2. Give short answer.
(i) Define exothermic and endothermic reactions.
Answer:
Exothermic Reactions: Chemical reactions that release heat energy to the surroundings. The enthalpy change (ΔH) is negative. The products have lower energy than the reactants. Example: Combustion of methane, neutralization of acid
and base. Endothermic Reactions: Chemical reactions that absorb heat energy from the surroundings. The enthalpy change (ΔH) is positive. The products have higher energy than the reactants. Example: Melting of ice, photosynthesis, decomposition of calcium carbonate.
(ii) Define enthalpy of a chemical reaction.
Answer: The enthalpy of a chemical reaction (ΔH) is the heat change that occurs during a chemical reaction at constant pressure. It
is the difference between the total enthalpy of the products and the total enthalpy of the reactants: ΔH = Hproducts - Hreactants. A negative ΔH indicates an exothermic reaction, while a positive ΔH indicates an endothermic reaction. It is usually expressed in kJ/ mol.
(iii) What is anaerobic respiration?
Answer: Anaerobic respiration is the process of producing cellular energy without oxygen. It occurs in the cytoplasm of cells when oxygen is limited or absent. Glucose is partially broken down into lactic acid (in muscle cells) or ethanol and carbon dioxide (in yeast). It produces a small amount of ATP (2 ATP per glucose) compared to aerobic respiration (36-38 ATP). It is less efficient but allows for short bursts of energy.
(iv) Define activation energy.
Answer: Activation energy (Ea) is the minimum amount of energy that colliding reactant molecules must possess in order to undergo a chemical reaction and form products. It is the energy barrier that must be overcome for the reaction to proceed. A higher activation energy means fewer molecules have sufficient energy to react, resulting in a slower reaction rate.
(v) What is the role of a catalyst in a chemical reaction.
Answer: A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy. It is not consumed in the reaction and does not change the position of equilibrium or the enthalpy change of the reaction. It is required in small amounts and is highly specific.
(vi) Differentiate between aerobic and anaerobic respiration.
Answer:
| Feature | Aerobic Respiration | Anaerobic Respiration |
| Oxygen | Requires oxygen | Does not require oxygen |
| Location | Cytoplasm and mitochondria | Cytoplasm only |
| Products | CO₂, H₂O, ATP | Lactic acid (animals), ethanol + CO₂ (yeast) |
| ATP Yield | High (36-38 ATP) | Low (2 ATP) |
| Efficiency | Complete breakdown of glucose | Incomplete breakdown of glucose |
3. How can you determine the enthalpy of a chemical reaction?
Answer: The enthalpy of a chemical reaction can be determined experimentally using a calorimeter. The reaction is carried out in an insulated container (calorimeter) filled with a known mass of water. The heat released or absorbed by the reaction causes a temperature change in the water. By measuring the temperature change (ΔT), the mass of water (m), and knowing the specific heat
capacity of water (c = 4.18 Jg°C), the heat change (q) can be calculated using q = m × c × ΔT. The enthalpy change is then calculated
as ΔH = -q / moles of reactant.
4. Explain, how does the process of respiration provides us energy?
Answer: Respiration is the process by which cells break down glucose to release energy in the form of ATP (adenosine triphosphate). During aerobic respiration, glucose is oxidized in the presence of oxygen to produce carbon dioxide, water, and a large amount of ATP. The ATP is then used by cells to power various activities such as muscle contraction, active transport, and protein synthesis. The energy stored in the chemical bonds of glucose is transferred to ATP, which acts as the energy currency of the cell. Anaerobic respiration also produces ATP but in smaller amounts.
5. Draw labeled reaction pathway diagram for an exothermic and an endothermic reaction.
Answer: (Diagram Description)
Exothermic Reaction Diagram: Y-axis: Potential Energy. X-axis: Reaction Progress. Reactants start at a higher energy level. Products end at a lower energy level. The activation energy (Ea) is the difference between reactants and the transition state (peak). ΔH is negative (products lower than reactants). Endothermic Reaction Diagram: Y-axis: Potential Energy. X-axis: Reaction Progress. Reactants start at a lower energy level. Products end at a higher energy level. The activation energy (Ea) is the difference between reactants and the transition state. ΔH is positive (products higher than reactants).
6. Calculate the enthalpy of reaction between hydrogen and chlorine to form hydrogen chloride from the given bond energy data. Bond energy of H-H, Cl-Cl, H-Cl are 436kJ/mol, 243kJ/mol and 432kJ/mol respectively.
Answer:
Reaction: H₂ + Cl₂ [source symbol unavailable] 2HCl
Formula: ΔH = Σ(Bond energies of reactants) - Σ(Bond energies of products)
Reactants: H-H (436) + Cl-Cl (243) = 679 kJ/mol
Products: 2 × H-Cl = 2 × 432 = 864 kJ/mol
Calculation: ΔH = 679 - 864 = -185 kJ/mol The reaction is exothermic.
7. Justify the statement that the process of respiration is crucial for us.
Answer: Respiration is crucial for us because it provides the energy (ATP) required for all life processes. Without respiration, cells would not be able to perform essential functions such as muscle contraction, nerve impulse transmission, active transport, and synthesis of biomolecules. It is the process that converts the chemical energy stored in food into a usable form (ATP). Without a continuous supply of ATP, cells would die, and the organism would not survive.
