1. Describe an activity to demonstrate that the reaction between zinc and dilute hydrochloric acid is an exothermic reaction.

Activity 5

• Take 10 mL of hydrochloric acid in a conical flask. 
• Dip the bulb of a thermometer in the acid and clamp it in a vertical position (as shown in Fig.). 
• Observe the temperature in the thermometer. 
• Introduce a few pieces of zinc metal into the acid and again observe the temperature. 
  
   

Observations

• When zinc metal is added to the acid, small bubbles of hydrogen gas start coming out. After some time the speed of the bubbles increases.
• The temperature also increases.

Conclusion

• In the reaction of zinc with hydrochloric acid, hydrogen gas is formed as expressed by the following equation: 
   

The reaction is accompanied by the evolution of heat. Such reactions are called as exothermic reactions.

2. Describe an activity to demonstrate that the reaction between barium hydroxide  octahydrate and ammonium chloride  is an endothermic reaction. 

Activity 6

• Take about 2 g of solid barium hydroxide octahydrate (Ba(OH)₂.8H₂O) in a clean and dry boiling tube.
• Insert the bulb of a thermometer in the solid.
• Clamp the boiling tube and thermometer in a vertical position (as shown in Fig.).
• Observe the temperature and in the thermometer.
• Introduce about 2 g of solid ammonium chloride NH₄CI into the boiling tube and mix the solids properly with a clean and dry glass rod.
• Observe the temperature again in the thermometer. 

Observations

• When the two solids are mixed a reaction starts and the cooling is observed so that temperature falls.
• As the reaction progresses, barium chloride and water are formed and ammonia gas comes out.
• Barium chloride dissolves in water and its aqueous solution is formed.
• Because of the cooling produced in the reaction the moisture of the air in contact with the outer surface of the boiling tube is deposited as water droplets.

Conclusions

The reaction is accompanied by the absorption of heat. Such reactions are called as endothermic reactions. The reaction between barium hydroxide and ammonium chloride is Endothermic.

The reaction is described by the equation. 

Note: If barium hydroxide is not available, it can be prepared by mixing solutions of barium chloride and sodium hydroxide. In place of ammonium chloride, the reaction can be performed with ammonium thiocyanate (NH₄SCN) or ammonium nitrate (NH₄NO₃).

3. What are exothermic and endothermic chemical reactions?

When a fuel (such as butane, LPG) burns, heat energy is produced. It is also an observation that when ice melts, cooling is observed. That is, in chemical reactions and physical changes usually heat is either given out or absorbed. Accordingly changes are classified as exothermic or endothermic.

EXOTHERMIC REACTIONS: A change is called exothermic when heat is given out during that change. The term exothermic is taken form Greek word which means heat goes out (exo = out, therm = heat). In an exothermic process the heat is given to the surroundings so that the temperature increases.

Examples:

1. When hydrogen combines with oxygen water is formed and heat is produced. So it is an example of exothermic process.

2. Burning of coke in air. During burning carbon combines with oxygen to form carbon dioxide and a large quantity of heat is liberated.

3. When methane (CH₄) is burnt, it reacts with oxygen of air to form carbon dioxide and water. During this combustion (burning) heat is produced. The reaction is represented as:

All combustion reactions are exothermic.

4. In our metabolic process, glucose undergoes combustion to form CO₂ and H₂O and a large quantity of energy is provided to our body. The reaction is exothermic.

5. When nitrogen and hydrogen combine ammonia is formed and heat is liberated. Thus, formation of ammonia is an example of exothermic reaction.

6. Neutralization reaction between hydrochloric acid and sodium hydroxide is exothermic by 57.5 kJ.

7. Reduction of ferric oxide into iron by aluminium in the thermite process is exothermic.

This reaction forms the basis of welding iron and rails.

ENDOTHERMIC REACTIONS: A change is called endothermic when heat is absorbed by the system (Endo = in, therm = heat). In an endothermic process heat is taken from the environment so that a decrease in temperature of the environment is observed.

Examples:

1. When nitrogen and oxygen combine to form nitric oxide (NO), heat is absorbed. The reaction is represented as:

Here heat appears on the reactant side, because it is consumed along with the reactants . 

