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A comparison of exothermic and endothermic reactions
Type of reaction Exothermic Endothermic
Energy absorbed or released Released Absorbed
Relative energy of reactants and products Energy of reactants greater than energy of product Energy of reactants less than energy of product
Sign of Δ H Negative Positive
Writing equations using Δ H

There are two ways to write the heat of the reaction in an equation

For the exothermic reaction C ( s ) + O 2 ( g ) C O 2 ( g ) , we can write:

C ( s ) + O 2 ( g ) C O 2 ( g ) Δ H = -393 kJ.mol - 1 or

C ( s ) + O 2 ( g ) C O 2 ( g ) + 393 kJ.mol - 1

For the endothermic reaction, C ( s ) + H 2 O ( g ) H 2 ( g ) + C O ( g ) , we can write:

C ( s ) + H 2 O ( g ) H 2 ( g ) + C O ( g ) Δ H = +131 kJ.mol - 1 or

C ( s ) + H 2 O ( g ) + 131 kJ.mol - 1 C O + H 2

The units for Δ H are kJ.mol - 1 . In other words, the Δ H value gives the amount of energy that is absorbed or released per mole of product that is formed. Units can also be written as kJ, which then gives the total amount of energy that is released or absorbed when the product forms.

Investigation : endothermic and exothermic reactions

Apparatus and materials:

Approximately 2 g each of calcium chloride (CaCl 2 ), sodium hydroxide (NaOH), potassium nitrate (KNO 3 ) and barium chloride (BaCl 2 ); concentrated sulfuric acid (H 2 SO 4 ) (Be Careful, this can cause serious burns) ; 5 test tubes; thermometer.

Method:

  1. Dissolve about 1 g of each of the following substances in 5-10 cm 3 of water in a test tube: CaCl 2 , NaOH, KNO 3 and BaCl 2 .
  2. Observe whether the reaction is endothermic or exothermic, either by feeling whether the side of the test tube gets hot or cold, or using a thermometer.
  3. Dilute 3 cm 3 of concentrated H 2 SO 4 in 10 cm 3 of water in the fifth test tube and observe whether the temperature changes.
  4. Wait a few minutes and then carefully add NaOH to the H 2 SO 4 . Observe any energy changes.
  5. Record which of the above reactions are endothermic and which are exothermic.

Results:

  • When BaCl 2 and KNO 3 dissolve in water, they take in heat from the surroundings. The dissolution of these salts is endothermic .
  • When CaCl 2 and NaOH dissolve in water, heat is released. The process is exothermic .
  • The reaction of H 2 SO 4 and NaOH is also exothermic .

Examples of endothermic and exothermic reactions

There are many examples of endothermic and exothermic reactions that occur around us all the time. The following are just a few examples.

  1. Endothermic reactions
    • Photosynthesis Photosynthesis is the chemical reaction that takes place in plants, which uses energy from the sun to change carbon dioxide and water into food that the plant needs to survive, and which other organisms (such as humans and other animals) can eat so that they too can survive. The equation for this reaction is: 6 CO 2 + 12 H 2 O + energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Photosynthesis is an endothermic reaction because it will not happen without an external source of energy, which in this case is sunlight.
    • The thermal decomposition of limestone In industry, the breakdown of limestone into quicklime and carbon dioxide is very important. Quicklime can be used to make steel from iron and also to neutralise soils that are too acid. However, the limestone must be heated in a kiln at a temperature of over 900 0 C before the decomposition reaction will take place. The equation for the reaction is shown below: CaCO 3 CaO + CO 2
  2. Exothermic reactions
    • Combustion reactions - The burning of fuel is an example of a combustion reaction, and we as humans rely heavily on this process for our energy requirements. The following equations describe the combustion of a hydrocarbon such as methane (CH 4 ): Fuel + Oxygen Heat + Water + Carbon Dioxide CH 4 + 2 O 2 Heat + 2 H 2 O + CO 2 This is why we burn fuels for energy, because the chemical changes that take place during the reaction release huge amounts of energy, which we then use for things like power and electricity. You should also note that carbon dioxide is produced during this reaction. Later we will discuss some of the negative impacts of C O 2 on the environment. The chemical reaction that takes place when fuels burn therefore has both positive and negative consequences.
    • Respiration Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + energy In the reaction above, glucose (a type of carbohydrate in the food we eat) reacts with oxygen from the air that we breathe in, to form carbon dioxide (which we breathe out), water and energy. The energy that is produced allows the cell to carry out its functions efficiently. Can you see now why you are always told that you must eat food to get energy? It is not the food itself that provides you with energy, but the exothermic reaction that takes place when compounds within the food react with the oxygen you have breathed in!

Questions & Answers

What does Redox reactins mean?
kholeka Reply
Reaction in which there is transfer of electrons between elements
sibahle
how to calculate a vector
sakhumzi Reply
state Faraday's law in word
Emkay Reply
which rules do you have to follow to identify oxidation numbers
sessy Reply
Why do vectors have arrows?
vanessa Reply
vector quantity has both direction and magnitude so arrows illustrate the direction of the vector
Mhleli
how to calculate net electric field at a point
Bazel Reply
what are controlled variables
Link Reply
Independent variables in an experiment which influence the outcome results
Boss
Such as temperature when dealing with ohms law
Boss
variables whose amount is determined by the person doing the experiment
Clemencia
how to calculate exodation number
Cphiwe Reply
Calculate relative atomic mass
Lindo Reply
what is chemical bonding?
Lefa Reply
Chemical Bonding is a mutual attraction between two atoms resulting from the simultaneous attraction between their nuclei and the outer elections
Sydney
How would you go about finding the resistance of an unknown resistor using only power supply, a voltmeter and a known resistance
Lebona
who is the scientist who discovered electromagnetism
Zivele Reply
what happens to the galvanometer when the switch is closed ?
Zivele
how do we identify an oxidising or reducing agent in a reaction?
Zivele
hey
Banele
@zivele in your products the element with an oxidation number less than 0 is a reducing and the one with an oxidation number greater than 0 is oxidising
sessy
what is electricity
Vihanga Reply
Formula for concentration
Kabelo Reply
c=m/MV
Zipho
if given number of moles and volume , can use c=n/V
sibahle
Chemistry term three topic is stressing me out, I really need a huge help
Ziyanda
on what
Olebogeng
during a snooker competition ,a 200g ball A m moving with velocity va collide head on with a identical ball B that was at rest.A after the collision ball A remains at rest wile ball B moves on with a velocity of 4m/s? With what speed was ball a moving before the collision
mathew Reply
a vector can be resolved into a horizontal component only
Lizoh 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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