Hydrogen reacts with chlorine to form hydrogen chloride \(\left( HCl ( g ), \Delta H_{ f }=-92.3 kJ / mol \right)\) according to the reaction below
\(H_2(g)+Cl_2(g) \rightarrow 2 HCl(g)\)
Which statement is correct?
Use \(\Delta H_{\text {pon }}=\sum\left(\Delta H_{\text {f.products }}\right)-\sum\left(\Delta H_{\text {f.reactants }}\right)\).
A. The enthalpy of the reaction is -184.6 kJ, and the reaction is exothermic.
B. The enthalpy of the reaction is -184.6 kJ, and the reaction is endothermic.
C. The enthalpy of the reaction is 184.6 kJ, and the reaction is endothermic.
D. The enthalpy of the reaction is 184.6 kJ, and the reaction is exothermic.
Answer (1)
Calculate the enthalpy of the reaction using the formula: Δ H r x n = ∑ Δ H f , p ro d u c t s − ∑ Δ H f , re a c t an t s .
Substitute the given values: Δ H r x n = 2 × ( − 92.3 kJ/mol ) − ( 0 + 0 ) = − 184.6 kJ/mol .
Since Δ H r x n is negative, the reaction is exothermic.
The enthalpy of the reaction is − 184.6 kJ , and the reaction is exothermic.
Explanation
Understanding the Problem We are given the reaction: H 2 ( g ) + C l 2 ( g ) → 2 H Cl ( g ) . The enthalpy of formation of H Cl ( g ) is Δ H f = − 92.3 kJ/mol . We need to find the enthalpy of the reaction and determine if it is exothermic or endothermic.
Enthalpy of Reaction Formula The enthalpy of reaction is calculated using the formula: Δ H r x n = ∑ Δ H f , p ro d u c t s − ∑ Δ H f , re a c t an t s The enthalpy of formation of elements in their standard states ( H 2 and C l 2 ) is zero.
Calculating Enthalpy of Reaction Δ H r x n = 2 × Δ H f , H Cl − ( Δ H f , H 2 + Δ H f , C l 2 ) Substituting the given values: Δ H r x n = 2 × ( − 92.3 kJ/mol ) − ( 0 + 0 ) = − 184.6 kJ/mol
Determining Exothermic or Endothermic Since the enthalpy of the reaction is negative ( Δ H r x n = − 184.6 kJ/mol ), the reaction is exothermic. This means the reaction releases heat.
Final Answer Therefore, the correct statement is: The enthalpy of the reaction is -184.6 kJ, and the reaction is exothermic.
Examples
Understanding enthalpy changes is crucial in many real-world applications. For example, when designing engines or chemical reactors, engineers need to know whether a reaction will release or absorb heat. Exothermic reactions, like the one described, release heat and can be used to generate energy. In contrast, endothermic reactions require heat input to proceed. Knowing the enthalpy change helps in managing energy efficiently and safely in various industrial processes.
