Question

38.55 kJ of heat is absorbed when 6.0 g of ${{O}_{2}}$ react with $ClF$ according to the reaction,
$2ClF(g)\,+\,{{O}_{2}}(g)\,\to \,\,\,C{{l}_{2}}O(g)\,\,+\,O{{F}_{2}}(g)$
What is the standard enthalpy of the reaction?
($\Delta {{H}^{o}}$ = + 205.6 kJ )

Answer 1

Hint: An attempt to this question can be made by understanding the physical quantity enthalpy. Now determine the relation between the heat given for 6 g of oxygen and 1 mole of oxygen gas. With this you can calculate the standard enthalpy of the reaction which is the amount of heat released in a chemical reaction.

Complete Solution :
- Enthalpy is a property of a thermodynamic system, that is a convenient state function used as a means of measurement in various chemical and biological systems at constant pressure.
- A state function is a property such that its value does not depend on the path taken by the physical quantity to reach a specific value. Enthalpy is one such state function.
- It is given to us that 6 g of oxygen when used in the reaction releases 38.55 kJ of heat. We will now calculate the heat released for 1 mole of oxygen gas.
1 mole of oxygen has a mass of 32 g.
- So, the amount of energy released by 1 mole of oxygen is,
$\dfrac{32}{6}\text{ x 38}\text{.55}$ = 205.6 kJ.
- Since only 1 mole of oxygen is required for reaction to occur, the standard enthalpy of the reaction above is 205.6 kJ.

Additional Information:
- Thermodynamics is a branch of physics that deals with the physical quantities heat, work and temperature and their relation to radiation and energy released.
- The behaviour of these quantities is in accordance with the laws of thermodynamics. The 3 laws of thermodynamics give a quantitative description using the above physical quantities at a microscopic level.
- In contrast to state function, functions that depend on the path taken by the quantity to go between two values are called path functions. Both these functions are often used in thermodynamics.

Note: We can never define the absolute enthalpy of a substance or a reaction. This is the reason why we consider change in enthalpy for a reaction like enthalpy of formation, enthalpy of atomisation etc.