1.7 Energy changes in chemical reactions

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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:

$2{H}_{2}+{O}_{2}\to 2{H}_{2}O$

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:

$\mathrm{Reactants}\to \mathrm{Product}+\mathrm{Energy}$

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:

$\mathrm{Reactants}+\mathrm{Energy}\to \mathrm{Product}$

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

explain why HCL is a Bronstead lowry acid . use an equation of HCL in water to explain
Mihlali Reply
what is electron. ( you should not allowed to tell it properties in it defination.)
vigyan Reply
a charged particle.
Siyabonga
investigative question
Obakeng Reply
how do you create a bomb
Nasty Reply
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
if you want to create bomb use this chemical substance oxidised, sulphuric acid,tobacco,gas and flammable substance , mix all this substance together and put in strong mental bottle like gas bottle put in . you must close the bottle very parked and carry that bottle carefully and shake in 5 minutes
Aaron
after shake put this bottle were the temperature is 100 degrees Celsius after moment that bottle will be high explosion like bomb
Aaron
what is mass
asmerom Reply
is object amount matter contain by object
Aaron
what is hypothesis
Aaron
hypothesis is a tentative explanation that does not have facts and it can be proven to be wrong or right
MPHO
wath is physical science
Siphelo Reply
what is physics
Dineo Reply
the branch of science concerned with the nature and properties of matter and energy
Thabisto
use a vector scale diagram and represent the two displacements(5m east and 7m west)
itumeleng Reply
what os covalent?
Shellah Reply
can i ask something?
Uapfa Reply
Ask
Precious
how to calculate enthalpy on a reaction?
Uapfa
how do you learn this chapter festet
Lisakhanya Reply
it's not all about learning it fast but understanding the basics, the concepts. formulas .. if you juss understand the basics you know the chapter .understand it
Precious
you are right precious
MPHO
can i ask smething.?
Uapfa
electric field around a point
Sameer Reply
The force per positive unit charge
Mpho
what is Kevlar made of ?
Crazy Reply
How do I determine the number of valence electrons for an atom
Kamohelo Reply
look at its group number on periodic table. group 2 elements will have 2 valence electrons and so on for all the groups. (correct me if I'm wrong but this is how I remeber it )
Siyabonga
how to find a point between to charges that is zero?
Thyrin Reply

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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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