Question

A mixture of 2 moles of carbon monoxide and one mole of oxygen in a closed vessel is ignited to get carbon dioxide. If $\Delta H$ is the enthalpy change and $\Delta U$ is the change in internal energy, then:
A.$\Delta H > \Delta U$
B.$\Delta H < \Delta U$
C.$\Delta H = \Delta U$
D.Can’t be predicted

Answer 1

The heat and other forms of energy are a part of thermodynamics and the laws of thermodynamics describe how the energy can be interconverted. The branch of thermodynamics that deals with the chemical reactions is known as chemical thermodynamics.

Complete step by step answer:
At constant pressure, the amount of heat absorbed or emitted is known as enthalpy change and it is represented by $\Delta H$. It is represented as:
$\Delta H = \Delta U + \Delta nRT$
Here, $\Delta U$ represents the change in internal energy and $\Delta n$ represents the difference between gaseous molecules of product and reactants. The gas constant is defined by R and T is temperature.
It is given that number of moles of $CO$ is 2 and that of Oxygen(${O_2}$) so for the above reaction, $2CO(g) + {O_2}(g) \to 2C{O_2}(g)$
The change in the number of moles $\Delta n$ = 2-3=-1. Therefore, the enthalpy change is defined as
$\Delta H = \Delta U - RT$. Therefore, $\Delta H < \Delta U$.

Hence, option B is correct.

Additional information: The conditions at which $\Delta U = \Delta H$ are:
When the reaction is carried out in a closed vessel, so that volume remains constant.
There is no change in the number of moles of gaseous reactants and products
The reactant and product are in the solid or liquid phase as in the case of solid or liquid phase the number of moles doesn’t change during the reaction.

Note:
Enthalpy of a reaction also known as the heat of reaction defines the amount of heat liberated or absorbed in a chemical reaction. When heat is liberated, the reaction is exothermic, also when the heat is absorbed in a reaction, it is called an endothermic reaction.