Exothermic and endothermic reactions

Exothermic and endothermic reactions

During a chemical reaction, there tend to be a change in energy

of the system. The energy change in chemical reactions are

usually in the form of heat energy, however, they may also be

in the form of light, electrical or nuclear.

Reactions either absorb energy (endothermic) or release

energy (exothermic).

An exothermic reaction releases energy to its surroundings,

causing the surroundings to get hotter.

Characteristics of Exothermic Reactions

 Heat is given out and transferred from the chemical to the

surrounding environment.

 The temperature of the mixture rises, the container will

therefore feel hot.

In an exothermic reaction, the temperature will rise until the

highest temperature is reached. When the reaction is

complete, the temperature will fall until it reaches room

temperature.

Examples of exothermic reactions include:

 The combustion of fuels.

 The rusting of iron.

 The corrosion of metals.

 The neutralization reaction between acid and alkali

 Respiration

 Dissolving some salts like NaOH

An endothermic reaction absorbs energy from its surroundings

causing the surroundings to get colder.

Characteristics of Endothermic Reactions

 Heat energy is absorbed and transferred from the

surrounding environment to the reactants.

 The temperature of the mixture falls, the container will

therefore feel cold.

Examples of Endothermic Reactions

 Photosynthesis.

 The action of light on silver bromide in photographic film.

 Thermal decomposition.

 Dissolving many salts like KNO 3 and NH 4 Cl

The temperature of the reaction mixture falls until the lowest

temperature is reached. When the reaction is completed, the

temperature of the mixture will rise until it reaches room

temperature.

Why is heat released or absorbed in a chemical reaction?

In any chemical reaction, chemical bonds are either broken or

formed. When chemical bonds are formed, heat is released

(exothermic), and when chemical bonds are broken, heat is

absorbed (endothermic).

In other words, when the original bonds are broken in the

reactants, energy is absorbed. When new bonds are formed in

the product, energy is released.

Molecules inherently want to stay together, so formation of

chemical bonds between molecules requires less energy as

compared to breaking bonds between molecules, which

requires more energy and results in heat being absorbed from

the surroundings.

What is enthalpy of a reaction?

The energy content of a substance is called its enthalpy and is

denoted by the symbol H. The enthalpy of a substance cannot

be measured directly, but it is possible to measure the change

that occurs during a chemical reaction. This is known as

enthalpy change and is given the symbol (ΔH)

Enthalpy of a reaction is defined as the heat energy change

(ΔH) that takes place when reactants go to products. Units are

kJmol -1 . If heat is absorbed during the reaction, ΔH is positive; if

heat is released, then ΔH is negative.

ΔH = total enthalpy of products – total enthalpy of reactants

ΔH reaction = H products - H reactants

Every chemical bond has a specific amount of energy, no

matter what compound it is in, known as bond energy. The

amount of energy which has to be absorbed to break the bond

is the same as the amount of energy released when that bond

is formed.

Exothermic reactions

For exothermic reactions, the energy absorbed to break the

existing bonds in the reactants is less than the energy released

when forming new bonds in the products. The extra energy is

released to the surroundings and the temperature of the

surroundings increases. Therefore the enthalpy of the products

is less than the enthalpy of the reactants:

H products < H reactants

Since the reaction lose energy, ΔH is negative i.e. ΔH - ve which

means that ΔH has a value less than zero.

An example of an exothermic reaction is the neutralization of

sodium hydroxide by hydrochloric acid.

NaOH (aq) + HCl (aq) → NaCl (aq) + H 2 O (l) ΔH = - 56.28 kJmol -1

Endothermic reactions

In endothermic reactions, the energy absorbed to break the

existing bonds in the reactants is greater than the energy

released when forming new bonds in the products. The extra

energy is absorbed from the surroundings and the temperature

of the surroundings decreases. Therefore the enthalpy of the

products is less than the enthalpy of the reactants. Therefore,

the enthalpy of the products is greater than the enthalpy of the

reactants:

H products > H reactants

Since the reaction gain energy, ΔH is positive i.e. ΔH + ve which

means that ΔH has a value greater than zero.

An example of an endothermic reaction is the reaction between

hydrogen and iodine:

H 2(g) + I 2(g) → 2HI (g) ΔH= + 26.50kJmol -1

Depiction of an energy profile diagram

An energy diagram can be defined as a diagram showing the

relative potential energies of reactants, transition states, and

products as a reaction progresses with time. Most reactions

need some energy to get them started. The diagram shows the

enthalpy, of the reactants and products, ΔH and activation

energy, Ea.

Exothermic reaction

Endothermic reaction

When drawing the energy profile diagram for a specific reaction

it is important that you include the following information on

your diagram:

 The formulae of reactants

 The formulate of products

 Arrows indicating the activation energy and ΔH

 The value of ΔH

Catalysts and energy profile diagrams

You learnt that a catalyst is a substance that can increase the

rate of a reaction. In terms of energy, a reaction in which a

catalyst is used has a lower activation energy, Ea. than the

same reaction without a catalyst.