Thermochemical equations Knowledge-Based Practice

Questions

1. 1.

   Why is it strictly necessary to specify the physical states of all reactants and products in a thermochemical equation?

   • A) State symbols are required exclusively to indicate whether the chemical transformation can attain dynamic equilibrium.
   • B) Omitting physical states prevents the stoichiometric balancing of atoms and violates mass conservation.
   • C) Specifying the phase is necessary only to determine whether the reaction rate is positive or negative.
   • D) The enthalpy of a substance depends on its physical state, so a change in phase directly alters the overall reaction enthalpy.
2. 2.

   In thermochemical equations, fractional stoichiometric coefficients such as $\frac{1}{2}$ are permissible and frequently used. Which of the following explains why fractional coefficients are valid in thermochemical equations?

   • A) The coefficients denote the relative counts of individual atoms or molecules reacting in a microscopic event.
   • B) The coefficients represent exact amounts of substance in moles rather than individual particles.
   • C) The coefficients reflect the equilibrium partial pressures of each component at standard temperature.
   • D) The coefficients indicate the mass percentages of elements combined within each chemical species.
3. 3.

   Consider the thermochemical equation: $2\text{C}(s) + \text{H}_2(g) \rightarrow \text{C}_2\text{H}_2(g) \quad \Delta H = +227\text{ kJ}$ Which of the following statements regarding this reaction is correct?

   • A) The synthesis of acetylene is an exothermic process that releases 227 kJ of heat per mole of $\text{C}_2\text{H}_2(g)$ produced.
   • B) The formation of 2 moles of $\text{C}_2\text{H}_2(g)$ absorbs 113.5 kJ of heat under the same conditions.
   • C) The decomposition of 1 mole of $\text{C}_2\text{H}_2(g)$ into its constituent elements releases 227 kJ of heat.
   • D) The reaction absorbs 227 kJ of heat for every 1 mole of solid carbon consumed.
4. 4.

   Given the thermochemical equation: $\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l) \quad \Delta H = -890\text{ kJ}$ How much heat is released during the complete combustion of 4.0 g of $\text{CH}_4(g)$ in excess oxygen? (Molar mass of $\text{CH}_4 = 16\text{ g/mol}$)

   • A) 55.6 kJ
   • B) 445.0 kJ
   • C) 890.0 kJ
   • D) 222.5 kJ
5. 5.

   Given the thermochemical equation $2\text{SO}_{2}(g) + \text{O}_{2}(g) \rightarrow 2\text{SO}_{3}(g)$ with $\Delta H = -198\text{ kJ}$, what is the enthalpy change for the reaction $\text{SO}_{3}(g) \rightarrow \text{SO}_{2}(g) + \frac{1}{2}\text{O}_{2}(g)$?

   • A) +99 kJ
   • B) -99 kJ
   • C) +198 kJ
   • D) -396 kJ

Answers and step-by-step solutions appear after you finish the test.