Exothermic and endothermic reactions

Law of conservation of energy

Energy cannot be created or destroyed but can be transferred or dissipated to the surroundings.

How to measure heat content

Thermometer

Two types of energy created

1) Exothermic Reactions

2) Endothermic Reactions

Exothermic reaction

They release energy, and the temperature of the surroundings increases.

Examples of exothermic reaction

• Combustion Reactions (fires/burning)

• Many oxidation reactions

• Most neutralisation reactions

Everyday example of exothermic reaction

Self-heating cans and hand warmers.

Endothermic reaction

Absorbs energy, the temperature of the surroundings decreases.

Example of endothermic reaction

• Thermal decomposition reactions

• The reaction of citric acid and sodium hydrogen carbonate

Everyday example of endothermic reaction

Instant ice packs which can be used to treat sports injuries.

Some diagrams of an exothermic and endothermic reaction

Good way to show exothermic/endothermic reactions

Energy level diagram

Energy level diagram for an exothermic reaction

Energy level diagram for an endothermic reaction

Calorimetry

A technique used to measure changes in enthalpy of chemical reactions (how much heat is released/absorbed by a chemical reaction)

Enthalpy change?

The amount of heat as a result of a chemical reaction, such as combustion and neutralisation.

Two calorimetry experiments

• Enthalpy changes of reactions in solution

• Enthalpy changes of combustion

What is the enthalpy changes of reactions in solution suitable for?

Dissolving, displacement and neutralisation

Things you have to do to make sure the experiment is accurate for enthalpy changes of reactions in solution

• That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g^-1 K^-1

• That the density of the solution is the same as pure water, i.e. 1 g cm^-3

• The specific heat capacity of the container is ignored

• The reaction is complete

• There are negligible heat losses

What do you test enthalpy changes of reactions in solution in

A calorimeter (can be made up of a polystyrene drinking cup, a vacuum flask or a metal can), It will have an in

How to test for enthalpy changes of reactions in solution

• A fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer

• An excess amount of the second reagent is added and the solution is stirred continuously

• The maximum temperature is recorded and the temperature rise calculated

To see this reaction in action and how to set up (Optional)

What do you need to test for Enthalpy of Combustion

• Thermometer

• Draught shield

• Spirit burner

• Typical calorimeter

Setup for an Enthalpy of Combustion experiment

Other calorimeter setup

Possible faults in either experiment

• Not all the heat produced by the combustion reaction is transferred to the water

  • Some heat is lost to the surroundings

  • Some heat is absorbed by the calorimeter

• To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter

• Shielding can be used to reduce draughts

• In this experiment the main sources of error are

  • Heat losses

  • Incomplete combustion

What is specific heat capacity

Describes how much energy it takes to raise the temperature of 1kg of a substance by 1°C

Specific heat capacity of 1kg water?

4200J

How to work out energy needed?

Energy = Mass x heat capacity x change in temperature

or

M x c x △T

Work out the energy needed to make 3kg of water rise in temperature by 5°C

(M x c x △T)

63,000 J

Formula triangle for specific heat capacity