Energy Changes in Reactions

Aims of lesson:

• To discuss energy changes in chemical reactions.

• To draw potential energy diagrams for exothermic and endothermic reactions.

• Chemical reactions usually involve a change in energy.

• Exothermic reaction: Heat energy is released to surroundings

• Endothermic reaction: Heat energy is absorbed (taken in) from surroundings

During chemical reactions, bonds are broken and then bonds are formed.

REACTANTSBonds broken

Bond breaking requires energy (endothermic).

PRODUCTSBonds formed

Bond formation releases energy (exothermic).

A reaction is exothermic or endothermic depending on which process predominates.

Potential Energy Level Diagrams

Reactants

Products

Activated Complex

• The difference in Potential Energy (P.E.) between reactants and products is the ENTHALPY CHANGE (H).

• H= HP-HR

• For an exothermic reaction, H is the amount of heat energy lost to the surroundings.

Chemical energy Heat Energy

• Activated complex (2) is the unstable arrangement of atoms formed at the maximum of the potential energy barrier.

P.E Diagram for an Exothermic Reaction

• Chemical energy Heat energy.

• Energy loss by reactants.

• Temperature increases.

• H is negative.• Exothermic

reaction.

MNEMONIC

N - HE -

X –

T –

U – Temperature goes up

P –

Exothermic

P.E Diagram for an Endothermic Reaction

• Heat energy Chemical energy.

• Energy gain by reactants.

• Temperature decreases.

• H is positive.• Endothermic

reaction.

Ea