Chapter 1 · Question 13

Differentiate between exothermic and endothermic reactions. Why is respiration considered an exothermic process? Give one example of each type of reaction.

Q13

Differentiate between exothermic and endothermic reactions. Why is respiration considered an exothermic process? Give one example of each type of reaction.

Answer Revealed

Direct Answer:

Exothermic reactions release heat energy to the surroundings, making the reaction mixture warm. Endothermic reactions absorb heat energy from the surroundings, making the reaction mixture cool. Respiration is an exothermic process because glucose (

\text{C}_6\text{H}_{12}\text{O}_6

) combines with oxygen in our body cells to release energy, which is used for life processes:

\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} + 6\text{O}_2\text{(aq)} \rightarrow 6\text{CO}_2\text{(aq)} + 6\text{H}_2\text{O(l)} + \text{energy}

. Example of exothermic reaction: burning of natural gas (

\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{Heat}

). Example of endothermic reaction: thermal decomposition of calcium carbonate (

\text{CaCO}_3 \xrightarrow{\text{Heat}} \text{CaO} + \text{CO}_2

Simple Explanation

Exothermic means 'heat goes out' — the reaction releases heat and the container gets warm, like when quicklime is mixed with water. Endothermic means 'heat comes in' — the reaction absorbs heat and the container gets cold, like when ammonium chloride is mixed with barium hydroxide. Respiration is exothermic because it breaks down glucose to release the energy our body needs to function. Every time you breathe, you are running an exothermic reaction that powers your cells.

Exam-Ready Structure

Exothermic and endothermic reactions are classified based on the direction of heat flow during the reaction: 1. Exothermic reactions: Reactions in which heat energy is released along with the formation of products. Characteristics: (a) The reaction mixture becomes warm/hot to touch. (b) Heat is shown on the product side of the equation. (c) Examples from the chapter: - Reaction of quicklime with water:

\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat}

. - Burning of natural gas:

\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(g)}

(Equation 1.17). - Burning of magnesium ribbon, respiration, decomposition of vegetable matter into compost. 2. Endothermic reactions: Reactions in which energy (heat, light, electricity) is absorbed. Characteristics: (a) The reaction mixture becomes cooler. (b) Energy is shown above the arrow as a condition. (c) Examples: - All decomposition reactions require energy and are endothermic. -

\text{CaCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}

(thermal decomposition). - The unnumbered activity: mixing barium hydroxide

\text{Ba(OH)}_2

with ammonium chloride

\text{NH}_4\text{Cl}

— the test tube becomes cold. 3. Respiration as an exothermic process: (a) During digestion, complex carbohydrates (from food like rice, potatoes, bread) are broken down into simple glucose. (b) This glucose is transported to cells where it combines with inhaled oxygen. (c) The reaction

\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}

releases energy that is used for all life processes — movement, growth, repair, and maintenance of body temperature. (d) Since energy is released (given out), respiration is classified as an exothermic reaction. 4. Key distinction: Energy (heat) is a product in exothermic reactions but a requirement (reactant/condition) in endothermic reactions.

Key Points

Exothermic reactions: release heat → products include '+ Heat'; mixture becomes warm
Endothermic reactions: absorb heat/energy → energy written above the arrow; mixture becomes cold
Respiration is exothermic: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$
Exothermic examples: burning of natural gas, CaO + H₂O, burning of magnesium
Endothermic examples: decomposition of CaCO₃, Ba(OH)₂ + NH₄Cl (test tube turns cold)

Common Mistakes

Writing respiration as an endothermic process — respiration RELEASES energy; it is exothermic
Confusing the direction of heat flow — in exothermic, heat goes out (EXO = exit); in endothermic, heat goes in (ENDO = enter)

Differentiate between exothermic and endothermic reactions. Why is respiration considered an exothermic process? Give one example of each type of reaction.