Calorimetery

Calorimetery is the science of measuring the heat change associated with either a physical or chemical process.

When an object is heated, its temperature increases and when the object cools, its temperature decreases.

Heat capacity (C) is the amount of heat (q) required to raise the temperature of an object one degree Celsius.

The units for heat capacity are J/^oC.

(The unit is read as Joules per degree Celsius).

The equation which describes this relationship is:

            C = q/ΔT

                        C = is the heat capacity of the object

                        q = is the amount of heat entering or leaving the object

                        The change in temperature (ΔT) of the object is defined as:

                        ΔT = T_final - T_initial

A positive value for heat (+q) means that heat is entering the object.  In this case, the final temperature of the object will be higher than the initial temperature.

A negative value for heat (-q) means that heat is leaving the object.  In this case, the final temperature of the will be lower than the initial temperature.

Example 1

How much heat is required to raise the temperature of an object from 25.00^oC to 60.00^oC, if the heat capacity of the object is 0.755 J/^oC?

Example 2

An object with a heat capacity of 38.25 J/^oC lost 4.593 kJ of heat.  If the initial temperature was 235.19 ^oC, calculate the final temperature after the cooling process.

Specific heat capacity (or simply specific heat) is the amount of heat required to raise the temperature of one gram of a substance one degree Celsius.

The mathematical equation which describes this relationship is:

            q = (mass of the substance)(specific heat of the substance)(ΔT)

Example 3

What amount of heat is released when a 100.0 gram sample of copper cools from 95.00°C to 65.00°C?  The specific heat capacity of copper is 0.382 J/g×°C.

Calorimeter

A calorimeter is a device used to measure heat changes that accompany physical or chemical processes.

Exothermic processes carried out inside a calorimeter result with an increase of the calorimeter temperature. 

Endothermic processes carried out inside a calorimeter result with a decrease of the calorimeter temperature. 

The heat given off in an exothermic process will be the same amount of heat gained by the calorimeter.

The heat absorbed in an endothermic process is the same amount of heat lost by the calorimeter.

The heat change of the process is therefore equal to, but opposite in sign to the heat change of the calorimeter.

            - q_process  = q_cal

Example 4

A 123.57 g sample of iron was heated to 100.00^oC and then placed in a calorimeter with a heat capacity of 209.2 J/^oC.  The temperature of the calorimeter rose from 20.00^oC to 36.80^oC.  Calculate the specific heat capacity of iron.

When a chemical reaction is carried out inside a calorimeter, the heat lost or absorbed by the reaction (q_rxn) is equal to but opposite in sign to the heat gained or lost by the calorimeter (q_cal).

            - q_rxn  = q_cal

Example 5

A certain calorimeter has a heat capacity of 8.952 kJ/^oC.  When a 3.500 g sample of carbon was burned to CO₂ in the calorimeter, the temperature of the calorimeter rose from 25.00 ^oC to 38.18 ^oC.  What is the heat of combustion of carbon in joules per gram?