Question

Consider the reaction: M + 2HCl → MCl2 + H2 When 0.85 mol of the metal, M, reacted with an aqueous HCl solution (the HCl is in excess), the temperature of the solution rose because the reaction produced 4035 J of heat. What is ∆H in kJ per mol of M for this reaction? (Hint: Is this reaction exothermic or endothermic?) It is possible that your answer could be either positive or negative. If it is negative, you must include the "minus" sign. Enter your answer as a decimal number.

Answer 1

Answer : The enthalpy change of the reaction is -4.747 kJ/mol and this reaction is exothermic.

Explanation :

First we have to calculate the enthalpy change of the reaction by using the equation:

$\Delta H_{rxn}=\frac{q}{n}$

where,

q = amount of heat released = -4035 J

n = number of moles of metal M = 0.85 moles

$\Delta H_{rxn}$ = enthalpy change of the reaction

Now put all the given values in above equation, we get:

$\Delta H_{rxn}=\frac{-4035J}{0.85mol}=-4747.0J/mol=-4.747kJ/mol$

Conversion used:   (1 kJ = 1000 J)

Sign convention of heat:

When heat is absorbed then the sign of heat is taken to be positive and is written on the reactant side and is considered as an endothermic reaction.

When heat is released then the sign of heat is taken to be negative and is written on the product side and is considered as an exothermic reaction.

Thus, the chemical reaction will be:

$M+2HCl\rightarrow MCl_2+H_2+4.477kJ$

Hence, the enthalpy change of the reaction is -4.747 kJ/mol and this reaction is exothermic.