Energy

Brightrose 14

Energy Diagrams

Exothermic reactions

Endothermic Reactions:

When reactions occur there is a change in energy – this is called enthalpy change and is a negative or positive value depending on if it is an exothermic or endothermic reaction.

Endothermic reaction - takes in more heat (energy) then it gives out.

Exothermic reaction – gives more heat (energy) out then it takes in.

Because the enthalpy change is what happens within the system the enthalpy change for an endothermic reaction is positive as it gains energy and for an exothermic reaction it is negative as the system loses energy.

Energy Transferred Calculations

An energy transfer calculation has several steps. The main equation is:

Specific heat capacity is a set value of each substance and will usually be given if not then water is 4.12kJmol-1

The next step is working out the number of moles in the reaction.

One way of working out number of moles is by using the mass and the relative molecular or atomic mass of the element or compound.

The final step is working out the energy transferred per mole. To do that you divide the energy in joules by the number of moles:

The value is going to be big and in Jmol-1, but it can be divided by 1000 to give kJmol-. The sign depends if the reaction is endothermic or exothermic which can be determined through the change in temperature – negative change is endothermic so the enthalpy change will be positive and if the temperature change is positive then the reaction is exothermic and the ethalpy change is negative.