Energy changes in Chemical Reactions

Lesson Plan of Energy changes in Chemical Reactions

General Science Grade VIII

Students’ Learning Outcomes

·         Explain the energy changes in chemical reactions.
·         Differentiate between exothermic and endothermic reactions.
·         Describe the importance of exothermic reactions in daily life.

Information for Teacher

Concept Map

·         A reaction in which chemical changes occurs is called a chemical reaction.
·         A Chemical reaction, a method in which one or more substances, the reactants, are transformed to one or more different substances, the products.
Energy changes in Chemical Reactions
·         A chemical reaction occurs when energy is released or absorbed. E.g. the reaction between an acid and base to form salt and water is a chemical reaction.
·         A chemical reaction is also irreversible, because we can’t get the original substance back. E.g: if we bake a cake with milk, flour and sugar we can’t get each substance back therefore it is irreversible and a chemical reaction.
·         A chemical reaction or change that releases heat or energy is called exothermic reaction e.g. combustion reactions of fuels, neutralization and burning.
·         Endothermic reaction takes place when heat is absorbed by the reactions to form products e.g. melting ice cubes and solid salts.
·         Exothermic reactions occur in many phenomena and applications of every-day life
·         The most common application is the use of Cold Packs in our daily lives.
·         Energy changes are either occurring naturally (combustion/burning, photosynthesis, respiration, boiling, freezing, etc.) or being employed by mankind to make our lives better through its numerous application.
·         Visible evidence of a chemical reaction:
1)      Change in color
2)      Formation of gas
3)      Formation of a solid
4)      Temperature change

Material / Resources

Candle, match box, water, beakers glasses, jug, sodium hydroxide (few pellets), stirrer, unsliced lime, urea

Worm up activity

·         Write the word “chemical reaction” on board and ask children to give their ideas about it.
·        Brainstorming:
o   After making the above spider map, direct students’ discussion towards the relation between energy and temperature of a chemical reaction by asking following questions and have a class discussion on these:
1)      When you heat water, are you giving heat to water or getting heat from water? If yes, why?
2)      Burning of a paper gives us heat or not? Give reasons 1-2.
o   After discussion: introduce exothermic and endothermic changes, by telling them their definitions, and elaborating through example.
o   Then ask students:
1)      Are these a type of chemical changes?
2)      Listen to their responses and explain that yes, these are the chemical changes in which new products are formed by losing or gaining heat. Write the following on board:
Endo = out (loose)
Thermic = energy (heat)
o   Ask a volunteer to define an endothermic change with the help of the words. “Endothermic change or reaction is that in which heat is absorbed by a chemical reaction”
    Endo = in (gain)
Thermic –energy (heat)
o   Ask a volunteer to define an endothermic change with the help of the words. “Endothermic change or reaction is that in which heat is absorbed by a chemical reaction.


Activity 1

·         Divide students in three groups:
          Group 1: give material to the 1st group (a candle for each group, a match box) instruct them to:
1)      Light a candle
2)      Observe what happens with the candle.
3)      Feel the heat which is given out by the burning candle.
Conclude the activity by asking:
1)      Can we return the products of burnt out wax into the original candle?
2)      What type of reaction is this? (Exothermic or endothermic)
·         After getting student responses then tell them that “ yes” “it is an exothermic reaction”

Activity 2

·         To the second group give material (Na OH) water, beaker/glass, stirrer) and instruct them to:
o   Half fill a beaker/glass with water.
o   Add 1 table spoon of sodium hydroxide pellets in the water and mix it with stirrer.
o   Touch the sides of the beaker.
o   What do you feel and why?

Activity 3

·         To the third group, give material (NH4 C l H2 O, stirrer, beaker) and instruct them to:
o   Half fill a beaker/glass with water.
o   Add half table spoon ammonium chloride in the water and mix it with a stirrer.
o   Touch the sides of the beaker and  conclude by asking:
1)      What is your observation?
2)      Is it an exothermic reaction or endothermic?
Ammonium chloride solution

Sum up / Conclusion

·         Conclude the lesson by telling students that they learnt:
o   Changes which can’t be reversed by physical methods are called chemical changes or chemical reactions.
o   Chemical reactions involve/use production of energy, as we have observed the production of heat energy in burning of candle and consumption of heat energy on dissolving ammonium chloride in water.
o   Burning of fuel, rusting of iron, digestion of food etc. are the examples of chemical reactions.


·         Assess students learning by asking these questions.
             Q: take a few drops of spirit on your palm and blow it. What do you feel and why?
                  (Expected response: we feel cooling effect as spirit evaporates- Evaporation is an   
                    Endothermic process)
               Q: is respiration an exothermic or endothermic change?
                  (Respiration is exothermic as energy is released during the process)

Follow up

·         Ask students to write answer of the following question:
1)      Sometimes burning results into damage, e.g. a house on fire. How can you control such situation?
2)      Dissolve 5g-15g urea in a glass/beaker. Half filled with water. Observe the sides of the glass. Why is it so?


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