Question

Carbon reacts with sulphur to give carbon disulphide, which is an endothermic reaction, where $92kJ/mol$ of heat is absorbed. What will be the value of $\Delta H$, when $24$ grams of carbon reacts?
A.$- 92kJ$
B.$+ 92kJ$
C.$- 184kJ$
D.$+ 184kJ$

Answer 1

An endothermic process or reaction is one in which there is an increase in the enthalpy $H$ (or internal energy $U$) of the system. In thermodynamics, in such a process, a closed system usually absorbs thermal energy from its surroundings, which is heat transferred into the system. Endothermic reactions generally involve the formation of chemical bonds through this absorption of heat from the surroundings.

Complete answer:
The above reaction can be represented as:
$C + 2S\xrightarrow{\Delta }C{S_2}$
As we can see, $1$ mol of carbon gives $1$ mol of carbon disulphide. It is given that $92kJ$ of energy is absorbed in this reaction by $1$ mol or $12$ grams of carbon.
Energy absorbed by $24$ grams or $2$ mol of carbon is $92 \times 2$ which equals $184kJ$. Since, energy is absorbed here, therefore the sign of $\Delta H$ is positive, that is, $+ 184kJ$.
And hence option D is the correct answer.

Additional information:
Carbon disulphide is a colourless volatile liquid and is frequently used as a building block in organic chemistry as well as an industrial and chemical non-polar solvent. It has an "ether-like" odour, but commercial samples are typically contaminated with foul-smelling impurities. It is highly flammable and its combustion gives out sulphur dioxide.

Note:
There is an opposite to endothermic reaction which is called exothermic reaction. As the name suggests, energy is released from the system in the form of heat into its surroundings. This results in an increase in entropy of the surrounding. Here, the enthalpy change ($\Delta H$) is negative.
The energy which is supplied or released in an endothermic or exothermic reaction can be of various forms like heat, light or electricity.