Exothermic andEndothermic Reactions

Key Differences Between theTwo Types of Reactions

Systems & Surroundings

In thermodynamics, the world is divided into a system and its surroundings.

A system is the part of the world we want to study (e.g. a reaction mixture in a flask).

The surroundings consist of everything else outside the system.

SYSTEM

SURROUNDINGS

SYSTEM

SURROUNDINGS

The heat supplied/removed is equal to the change in a thermodynamic property called enthalpy (H), which

comes from a Greek word meaning “heat inside.”

H = q•this relation is only valid at constant

pressure.Since most reactions in chemistry take place at

constant pressure, we can say that:

A change in enthalpy = A change in the heat supplied/released

ENTHALPY (H)

Endothermic and exothermic reactions

Step 1: Energy must be SUPPLIED to break chemical bonds:

Step 2: Energy is RELEASED when new chemical bonds are made:

A reaction is EXOTHERMIC if more energy is RELEASED than is SUPPLIED. A reaction is ENDOTHERMIC if more

energy is SUPPLIED than is RELEASED.

Exothermic• The sum of the bond energies of products is

greater than the sum of the bond energies of reactants.

• The enthalpy value for the products is lower than the enthalpy value of the reactants.

• Negative ΔH value.

• Heat flows to the surroundings.

• Typically, the reaction vessel feels warm.

• The temperature of the water in the calorimeter rises.

EXOTHERMIC REACTIONSEXOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases heat.

A release of heat corresponds to a decrease in enthalpy.

Exothermic process: H < 0 (at constant pressure)

Burning fossil fuels is an exothermic reaction.

EXOTHERMIC REACTIONSEXOTHERMIC REACTIONS

The PE diagram goes downhill.

Endothermic• The sum of the bond energies of products is less

than the sum of the bond energies of reactants.

• The enthalpy value for the products is higher than the enthalpy value of the reactants.

• Positive ΔH value.

• Heat flows into the system from the surroundings.

• Typically, the reaction vessel feels cold.

• But, can be very hot from an added heat source!!

• Temperature of the water in the calorimeter falls.

Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.

An input of heat corresponds to an increase in enthalpy.

Endothermic process: H > 0 (at constant pressure)

Photosynthesis is an endothermic reaction

(requires energy input from sun)

Forming Na+ and Cl- ions from NaCl is

an endothermic

process

ENDOTHERMIC REACTIONSENDOTHERMIC REACTIONS

ENDOTHERMIC REACTIONSENDOTHERMIC REACTIONS

The PE diagram goes uphill.

ENDOTHERMIC REACTIONSENDOTHERMIC REACTIONS

The PE diagram goes uphill.

Measuring HeatMeasuring Heat

reaction

reaction

Exothermic reaction: heat Exothermic reaction: heat given off & temperature of given off & temperature of

water rises.water rises.

Endothermic reaction: Endothermic reaction: heat taken in & heat taken in &

temperature of water temperature of water drops.drops.

VaporizationVaporization

Energy has to be supplied to a liquid to enable it to overcome forces that hold molecules together.

• endothermic process (H positive)

Melting

Energy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and

flow as a liquid.

• endothermic process (H positive)

Freezing

Liquid releases energy and allows molecules to settle into a lower energy state and form a solid.

• exothermic process (H negative)

(we remove heat from water when making ice in freezer)

Reaction EnthalpiesReaction Enthalpies

All chemical reactions either release or absorb heat.

Exothermic reactions:

Reactants products + energy as heat (H -ve)

Endothermic reactions:

Reactants + energy as heat products (H +ve)

e.g. burning fossil fuels

e.g. photosynthesis

Reaction ExamplesReaction Examples

Exothermic reactions:

Reactants products + energy as heat (H -ve)

Reaction ExamplesReaction Examples

Exothermic reactions:

Reactants products + energy as heat (H -ve)

Reaction ExamplesReaction Examples

Exothermic reactions:

Reactants products + energy as heat (H -ve)

Reaction ExamplesReaction Examples

Endothermic reactions:

Reactants + energy as heat products (H +ve)

Reaction ExamplesReaction Examples

Endothermic reactions:

Reactants + energy as heat products (H +ve)

The contents of The contents of the crucible are the crucible are the “system.” the “system.” The Bunsen The Bunsen

burner that is burner that is providing the providing the

heat is the heat is the “surroundings.”“surroundings.”

Reaction ExamplesReaction Examples

Endothermic reactions:

Reactants + energy as heat products (H +ve)

The contents of The contents of the test tube are the test tube are

the “system.” the “system.” The Bunsen The Bunsen

burner that is burner that is providing the providing the

heat is the heat is the “surroundings.”“surroundings.”

Exothermic/Endothermic

• For exothermic reactions, heat is released.

• For endothermic reactions, heat is absorbed.