ย 

Energy changes for SQA Higher Chemistry

Energy changes

This page covers the following topics:

1. Thermal energy
2. Enthalpy of combustion
3. Exothermic and endothermic reactions
4. Reaction profile

Calorimetry measures enthalpy changes from combustion using the equipment shown. It can also be used to measure other reactions using a beaker with a cork stopper and thermometer.

Thermal energy

Energy is conserved in reactions, meaning it is never created or destroyed but rather transferred to or from a reaction environment. Energy is gained and lost from a reaction mixture as bonds are broken or made.

Enthalpy of combustion

If a reaction gives out heat it is exothermic and if it draws in heat it is endothermic.

Exothermic and endothermic reactions

Energy change of reaction may need to be calculated to determine whether a reaction is exothermic or endothermic and sketch the reaction profile.

Reaction profile

1

Does the reaction profile diagram shown show an endothermic or exothermic reaction?

Endothermic, the energy of the reactants is lower than that of the products so energy was gained.

Does the reaction profile diagram shown show an endothermic or exothermic reaction?

2

Is activation energy higher in endothermic or exothermic reactions?

endothermic

Is activation energy higher in endothermic or exothermic reactions?

3

Given that ฮ”H for the breakdown of ammonia is +92.9 kJ/mol, which reaction profile best fits this reaction? Why?

Graph 2, in which case the products have more energy than the reactants and ฮ”H is indicated by an upwards arrow.

Given that ฮ”H for the breakdown of ammonia is +92.9 kJ/mol, which reaction profile best fits this reaction? Why?

4

Is energy absorbed or released when breaking bonds?

Absorbed

Is energy absorbed or released when breaking bonds?

5

Sketch an endothermic reaction profile and indicate ฮ”H.

image

Sketch an endothermic reaction profile and indicate ฮ”H.

End of page

ย