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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

what is tail to tail method
Thobile Reply
a person was standing in a stationary lift / elevator and scale shown is 490N and then 470N when the lift started to move. Did the lift go up or down?
Kamva Reply
down
Fabzeey
How do you state snell's law
Mlondi Reply
The ratio of the sine of the angle of incidence in one medium to the sine of the angle of refraction in the other medium is constant
Boss
who was the first person to discover nuclears bomb
Ismael Reply
how to calculate resistance in grade 11
Ngomane Reply
last year memo in 2018 June exam
Ngomane
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Mike
yes
Ismael
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Amanda
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Patricia
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Patricia
states the Newton's second law of motion
Shallin Reply
In words it says:" when a net force is applied to an object of mass, It accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass".
Buhle
And in symbols: Fnet = ma
Buhle
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Shallin
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Buhle
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Mzondi
hlw
manu
which degree is usually be a refractive angle before it complete a totally reflection?
Mzondi
90°
Boss
yes obviously 90 degree
manu
fnet=ma
Fabzeey
how does hydrogen bonds differ from London force
Madzivha Reply
Hydrogen bonds are the strongest intermolecular forces and London forces are the weakest. Hydrogen bonds exist between polar molecules ( they are a special case of dipole-dipole forces ), while London forces occur between non-polar molecules.
Kagiso
how come the resultant force is 0
Andrew Reply
It's when you have equivalent forces going different directions then your resultant will be equal to zero
Temosho
describe what john's experiment proves about water molecules?
Fanozi Reply
Newton's first law of motion
Ayabonga Reply
hy
Madzivha
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Ayah
hey
Dire
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Precious
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Precious
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Dire
An unknown gas has pressure,volume and temprature of 0.9atm,and 120°C.how many moles of gas are present?
Chrislyn Reply
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Bradley Reply
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heath Reply
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Kgaugelo Reply
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Sphe Reply
It depends
Mbongeni
Please state the Newton third low
Malwandla
hy
Nelito
Newton's Third law states that to every force applied, there's an equal but opposite reaction
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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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