Energetics MCQs PDF

1. What is the de nition of enthalpy change (ΔH)? a) The energy required to convert one mole of substance into ions b) The heat energy change measured under conditions of constant pressure c) The thermal energy absorbed during melting d) The energy change during chemi...

1. What is the de nition of enthalpy change (ΔH)? a) The energy required to convert one mole of substance into ions b) The heat energy change measured under conditions of constant pressure c) The thermal energy absorbed during melting d) The energy change during chemical equilibrium formation Answer: b) The heat energy change measured under conditions of constant pressure Explanation: ΔH refers to the change in heat energy under constant pressure. 2. What is the molar enthalpy change of combustion? a) Heat change when 1 mole of a compound is formed from its elements b) Heat change when 1 mole of a substance is burned completely in oxygen c) Heat change when 1 mole of solute is dissolved in water d) Energy absorbed per mole of reaction Answer: b) Heat change when 1 mole of a substance is burned completely in oxygen Explanation: The enthalpy of combustion is the heat released when one mole of substance undergoes full combustion in oxygen. 3. A reaction where the system absorbs energy from the surroundings is called: a) Exothermic b) Endothermic c) Combustion d) Neutralisation Answer: b) Endothermic Explanation: In endothermic reactions, energy is absorbed from the surroundings, resulting in a positive ΔH. 4. Which type of reaction releases energy to the surroundings, making the surroundings hotter? a) Endothermic b) Exothermic c) Catalytic d) Photochemical Answer: b) Exothermic Explanation: Exothermic reactions release heat to the surroundings, leading to an increase in temperature. 5. De ne standard enthalpy of formation (ΔHf°). a) Heat change when 1 mole of a substance is burned in oxygen b) Heat change when 1 mole of a compound is formed from its elements in their standard states c) Energy released during complete ionisation d) Heat energy change observed during vaporisation Answer: b) Heat change when 1 mole of a compound is formed from its elements in their standard states Explanation: ΔHf° is the enthalpy change for the formation of 1 mole of compound from elements in their standard states. 6. What does Hess' Law state? a) Energy cannot be created or destroyed b) The enthalpy change of a reaction is independent of the pathway taken c) The total entropy of an isolated system can never decrease d) Activation energy always stays constant Answer: b) The enthalpy change of a reaction is independent of the pathway taken fi fi  Explanation: Hess' Law indicates that the enthalpy change is the same regardless of the steps taken between reactants and products. 7. Calculate the enthalpy change for the reaction: \( C(s) + O_2(g) \to CO_2(g) \) using the provided enthalpy changes: \( \Delta H_{f}^\circ (CO_2) = -394 \) kJ/mol. a) -484 kJ/mol b) +394 kJ/mol c) -394 kJ/mol d) +484 kJ/mol Answer: c) -394 kJ/mol Explanation: This reaction is directly the formation of \( CO_2 \) from its elements, given as -394 kJ/mol in standard state enthalpy form. 8. Which equation is used to calculate the heat change (q) in calorimetry? a) \( q = \Delta H \cdot n \) b) \( q = mc\Delta T \) c) \( q = \Delta H_f \) d) \( q = P \Delta V \) Answer: b) \( q = mc\Delta T \) Explanation: The heat change (q) can be calculated using the mass of the substance (m), speci c heat capacity (c), and the change in temperature (ΔT)】. 9. What is the activation energy of a reaction? a) The minimum energy required for a reaction to occur b) The total energy change of the reaction c) The energy required to complete combustion d) The energy released during ionisation Answer: a) The minimum energy required for a reaction to occur Explanation: Activation energy is the minimum energy that reactant molecules need to succeed in a reaction. 10. When determining activation energy, what does a large activation energy indicate? a) The reaction occurs spontaneously b) The reaction has a high activation barrier c) The reaction releases a small amount of energy d) The reaction occurs quickly Answer: b) The reaction has a high activation barrier Explanation: A large activation energy indicates that a signi cant amount of energy is required to initiate the reaction, leading to a high activation barrier. 11. How can Hess' law be applied using formation values? a) By using heat capacity b) By accounting for total product mass c) By inserting each reactant and product formation enthalpies in a cycle d) By balancing atomic numbers Answer: c) By inserting each reactant and product formation enthalpies in a cycle Explanation: Hess' Law is applied using formation enthalpies by constructing a cycle with known formation values and calculating routes to nd unknowns. 