Calorimetry

Calorimetry

Mr. DvorskySCH 4U1

• A calorimeter is an object used for measuring the heat of a chemical reaction or physical change.

• Can be as simple as a pair of cups and a thermometer, or as complex as a device attached to the Large Hadron Collider.

• Ideally, the calorimeter does not allow system to exchange either matter or energy with the surroundings (isolated).

• There is water inside the calorimeter, and also a reaction or process of interest.

• You can view it as water being one system and the process occurring in another system. They are in thermal contact but isolated from the rest of the universe.

Recall: The Laws of Thermodynamics

• 1st Law – Energy of the Universe is constant so any energy added to the system came from the surroundings, and any energy lost by the system goes to the surroundings.

• 2nd Law – Heat will pass from an object at higher temperature to cooler temperature until they reach thermal equilibrium.

Tying this to Calorimetry

• A chemical reaction occurs in the calorimeter. • This results in a change in temp in the water

system surrounding the reaction. • The enthalpy of reaction can be calculated

based on the thermal energy transfer in the calorimeter.

Simple Calorimeter

• -two stacked styrofoam• cups, lid & thermometer• -the reaction happens in • the inner cup with known • amount of water. • -the chemical reaction • usually occurs in the water

Key Assumptions

• The system is isolated – no outside exchange of energy

• The thermal energy exchanged to the cup and thermometer itself is so small it can be ignored.

• If something reacts or dissolves in water in the calorimeter, the resulting solution still retains the properties of water (i.e. specific heat capacity).

• Occurs at constant pressure

Once these assumptions are made…

• The following applies:Thermal energy released by the system = -thermal energy absorbed by the surroundings

Heat lost (or gained) by the system = -heat gained (or lost) by the surroundings.

System = chemical change you are studying, and the surroundings = water in the calorimeter. Since the mass of water and its specific heat capacity is known, you can calculate the change in thermal energy using Q = mcΔT

• How to use a simple calorimeter – see top of page 302.

• Show clip

Using calorimetry data to determine the enthalpy of reaction

• Ideal for determing ΔH of reactions in dilute aqueous solutions

• The solution itself absorbs or releases the energy.

ΔH = nΔHr

Flame and Bomb Calorimeters

• Flame calorimeter can measure enthalpy of combustion of a substance that is burning.

• Not all assumptions are the same, i.e. the heat absorbed by the calorimeter itself should be included in calculations

• For pure substances, molar enthalpy of combustion is used. For foods, enthalpy of combustion is often expressed as kJ/g rather than kJ/mol. –see Figure 5.18 page 306

• Bomb calorimeter – very precise, reaction takes place in a bomb.

-see clip.