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Exothermic & Endothermic REACTIONS

Energy in Reactions

Some chemical reactions release heat to the surroundings

• These are called EXOTHERMIC reactions• Heat EXITS the reaction

Some chemical reactions absorb heat from the surroundings

• These are called ENDOTHERMIC reactions• Heat ENTERS the reaction

Energy in Reactions

All substances have a type of chemical potential: • The total energy (J or kJ) contained in a system • Chemical potential energy in a substance• The symbol for enthalpy is H• ΔH = Hproducts – Hreactants

Energy in Reactions

An exothermic reaction converts the enthalpy stored in a substance into HEAT which is released to the surroundings

N2 + 3H2 2 NH3+ heat

Exothermic Reactions

Has higher enthalpy

Has lower enthalpy

Exothermic Enthalpy Diagram

Products

ReactantsEnthalpy (H)

Reaction Proceeds

H is ΔNegative

1. The “heat term” written on the PRODUCTS side

N2 + 3H2 2 NH3 + 46.2 kJ

2. ΔH written beside the equation is NEGATIVE.

N2 + 3H2 2 NH3 ΔH = - 46.2 kJ

Exothermic Reactions

Exothermic Reactions:• ΔH is negative• Products are lower than reactants on the

enthalpy diagram• Heat is released to the surroundings

In Summary…

An endothermic reaction absorbs heat from the surroundings and converts into enthalpy

2C + 2 H2 + heat C2H4

Endothermic Reactions

Has higher enthalpy

Has lower enthalpy

Endothermic Enthalpy Diagram

ΔH is POSITIVE

Products

Reactants

Enthalpy (H)

Reaction Proceeds

1. The “heat term” written on the REACTANTS side.

2 C + 2H2 + 52.3 kJ C2H4

2. ΔH written beside the equation is POSITIVE.

2 C + 2 H2 C2H4 ΔH = + 52.3 kJ

Endothermic Reactions

Endothermic Reactions:• ΔH is positive• Products are higher than reactants on the

enthalpy diagram• Heat is absorbed from the surroundings

In Summary…

In Class Work

Please collect the Endo/Exo Worksheet from the front of the room, finish this worksheet by next class

Next Class: Begin Stoich