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Introduction

All chemical reactions involve energy changes. In some reactions, we are able to observe these energy changes as either an increase or a decrease in the overall energy of the system.

What causes the energy changes in chemical reactions?

When a chemical reaction occurs, bonds in the reactants break , while new bonds form in the product. The following example may help to explain this. Hydrogen reacts with oxygen to form water, according to the following equation:

2 H 2 + O 2 2 H 2 O

In this reaction, the bond between the two hydrogen atoms in the H 2 molecule will break , as will the bond between the oxygen atoms in the O 2 molecule. New bonds will form between the two hydrogen atoms and the single oxygen atom in the water molecule that is formed as the product.

For bonds to break , energy must be absorbed . When new bonds form , energy is released . The energy that is needed to break a bond is called the bond energy or bond dissociation energy . Bond energies are measured in units of kJ.mol - 1 .

Bond energy

Bond energy is a measure of bond strength in a chemical bond. It is the amount of energy (in kJ.mol - 1 ) that is needed to break the chemical bond between two atoms.

Exothermic and endothermic reactions

In some reactions, the energy that must be absorbed to break the bonds in the reactants, is less than the total energy that is released when new bonds are formed. This means that in the overall reaction, energy is released as either heat or light. This type of reaction is called an exothermic reaction. Another way of describing an exothermic reaction is that it is one in which the energy of the product is less than the energy of the reactants, because energy has been released during the reaction. We can represent this using the following general formula:

Reactants Product + Energy

Exothermic reaction

An exothermic reaction is one that releases energy in the form of heat or light.

In other reactions,the energy that must be absorbed to break the bonds in the reactants, is more than the total energy that is released when new bonds are formed. This means that in the overall reaction, energy must be absorbed from the surroundings. This type of reaction is known as an endothermic reaction. Another way of describing an endothermic reaction is that it is one in which the energy of the product is greater than the energy of the reactants, because energy has been absorbed during the reaction. This can be represented by the following formula:

Reactants + Energy Product

Endothermic reaction

An endothermic reaction is one that absorbs energy in the form of heat or light.

The difference in energy (E) between the reactants and the products is known as the heat of the reaction . It is also sometimes referred to as the enthalpy change of the system.

Demonstration : endothermic and exothermic reactions 1

Apparatus and materials:

You will need citric acid, sodium bicarbonate, a glass beaker, the lid of an ice-cream container, thermometer, glass stirring rod and a pair of scissors. Note that citric acid is found in citrus fruits such as lemons. Sodium bicarbonate is actually bicarbonate of soda (baking soda), the baking ingredient that helps cakes to rise.

Questions & Answers

how to calculate the magnitude of the force of repulsion
Vicky Reply
what is resistance
Girma Reply
how to calculate external resistance in the circuit
Geofrey Reply
it is the bonding when two or electrons pairs are shared between two non metal atoms
Pride Reply
what is covelet bonding
Angel Reply
explain the angle created by the wire n the flux
Tony Reply
what's the meaning of flux
Tony
What's the highest hearing capacity of human ear (in Hz)?
Pride Reply
20 to 20 000Hz then under laboratory conditions 12Hz to 28kHz
Mpho
28kHz = 28000Hz
Mpho
is 15000 good hearing
Bhuboy
as in ? cz it differs according to what ur dealing with
Pride Reply
when we are dealing with Newton ' forth law of motion ( universal gravitational) and Coulomb' law , we convert everything to metres
Pride
coulombs, meters right?
Unako
thank you
Unako
is r measured in cm, mm,m or km?
Unako Reply
hi guys I think it should be in metres
Luyanda
show
Wiseman
yes
Michael
yes
Mpho
yes
Katlholo
what is matter
Chris Reply
matter is any sunbstance that has mass and take up spaces by having vuliom.
Bhuboy
how to calculate the potential difference
Scott Reply
I=V/R
Tony
E=kQ1Q2 divided by a square distance between the two charges
ANDISWA Reply
why did you use 8.99×10 to the power 9 instead of 9.10×10 to the power 9
Lihle Reply
and how did you get 3.37×10 to the power negative-8
Lihle
what is the fumula to calculate the theoretical yield ?
Pride Reply
the Lewis structure for NH3
Elihle Reply
.
DIPOLELO

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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