Question

Which of the following is an exothermic reaction?
A. ${H_2}_{\left( g \right)} + C{l_{2(g)}} \to 2HC{l_{\left( g \right)}}$ $\Delta H = 184.6kJ$
B. ${N_2}_{\left( g \right)} + {O_{2(g)}} \to 2N{O_{\left( g \right)}}$ $\Delta H = 180.8kJ$
C. ${C_{graphite}} + {H_2}{O_{\left( g \right)}} \to C{O_2}_{\left( g \right)} + {H_{2(g)}}$ $\Delta H = - 131.4kJ$
D. ${C_{graphite}} + 2{S_{\left( g \right)}} + 91.9kJ \to C{S_{2(l)}}$

Answer 1

Hint: We need to know that the processes that require or absorb heat will be restricted to changes of state, known as phase changes, and changes in compound constitution, or chemical reactions. We know that exothermic reactions are those reactions, which eliminates heat energy. Endothermic reactions are those reactions where heat energy is absorbed. We could predict the types of chemical reaction using the signs of enthalpy.

Complete step by step answer:
Now we discuss the endothermic reaction as those reactions which absorb heat energy from the surroundings are known as endothermic reactions. When chemical bonds are broken heat energy is absorbed.
Example: In the photosynthesis process, carbon dioxide and water are altered to free oxygen and glucose by absorbing heat energy.
$6\,C{O_2} + 6\,{H_2}O + 2519kJ \to {C_6}{H_{12}}{O_6} + 6{O_2}$
Now we can discuss the exothermic reaction as those reactions that release heat energy to the surroundings are called exothermic reactions. Chemical bonds are formed by liberation of heat energy.
Example: In the combination reaction of solid carbon and gaseous oxygen, development of gaseous carbon dioxide occurs with the evolution of heat energy.
$C\left( s \right) + {O_2}\left( g \right) \to C{O_2}\left( g \right) + 393kJ$
For endothermic reaction, the change in enthalpy is a positive and for exothermic reaction, the change in enthalpy is a negative. Among the options, the chemical reaction that has negative enthalpy is option (C). So, the exothermic reaction is,
${C_{graphite}} + {H_2}{O_{\left( g \right)}} \to C{O_2}_{\left( g \right)} + {H_{2(g)}}$ $\Delta H = - 131.4kJ$

So, the correct answer is Option C.

Note: We need to know that in endothermic reaction, the enthalpy of reactants is not exactly the enthalpy of products, since heat is absorbed in endothermic response. Consequently, in endothermic reaction change in enthalpy is positive. Some of the exothermic reactions are rain, and combustion. The enthalpy of products is less than that enthalpy of reactants and the change of enthalpy is negative for exothermic reactions.