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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

why forces are classified as vectors
Sfundo Reply
because it has both size and the direction
Tobias
you can jus pull or push something and jus tell that you were pulling it in no direction
Tobias
Tobias Louw I highly appreciate that 😊
Sfundo
happy to help
Tobias
what is a molecular?
Kamogelo Reply
A molecule is covalently bonded. Meaning is it is a bond between two or more non-metals.
Sphiwe
I mean what is a resulted vector
Njabulo Reply
is the vector quantity that has the same effects as 2 or more vectors
junior
It is the combination of two or more vectors
Sisipo
is the vector sum of two or more vectors
Precious
thanks
Njabulo
what are compartments of vectors
Dineo
yes junior
Clinton
hey how do I calculate the unknown force using algebraic calculation
Lizzy Reply
I also need to know that help me please
Debra
help me too n calculating it
Alfred
I need to know more about components of vectors
Dineo
help me too, i dont know to
junior
what is a vector
Njabulo
what is covalent bonding
Jeanine Reply
is the sharing of atoms between two or more substances
Andiswa
Draw a 100 N force vector acting towards the right
Welcome Reply
Two forces act on an object. The first is a horizontal 50N force. The second is a 40N force acting at an angle to the horizontal 1.1 use a graphical method drawn to 1mm:1N to find the resultan if 40N force is inclined at: 1.1 30° to the horizontal 1.2 60° to the horizontal
Maya
A gardener mounts a pot plant on the wall using a metal ring and chain. The ring exerts a horizontal force of 20 N on the pot plant and the weight of the pot plant is 30 N Determine the magnitude of the tension in the chain by calculation and construction
Amelia Reply
what is a head to tail methods?
Karabo Reply
thats simply involves drawing the first vector and then joining the second vector to its tail then join the head of the firstvector
Andiswa
what is magnitude
Mfundo Reply
Magnitude is size
Thabiso
magnitude is the maximum or greater extent of size and the direction of an object, it describes the direction of vectors and scalars.
Siphosethu
So I got a question that follows: two forces of 60N and 80N respectively act simultaneously at a point, sketch and determine their resultant by using the tail to head method
Mbuli Reply
How do we calculate a resultant in a vector
Thembi Reply
Where are the answers to the questions on this page
Dzuneko Reply
What are the name of x ✖ and y?
Dzuneko
Hello
Jeanine
hi
Alfred
How do I determine the magnitude of the resultant by using tail to tail, head to tail, pythogolas theorem of fx=5N and fy=7N
mamokete Reply
Fr=Fx+Fy ~ 5N+7N=12N This applies for tail-tail and head-tail.Theorem of Pythagoras you square them(check the format)
Sphiwe
5^2+7^2
Anele
how chemicals change
Laymark Reply
what does the word extension mean or a synonym
Snothile Reply
it means maybe to Extend something
Debra
hi guys I'm new here
Ezile Reply
feel at home 😊
Sfundo

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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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