Chemical reactions involve breaking and making of bonds between atoms to produce new substances.
A reaction in which a single product is formed from two or more reactants is called a combination reaction.
Example:
(i) Burning of coal C(s) + O2 (g) → CO2 (g)
(ii) Formation of water from H2 and O2. 2H2 (g) + O2 (g) → 2H2O (I)
Now, let's do an activity to understand the combination reaction a little more.
Aim: Perform an activity to illustrate a combination reaction.
Materials required: Beaker (500 mL ), water, calcium oxide ( quick lime).
Procedure:
(ii) Take about 50 g of quick lime in it.
(iii) Add about 100 ml water to it carefully.
(iv) Quick lime (calcium oxide) reacts vigorously with water to form a single product slaked lime Ca(OH)2 and a large amount of heat is evoked.
CaO (s) + H2O (l) → Ca(OH)2 (aq) + Heat
Some more examples of a combination reaction are: Burning of coal
C(s) + O2 (g) → CO2 (g)
Formation of water from H2 (g) and O2 (g)
2H2 (g) + O2 (g) → 2H2O (l)
Observations:
Result:
White Washing of Buildings
A solution of slaked lime Ca(OH)2 is applied on the walls and roofs. After 2 or 3 days, slaked lime reacts with atmospheric carbon dioxide and it changes into calcium carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Calcium carbonate gives a shiny white finish to the walls.
Reactions which take place with the evolution of heat (increase of temperature) are called exothermic reactions.
Evolution of heat in Exothermic reaction
Example: Burning of natural gas and respiration are exothermic reactions.
Reactions which take place with absorption of heat (lowering of temperature) are called endothermic reactions.
Absorption of heat in Endothermic reaction
Example:
When a single reactant breaks down to give simpler products, it is called a decomposition reaction. For example:
Let's do an activity to understand the decomposition reaction more.
Aim: Perform an activity to illustrate the decomposition reaction.
Materials required: Lead nitrate powder, boiling tube, test tube holder, burner.
Procedure:
(i) Take a clean Pyrex glass boiling tube.
(ii) Add to it about 2 g powdered lead nitrate.
(iii) Using a test tube holder or pair of tongs, heat the test tube on a Bunsen burner.
(iv) Note the change that takes place.
Observation: We observe that brown fumes are emitted from the test tube.
The following reaction takes place in this activity:
Result:
In the above reaction, lead nitrate breaks down into simpler products proving that this is the decomposition reaction.
Similarly, we can perform the activities of heating ferrous sulphate crystals and calcium carbonate to illustrate decomposition reactions.
Aim: Perform the electrolysis of water experiment to illustrate the decomposition reaction.
Materials required: A plastic mug, two graphite rods, two test tubes, a 6 V battery, a switch, two rubber stoppers, and sulphuric acid.
Electrolysis of Water
Procedure:
(i) Set up the apparatus as shown in Fig. above.
(ii) Fill the mug with water such that graphite rods are immersed. Add a few drops of sulphuric acid.
(iii) Connect the electrodes to a 6 V battery and switch on the current. Leave the apparatus undisturbed for some time.
(iv) Formation of bubbles at both electrodes will take place.
(v) The reaction that takes place is as follows:
2H2O (l) → 2H2 (g) + O2 (g)
Observation:
Result: As we have seen in the reaction that water is converted into very simple products like hydrogen and oxygen hence it is a decomposition reaction.
Aim: Perform an activity (decomposition of silver salt) to illustrate the decomposition reaction.
Materials required: China dish, silver chloride.
Procedure
(i) Take about 2 g silver chloride in a china dish (Fig. 1.6).
(ii) Place the china dish in sunlight for some time.
(iii) You will observe that silver chloride becomes grey after some time.
Observation:
Silver chloride becomes grey after some time because silver chloride undergoes decomposition by the action of sunlight (sunlight is also a form of energy).
Result:
As we have seen in the above reaction Silver chloride breaks down into simpler products like Silver and chlorine gas, hence proving that this is a decomposition reaction.
A reaction in which one substance displaces another from the aqueous solution is called displacement reaction.
A general reaction of this type may be represented as:
Thus C has displaced B in the above reaction.
Aim: Perform an activity to illustrate displacement reaction.
Materials required: Test tube, thread, stand with clamp, copper sulphate solution, iron nails.
Procedure:
(i) Take three iron nails and clean them by rubbing them with sandpaper.
(ii) Take two test tubes. Mark them A and B. In each test tube take about 10 mL of copper sulphate solution.
(iii) Tie two nails with a thread and immerse them in the copper sulphate solution in test tube B for about 20 minutes Fig. 1.7(a).
Keep one nail aside for comparison.
(iv) Take out the iron nails from the copper sulphate solution in test tube B.
(v) Compare the colour of the copper sulphate solution in the two test tubes.
Observation:
Iron nails and copper sulphate solutions compared before and after the experiment
Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
Aim: Perform an activity to illustrate a double displacement reaction.
Materials required: Two test tubes, sodium sulphate solution, and barium chloride solution.
Procedure:
(i) Take two 20 mL clean test tubes.
(ii) Take 5 mL barium chloride solution in one test tube and 5 mL sodium sulphate solution in the other test tube.
(iii) Add sodium sulphate solution to the test tube containing barium chloride solution as shown in Fig. 1.8.
Observation:
Reactions in which a substance gains oxygen or loses hydrogen is called an oxidation reaction.
Similarly, a reaction in which a substance loses oxygen or gains hydrogen is called a reduction reaction.
Aim: Perform an activity to illustrate oxidation.
Materials required: China dish, copper powder, wire gauze, tripod stand, burner.
Procedure:
(i) Take about 2 g copper powder in a clean china dish (Fig.1.9)
(ii) Place it on a wire gauze supported on a tripod stand.
(iii) Heat it for about 15 minutes.
Observation:
When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is known as corrosion.
(ii) Rancidity
When fats and oils are oxidised (or kept in open for some days), their smell and taste changes. We say that they have gone rancid. This phenomenon is called rancidity.
Rancidity
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