2. The reaction between hot coke and steam is endothermic by 128 kJ. Thus.

3. Thionyl chloride (SOCl₂) is a useful drying agent. Its reaction with water is endothermic by about 50 kJ.

4. The reaction between solid barium hydroxide and solid ammonium chloride is highly endothermic.

4. Why and when a chemical reaction is exothermic or endothermic?

In a chemical reaction the chemical bonds in reactants are broken. The bonds are broken if energy is supplied. On the other hand, when new type of bonds are formed in the product, energy is released. 
Exothermic reaction: When the energy released due to bond formation in the products is greater than the energy supplied to break the bonds in reactants, the reaction is exothermic.
Endothermic reaction:  When the energy required to break the bonds in all reactants is greater than the energy released in the bond formation of the products, the additional energy is taken from the surroundings. Hence, the reaction is endothermic.

5. What is respiration and why is it considered as an exothermic reaction?

We need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. Food mainly contains carbohydrates, which are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration.
In the process of respiration oxygen is inhaled which oxidizes glucose and produces carbon dioxide, water and energy in the form of ATP (adenosine triphosphate) in the living systems. It is thus an exothermic reaction.

6. What are the various types of reactions?

To simplify the study of the reactions, they are classified as
1. Combination Reactions
2. Displacement Reactions
3. Decomposition Reactions
4. Oxidation reduction (Redox) Reactions

7. Describe an activity which shows a combination reaction.

Activity 7

• Take a small amount of calcium oxide or quick lime in a beaker and put a thermometer with the help of a stand. (Do not touch quick lime with your hand, it causes burns.)
• Slowly add water to this.
• Observe the temperature and touch the beaker.

Observations:

• The calcium oxide vigorously dissolves in water by making a hissing sound.
• Steam comes out. The temperature rises.

Conclusions

The calcium oxide reacts vigorously with water and forms calcium hydroxide (slaked lime).

Calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.
The reaction is accompanied by the evolution of heat. Such reactions are called as exothermic reactions.

8. What are the combination reactions? Give examples.

Reactions in which two or more substances combine to form a single substance are called combination reactions. 
Types of Combination Reactions
In combination reactions there are three possibilities about the substances combining. Thus, there are three types of combination reactions.

(A) Both are elements.

(B) Both are compounds.

(C) One is an element while the other is a compound. 

Examples:

• Hydrogen combines with chlorine to give hydrogen chloride in the presence of light.
  
• Magnesium and oxygen combine when heated to form magnesium oxide.
  
• Carbon burns in air to form carbon dioxide.
  
• Sodium combines with chlorine to form sodium chloride.
  
• Hydrogen burns in oxygen to form water
  
• Nitric oxide combines with oxygen of the air to form nitrogen dioxide, a brown gas.
  
• Sulphur dioxide combines with oxygen to produce sulphur trioxide.
  
• Ammonia and hydrogen chloride are colourless gases. They combine to form a solid, ammonium chloride.
  
• Calcium oxide reacts with sulphur trioxide to form calcium sulphate.
   

9. What is the role of quick lime and slaked Lime in white washing of the walls and roofs?

Quick lime is used for the purpose of white washing. When quick lime is dissolved in water, slaked lime is formed. The slaked lime is applied on the walls.


Calcium hydroxide of slaked lime reacts with the carbon dioxide of the atmospheric air. As a result of this reaction a thin layer of calcium carbonate is formed on the surface of the wall which gives a shining look to the wall.

Do you know that the chemical formula of marble is also CaCO₃

10. Describe an activity to show that heating of ferrous sulphate is a decomposition reaction. 

Activity 8

• Take about 2 g of solid ferrous sulphate in a clean and dry test tube and note the colour of the sample.
• Heat the sample of ferrous sulphate in the test tube over the flame of a burner.
• Observe.  

Observations

• The sample of ferrous sulphate is light green.
• As ferrous sulphate is heated a colourless gas is formed. The gas has irritating smell like burning sulphur
• The green sample has changed into a brown solid.
• Conclusions and Explanations
• When ferrous sulphate is heated it decomposes into ferric oxide and oxides of sulphur, i.e, sulphur dioxide and sulphur trioxide.
  
• This type of reaction in which one substance breaks down to produce two or more substances is called decomposition reaction.