12. Given: \( C(s) + 2H_2(g) \rightarrow CH_4(g) \), calculate ΔH using bond enthalpies (C-H = 412 kJ/mol, H-H = 436 kJ/mol). a) -52 kJ/mol b) +52 kJ/mol c) +166 kJ/mol d) -128 kJ/mol fi fi fi  Answer: b) +52 kJ/mol Explanation: Bonds broken: \( 2H_2 \times 436 \) = 872 kJ, Bonds formed: \( 4CH \times 412 \) = 1648 kJ. ΔH = Bonds broken - Bonds formed = 872 - 1648 = +776 kJ. 13. If burning 5g of ethanol raises water temperature 3 degrees Celsius, calculate the heat energy change Q (c = 4.18 J/K g, m = 100g H₂O). a) 628 J b) 1254 J c) 2500 J d) 3762 J Answer: a) 628 J Explanation: Q = mcΔT = 100g 4.18 J/gK 3°C = 1254 J. 14. What is the standard enthalpy of combustion of methane (\( CH_4 \))? a) \( \Delta H_c^\circ (CH_4) = -882 \) kJ/mol b) \( \Delta H_c^\circ (CH_4) = +882 \) kJ/mol c) \( \Delta H_c^\circ (CH_4) = -394 \) kJ/mol d) \( \Delta H_c^\circ (CH_4) = +394 \) kJ/mol Answer: a) \( \Delta H_c^\circ (CH_4) = -882 \) kJ/mol Explanation: Complete combustion of 1 mole of CH4 in oxygen forming carbon dioxide and water releases -882 kJ/mol. 15. In exothermic reactions, what is meant by a “negative“ enthalpy change (ΔH)? a) The products acquire higher enthalpy b) The reaction requires continuous heat supply c) The reactants release energy to surroundings d) Entropy signi cantly increases Answer: c) The reactants release energy to surroundings Explanation: Exothermic reactions release energy, resulting in a negative ΔH value indicating decreased system energy. 16. What does Hess' law state about the total enthalpy change (ΔH) in a reaction? a) It varies with the reaction path taken b) It remains constant regardless of the pathway taken c) It increases with temperature d) It decreases with higher pressure Answer: b) It remains constant regardless of the pathway taken Explanation: Hess' law implies that the total enthalpy change in chemical reactions is path- independent. 17. How is the mean bond enthalpy derived? a) It averages speci c enthalpy changes for individual reactions b) It averages bond enthalpies of the same type from di erent molecules c) It speci es energy released in incomplete combustion d) By averaging pressure change over time Answer: b) It averages bond enthalpies of the same type from di erent molecules Explanation: Mean bond enthalpy values are calculated by averaging bond dissociation energies from a range of compounds. 18. Calculate ΔH for the reaction given bond energies: \( N_2(g) + 3H_2(g) \to 2NH_3(g) \), \( N≡N \) = 945 kJ, 2 × \( N-H \) = 432 kJ. a) -224 kJ/mol b) -92 kJ/mol c) +431 kJ/mol fi fi fi ff ff  d) +710 kJ/mol Answer: b) -92 kJ/mol Explanation: Estimating \( ΔH = Bonds broken - Bonds formed \): Broken: \( 945 + 3 × 2 \times 432 \), Formed: \( 6 × 432 = 2592\), ΔH = Broken-Formed: 2247 - 2592 = - 92 kJ/mol. 19. De ne standard enthalpy (ΔHf°) with standard components and signi cance. a) Energy by dissolving 1 mole of solute into in nite dilution b) Heat change of forming one mole of compound from elements in standard states c) Molar energy equilibrium attained in uorescence d) Balancing atomic no.s and symbol components in bond energy transitions Answer: b) Heat change of forming one mole of compound from elements in standard states Explanation: ΔHf° de nes enthalpy change forming one mole compound fully from constituent elements in their standard states. 20. Why is a reaction step with lower activation energy faster catalysed? a) Increased pressure and temperature b) Reduced energy barrier required to initiate catalysis c) Signi cantly altered reaction progress end-steps d) Di erential bond dissociation energy conversions Answer: b) Reduced energy barrier required to initiate catalysis Explanation: Enhancing reaction rate is achieved by lowering activation energy within catalytic in uence. 21. Determine heat energy for 3g reactant burning heating water with ΔT measured 10-degree Celsius (m= (water) 50g; c= 4.18 J/g·K)) a) 209 J b) 1007 J c) 2090 J d) 420 J Answer: b) 1007 J Explanation: Computing calorimetry via \( q=mc\Delta \(q\}), mass of water = temperature altitude ΔT:50g4.18J/g·K10°C= 1007 Joules. 22. Given substantial negative \ \(Delta \(f_H°C\)( \(f_H°C)_} warms reaction surroundings unlike: a) Enthalpy exothermic b) Mean bond energy altering c) Vector -b values adjustments d) Determining signi cant entropic distortion Answer: a) Enthalpy exothermic Explanation: Substantially neg. ∆(fH°) reactions release intense energy, warming surrounding, classifying as exothermic not endothermic combinations. 23. Is a catalyst altering activation energy rather pathway changing equilibrium? a) NO-Only equilibrates lower reactants threshold b) NO-Speci c alternative lowered pathway initiation c) NONE INDUCES, retaining equilibrium state net-zero less d) Dynamic both equalify shown channel adjustments Answer: b) NO-Speci c alternative lowered pathway initiation Explanation: Catalyst action mechanism in series reduction induces lower activation energy pathway structural impact initiating faster occurrence. 24. For enthalpy combustion calculations Hess-Law cycle value signi cant ensuring a) Standardisation b) Path independence fl fi ff fi fi fi fi fi fl fi fi fi  c) Thermal regularisation d) Selective parameters establishing Answer: b) Path independence Explanation: Hess-Law Reaction Enthalpy calculated independent path is signi cant in cycle value notation validates consistent standardisation. fi