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Q9:
What does one mean by exothermic and endothermic reactions? Give examples.
Solution :
1. Exothermic Reactions: Definition and Analysis
An exothermic reaction is a chemical process that releases energy into the surrounding environment, primarily in the form of heat, light, or sound. In these reactions, the enthalpy of the reactants ($H_{reactants}$) is higher than the enthalpy of the products ($H_{products}$), resulting in a negative enthalpy change ($\Delta H < 0$).
General Formula:
$Reactants \rightarrow Products + Heat$
Example: The Combustion of Methane
The burning of natural gas (methane) in the presence of oxygen is a classic exothermic reaction used in domestic cooking appliances.
$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{Energy}$
Justification: [Per the Law of Conservation of Energy, the energy stored in the chemical bonds of the reactants is higher than that of the products; the excess energy is dissipated into the surroundings.]
2. Endothermic Reactions: Definition and Analysis
An endothermic reaction is a chemical process that absorbs energy from the surrounding environment to proceed. In these reactions, the enthalpy of the reactants is lower than the enthalpy of the products, resulting in a positive enthalpy change ($\Delta H > 0$).
General Formula:
$Reactants + Heat \rightarrow Products$
Example: Thermal Decomposition of Calcium Carbonate
Limestone decomposes into quicklime and carbon dioxide only when significant heat energy is supplied.
$CaCO_3(s) + \text{Heat} \xrightarrow{\Delta} CaO(s) + CO_2(g)$
Justification: [Per the First Law of Thermodynamics, the system absorbs thermal energy from the surroundings to break the strong ionic bonds within the reactant crystal lattice.]
3. Comparative Summary Table
| Feature | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Energy Flow | Released to surroundings | Absorbed from surroundings |
| Temperature Change | Surroundings become warmer | Surroundings become cooler |
| Enthalpy Change ($\Delta H$) | Negative ($-$) | Positive ($+$) |
4. Energy Profile Diagram
Final Solution: Exothermic reactions are processes that release energy (e.g., combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}$), while endothermic reactions are processes that require the absorption of energy (e.g., thermal decomposition: $CaCO_3 + \text{Heat} \rightarrow CaO + CO_2$).
More Questions from Class 10 Science Chemical Reactions and Equations EXERCISES
- Q1: Which of the statements about the reaction below are incorrect? $2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g)$ (a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.
- Q2: The reaction is an example of a $Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe$
- Q3: What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
- Q4: What is a balanced chemical equation? Why should chemical equations be balanced?
- Q5(a): Translate the following statements into chemical equations and then balance them. (a) Hydrogen gas combines with nitrogen to form ammonia.
- Q5(b): Translate the following statements into chemical equations and then balance them. (b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
- Q5(c): Translate the following statements into chemical equations and then balance them. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
- Q5(d): Translate the following statements into chemical equations and then balance them. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
- Q6(a): Balance the following chemical equations. (a) $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$
- Q6(b): Balance the following chemical equations. (b) $NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O$
- Q6(c): Balance the following chemical equations. (c) $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$
- Q6(d): Balance the following chemical equations. (d) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl$
- Q7(a): Write the balanced chemical equations for the following reactions. (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
- Q7(b): Write the balanced chemical equations for the following reactions. (b) Zinc + Silver nitrate → Zinc nitrate + Silver
- Q7(c): Write the balanced chemical equations for the following reactions. (c) Aluminium + Copper chloride → Aluminium chloride + Copper
- Q7(d): Write the balanced chemical equations for the following reactions. (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
- Q8(a): Write the balanced chemical equation for the following and identify the type of reaction in each case. (a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
- Q8(b): Write the balanced chemical equation for the following and identify the type of reaction in each case. (b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
- Q8(c): Write the balanced chemical equation for the following and identify the type of reaction in each case. (c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
- Q8(d): Write the balanced chemical equation for the following and identify the type of reaction in each case. (d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
- Q10: Why is respiration considered an exothermic reaction? Explain.
- Q11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
- Q12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
- Q13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.
- Q14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
- Q15: What do you mean by a precipitation reaction? Explain by giving examples.
- Q16(a): Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation
- Q16(b): Explain the following in terms of gain or loss of oxygen with two examples each. (b) Reduction
- Q17: A shiny brown coloured element ‘$X$’ on heating in air becomes black in colour. Name the element ‘$X$’ and the black coloured compound formed.
- Q18: Why do we apply paint on iron articles?
- Q19: Oil and fat containing food items are flushed with nitrogen. Why?
- Q20(a): Explain the following terms with one example each. (a) Corrosion
- Q20(b): Explain the following terms with one example each. (b) Rancidity
CBSE Solutions for Class 10 Science Chemical Reactions and Equations
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