Questions about Hess's Law of Heat Summation
Multiple-Choice Questions: Hess's Law of Heat Summation
1. What does Hess's Law state?
A) Enthalpy changes depend on the pathway of a reaction
B) Enthalpy changes are independent of the reaction pathway
C) Enthalpy changes are always zero
D) Entropy changes depend on temperature
E) Heat is absorbed during all chemical reactions
2. Hess's Law is based on the principle that enthalpy is a:
A) State function
B) Path function
C) Dependent variable
D) Constant value
E) Catalyst
3. Which of the following can Hess’s Law help calculate?
A) Activation energy
B) Total enthalpy change for complex reactions
C) Rate of reaction
D) Equilibrium constant
E) Reaction mechanism
4. If a reaction can be expressed as the sum of two or more steps, the total enthalpy change is:
A) The product of the enthalpy changes
B) The sum of the enthalpy changes of each step
C) Always zero
D) The difference between enthalpy changes
E) Unrelated to individual steps
5. Hess’s Law is useful because it allows calculation of enthalpy change for:
A) Only exothermic reactions
B) Reactions that are difficult to measure directly
C) Only combustion reactions
D) Photochemical reactions
E) Only reactions at equilibrium
6. In applying Hess’s Law, what happens if a reaction equation is reversed?
A) The enthalpy change is squared
B) The enthalpy change changes sign
C) The enthalpy change doubles
D) The enthalpy remains unchanged
E) The enthalpy becomes zero
7. When multiplying a chemical equation by a factor, how does the enthalpy change behave?
A) It remains constant
B) It is multiplied by the same factor
C) It is divided by the factor
D) It becomes negative
E) It becomes zero
8. What type of function is enthalpy?
A) Extensive
B) Intensive
C) Neither extensive nor intensive
D) Dependent on reaction path
E) Dependent on pressure only
9. Hess’s Law can be used to determine the enthalpy change for:
A) Phase changes only
B) Any chemical or physical process
C) Only gas-phase reactions
D) Only reactions involving solids
E) None of the above
10. Why is Hess’s Law valid?
A) Because heat energy is always conserved
B) Because enthalpy is independent of the reaction path
C) Because entropy is constant
D) Because temperature is always fixed
E) Because work done is always zero
11. Hess's Law allows calculation of enthalpy changes for reactions by using:
A) Only the products’ enthalpies
B) Only the reactants’ enthalpies
C) Enthalpy changes of related known reactions
D) Rate constants
E) Activation energies
12. Which of the following is NOT required to use Hess's Law?
A) Balanced chemical equations
B) Enthalpy changes of known reactions
C) Initial and final states of the system
D) Reaction rates
E) Knowledge that enthalpy is a state function
13. The sum of the enthalpy changes of the individual steps in a reaction sequence is:
A) Equal to the total enthalpy change of the overall reaction
B) Always zero
C) Less than the overall enthalpy change
D) Greater than the overall enthalpy change
E) Unrelated to the overall enthalpy change
14. How can Hess’s Law be applied to find the enthalpy change of a reaction that is difficult to measure directly?
A) By breaking the reaction into known steps with known enthalpy changes
B) By measuring pressure changes
C) By measuring the reaction rate
D) By increasing temperature
E) By calculating the equilibrium constant
15. If the enthalpy change of formation of CO₂(g) is known, and the enthalpy change of formation of CO(g) is known, Hess’s Law can be used to find:
A) Enthalpy of combustion of methane
B) Enthalpy of formation of carbon
C) Enthalpy change of the reaction CO(g) + 1/2 O₂(g) → CO₂(g)
D) Activation energy for CO oxidation
E) Reaction rate of CO₂ formation
16. Which of these laws is closely related to Hess’s Law?
A) Boyle’s Law
B) First Law of Thermodynamics
C) Charles’s Law
D) Avogadro’s Law
E) Dalton’s Law
17. When Hess’s Law is applied, enthalpy changes of reactions in the reverse direction are:
A) Ignored
B) Added without change
C) Added with sign reversed
D) Multiplied by zero
E) Multiplied by two
18. Hess’s Law is especially useful for reactions:
A) Occurring in a single step
B) With unknown enthalpy changes but known related reactions
C) Involving nuclear fusion
D) Only at high pressure
E) Only in gas phase
19. If a reaction is written as the sum of two reactions, and the enthalpy changes are -100 kJ and +40 kJ, what is the enthalpy change of the overall reaction?
A) -140 kJ
B) +140 kJ
C) -60 kJ
D) +60 kJ
E) 0 kJ
20. Hess’s Law implies that the enthalpy change for a reaction depends on:
A) The initial reactants only
B) The final products only
C) The difference between initial and final states only
D) The reaction mechanism
E) The rate of reaction
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Answers and Explanations
1. B – Hess's Law states that enthalpy changes are independent of the pathway.
2. A – Enthalpy is a state function; it depends only on initial and final states.
3. B – Hess’s Law helps calculate total enthalpy change for complex reactions by summing steps.
4. B – Total enthalpy change equals the sum of enthalpy changes of individual steps.
5. B – It helps determine enthalpy changes of reactions difficult to measure directly.
6. B – Reversing a reaction changes the sign of enthalpy change.
7. B – Multiplying a reaction by a factor multiplies ΔH by the same factor.
8. A – Enthalpy is an extensive property, dependent on amount of substance.
9. B – Hess's Law applies to any chemical or physical process.
10. B – Valid because enthalpy is independent of reaction path.
11. C – Uses enthalpy changes of related known reactions to calculate unknown ΔH.
12. D – Reaction rates are not needed to apply Hess's Law.
13. A – The sum of step enthalpies equals the overall reaction enthalpy change.
14. A – Break difficult reactions into known steps with known enthalpy changes.
15. C – The reaction CO + ½ O₂ → CO₂ can be evaluated via Hess’s Law.
16. B – The First Law of Thermodynamics underlies Hess’s Law.
17. C – Enthalpy changes in reverse reactions have opposite signs.
18. B – Useful when overall reaction enthalpy is unknown but component reactions are known.
19. C – Sum of -100 kJ and +40 kJ is -60 kJ.
20. C – Enthalpy change depends only on initial and final states.
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