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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.


  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.


  • 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

How do hydrogen and chlorine atoms bond covalently in a molecule of hydrogen chloride?
Tiny Reply
what is the parallel circuit
Philani Reply
how many moles of H2O can be formed if 12,5 moi CH4 reacts with sufficient NH3 and O2
Nomcebo Reply
what atoms form covalent bond
hehe Reply
The combination of two non metals
combined two non-metals 😊
what is a mole
Thandeka Reply
a mole is a measure of large quantities of small entities such as atoms, molecules etc
what does STP stand for?
STP stands for Standard Temperature and Pressure.
A car drives straight off the edge of a cliff that is 54m high. The police at the scene of the accident observed that the point of the impact is 130m from the base of the clif. Calculate the initial velocity of the car when it went ovet the clif.
hanyani Reply
wat happens to current if resistors are in parallel connection
Mosima Reply
more current flows from the source that would flow for any of them individually,so the total total resistance is lower
In the parallel circuit the current is divided among the resistors
what are the isotopes
Tlotlisang Reply
are different types of the same elemant but with different mass or atomic no.
i sotopes are different type of element with same atomic number but different mass number
yes i forgot some details😁
next quetion plz
why don't we insert the negative sign for 5 × 10 - 9 when substituting
Mpho Reply
Why does an enclosed gas exert pressure on the walls of a container
Palesa Reply
State the gay lussacs law
Anna Reply
what is the coefficient of Na in order to balanced the equation?_Na + MgCl2=2NaCl+Mg?
Arcel Reply
the combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.
Dealon Reply
download periodic table from play Store...it will explain everything to u
what are orbitals
Sphe Reply
examples of atoms whose Valence energy levels are not full and more to bond and become more stable
What is a valency
What a lone pairs...
Valency is the number of electrons than an atom must gain, lose or share to achieve noble gas configuration.
what is vacuum
Njabulo Reply
Vacuum, space in which there is no matter or in which the pressure is so low that any particles in the space do not affect any processes being carried on there. It is a condition well below normal atmospheric pressure and is measured in units of pressure (the pascal).

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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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