What is required for a correctly written thermochemical equation?
A. a balanced chemical equation that includes the enthalpy change and phase of each reactant and product
B. a balanced chemical equation that includes the entropy change
C. a balanced chemical equation that includes the phase of each reactant and product
D. a balanced chemical equation that includes the temperature change
The Correct Answer and Explanation is:
The correct answer is A. a balanced chemical equation that includes the enthalpy change and phase of each reactant and product.
Explanation:
A thermochemical equation is a chemical equation that includes not only the reactants and products involved in a chemical reaction but also the enthalpy change (ΔH) associated with the reaction. The enthalpy change represents the heat absorbed or released during the reaction at constant pressure. This type of equation provides important information about the energy changes in a reaction and is essential in thermodynamics.
The key requirements for a correctly written thermochemical equation are:
- Balanced Chemical Equation: Like any chemical equation, a thermochemical equation must be balanced, meaning the number of atoms of each element on the left (reactant) side of the equation must equal the number on the right (product) side. This ensures the law of conservation of mass is adhered to.
- Enthalpy Change (ΔH): The equation must include the enthalpy change (ΔH), which quantifies the heat change during the reaction at constant pressure. This value is typically given in joules (J) or kilojoules (kJ), and it reflects whether the reaction is exothermic (releases heat, ΔH is negative) or endothermic (absorbs heat, ΔH is positive). For instance, the equation for the combustion of methane would be written as:
[
CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) \quad \Delta H = -890 \, \text{kJ}
]
This shows that 890 kJ of heat is released during the combustion. - Phases of Reactants and Products: It is important to include the physical states (solid, liquid, gas, or aqueous) of each reactant and product in a thermochemical equation. This is necessary because the phase of a substance affects the enthalpy change. For example, the enthalpy change for the formation of a substance in a gas phase may differ from that in a solid or liquid phase due to differences in intermolecular forces and energy states.
In summary, a thermochemical equation must always be balanced, include the enthalpy change (ΔH), and show the phases of all substances involved to fully describe the thermodynamic properties of the reaction. Therefore, the correct answer is A.