeters are much more efficient than other mechanisms in that they aim to minimize heat loss through insulation, they cannot eliminate all heat loss. This is because:

• Energy is needed to heat or cool the calorimeter
• Energy may be used to heat the thermometer
• Energy may be used to heat the air inside the calorimeter
• Some heat is lost to the surroundings

However, it is possible to ‘account’ for this heat loss by establishing a relationship between heat energy transfer and temperature change. This process is known as calibration.

The calibration factor describes the amount of energy that is needed to increase the temperature of the water in the SPECIFIC calorimeter by one degree celcius / kelvin.

Calculating the calibration factor

To calibrate a calorimeter, we transfer a known quantity of energy to it and its contents and measure the associated temperature change. There are two methods that we can use to transfer energy to the calorimeter and determine the calibration factor;

• electrical calibration method
• chemical calibration method

Electrical calibration

An electrical heater is used to release a known quantity of thermal energy, and the resultant temperature rise is measured.

• The thermal energy released is calculated using the following formula;

E = V x I x t
• Energy (joules) = voltage (volts) x current (amps) x time (seconds)

• Therefore, to calculate the calibration factor using this method, the below formula is used:

CF = VIt/ΔT

Chemical calibration

A known quantity of energy can also be transferred to the calorimeter by performing a chemical reaction that releases a known quantity of thermal energy, then measuring the resultant temperature rise. The enthalpy change of the chemical reaction is used to calculate the amount of energy released according to:

E = n x ΔH

Therefore, CF = (n ×∆H)/∆T

*Note that the energy released is often measured in JOULES however the enthalpy values are often in KILOJOULES meaning you may need to convert between them if you want your answer to be in JOULES

Chemical calibration

As the calorimeter is not fully insulated, it can be difficult to obtain an accurate measurement of the change in temperature as the calorimeter slowly loses heat during and after the heater is operating. As well as this;

• The reaction takes time, and the energy needs time to diffuse through the water
• The thermometer needs time to respond

This inaccuracy can be improved by using temperature-time graphs which plots a graph of temperature against time, during and after calibration. The method involves:

1. Recording the temperature after the end of the reaction until the steady temperature drop due to heat loss is seen
2. Extrapolating the temperatyre recordings so the theoretical change in temperature that would have occurred if there was no heat loss can be determined. This is the temperature at reached by the extrapolated line when the reaction was initiated.
3. Temperature change is obtained using this above value – the original temperature of the calorimeter (TF – TI)