Questions about Thermochemical Equations
Multiple-Choice Questions: Thermochemical Equations
1. What is a thermochemical equation?
A) A balanced equation with molecular formulas only
B) A chemical equation that includes energy changes
C) An equation involving pressure and volume
D) A nuclear equation with decay terms
E) An ionic equation without charges
2. Which quantity is typically included in a thermochemical equation?
A) Enthalpy change (ΔH)
B) Molarity
C) Volume
D) Atomic radius
E) Temperature in Celsius
3. The enthalpy change, ΔH, in a thermochemical equation refers to:
A) Pressure change during a reaction
B) Volume of gases involved
C) Heat exchanged at constant volume
D) Heat exchanged at constant pressure
E) Work done on the system
4. An exothermic reaction in a thermochemical equation will have:
A) ΔH = 0
B) Positive ΔH
C) No temperature change
D) Negative ΔH
E) ΔH expressed in moles
5. Which of the following best represents an exothermic thermochemical equation?
A) H₂O(l) → H₂(g) + ½O₂(g) ΔH = +285.8 kJ
B) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = –890.3 kJ
C) N₂(g) + O₂(g) → 2NO(g) ΔH = +180.5 kJ
D) 2H₂O₂(aq) → 2H₂O(l) + O₂(g) ΔH = +196 kJ
E) HCl(g) → H⁺(aq) + Cl⁻(aq) ΔH = 0
6. In a thermochemical equation, the coefficient in front of a substance:
A) Indicates temperature
B) Multiplies the ΔH value proportionally
C) Refers only to mass
D) Has no effect on energy
E) Is always 1 for gases
7. Which of the following equations is properly balanced and includes a correct enthalpy change?
A) H₂ + O₂ → H₂O ΔH = –572 kJ
B) 2H₂ + O₂ → 2H₂O ΔH = –572 kJ
C) H₂ + O₂ → 2H₂O ΔH = –572 kJ
D) H₂ + ½O₂ → H₂O ΔH = –1144 kJ
E) 2H₂ + O₂ → H₂O ΔH = +572 kJ
8. What happens to the ΔH if you reverse a thermochemical equation?
A) It doubles
B) It stays the same
C) It becomes zero
D) It changes sign
E) It gets squared
9. If a thermochemical equation is multiplied by 2, what happens to ΔH?
A) It stays the same
B) It doubles
C) It becomes half
D) It becomes negative
E) It is squared
10. Which of the following is not a requirement for a valid thermochemical equation?
A) Balanced chemical equation
B) Physical states indicated
C) Enthalpy change listed
D) Pressure data included
E) Correct stoichiometry
11. A positive ΔH value indicates the reaction is:
A) Exothermic
B) Spontaneous
C) Endothermic
D) In equilibrium
E) Rapid
12. When writing a thermochemical equation, which of the following must be included for each substance?
A) Valence electrons
B) Molecular mass
C) Physical state
D) Color
E) Crystal shape
13. In a combustion reaction of a hydrocarbon, the ΔH is typically:
A) Zero
B) Positive
C) Negative
D) Unknown
E) Dependent on pressure
14. Which expression shows the correct unit for enthalpy change?
A) mol/L
B) atm
C) kJ
D) J/mol·K
E) L/mol
15. A reaction has ΔH = +150 kJ. Which of the following statements is true?
A) The system loses 150 kJ
B) It is an exothermic reaction
C) The surroundings gain 150 kJ
D) The reaction absorbs 150 kJ
E) Energy is destroyed
16. Which of the following reactions is endothermic?
A) Freezing of water
B) Combustion of methane
C) Melting of ice
D) Condensation of steam
E) Formation of ionic bonds
17. In a thermochemical equation, what does the enthalpy change depend on?
A) Type of calorimeter
B) Mass of the system
C) State and amount of reactants/products
D) Only the type of bonds
E) Volume of reaction vessel
18. If a thermochemical equation for 1 mol of H₂ combustion is:
H₂(g) + ½O₂(g) → H₂O(l) ΔH = –285.8 kJ
What would be ΔH for 2 mol H₂?
A) –571.6 kJ
B) +285.8 kJ
C) –142.9 kJ
D) +571.6 kJ
E) –285.8 kJ
19. Which term best defines a reaction that releases heat to surroundings?
A) Endothermic
B) Exothermic
C) Isothermal
D) Thermoneutral
E) Spontaneous
20. Which of the following changes must be included in a thermochemical equation to be accurate?
A) Mass of products
B) Temperature of reactants
C) Enthalpy change
D) Reaction rate
E) Catalyst used
Answers with Explanations
1. B – A thermochemical equation includes energy change (ΔH) along with the balanced chemical reaction.
2. A – ΔH (enthalpy change) is always included in thermochemical equations.
3. D – ΔH represents heat at constant pressure.
4. D – Exothermic reactions have a negative ΔH.
5. B – The combustion of CH₄ is exothermic and correctly shows a negative ΔH.
6. B – Coefficients affect the ΔH; doubling the equation doubles ΔH.
7. B – Balanced reaction with correct ΔH for 2 mol of water formed.
8. D – Reversing a reaction changes the sign of ΔH.
9. B – Multiplying a reaction multiplies ΔH by the same factor.
10. D – Pressure is not required in the equation itself, though relevant in some cases.
11. C – A positive ΔH indicates the reaction absorbs heat (endothermic).
12. C – Physical states are essential (e.g., g, l, s, aq).
13. C – Combustion releases heat → exothermic → ΔH is negative.
14. C – Enthalpy is measured in kilojoules (kJ).
15. D – Positive ΔH means 150 kJ is absorbed by the system.
16. C – Melting ice requires heat input (endothermic).
17. C – Enthalpy change depends on the amount and state of substances.
18. A – Doubling moles doubles the ΔH: –285.8 × 2 = –571.6 kJ.
19. B – Exothermic reactions release heat.
20. C – ΔH must be shown for thermochemical accuracy.
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