Question

What is the difference between standard enthalpy of combustion and standard enthalpy of formation?

Answer 1

In this we have to differentiate between standard enthalpy of combustion and standard enthalpy of formation on the basic of their physical and chemical properties through the definition, examples , change involved, whether exothermic or endothermic etc. Now answer the statement.

Complete answer:
We will differentiate between the two i.e. the standard enthalpy of combustion and standard enthalpy of formation in the tabular form because its easy way to understand them and learn them.
Their difference is as below-

Sr No.	Property	standard enthalpy of combustion	standard enthalpy of formation
1.	Definition	It may be defined as the amount of heat released when one mole of the substance is completely burnt in the presence of oxygen.	It is defined as the change in the enthalpy during the formation of one mole of the substance from its constituents elements involved in the reaction.
2.	Representation	It is denoted by $\Delta H_{c}^{\circ }$.	It is denoted by ${{\Delta }_{f}}{{H}^{\circ }}$.
3.	Presence of oxygen	It cannot occur in the presence of oxygen.	It can take place in the absence of oxygen.
4.	Change involved	It tells us about the amount of heat that is added or released during the chemical reaction.	It tells us about the change involved in the enthalpy during the chemical reaction.
5.	Exothermic/endothermic	It is always exothermic in nature.	It can be either exothermic or endothermic in nature.
6.	Examples	The standard enthalpy of formation of formation is;-$\begin{aligned} & {{C}_{2}}{{H}_{6}}(g)+\dfrac{1}{2}{{O}_{2}}(g)\to 2C{{O}_{2}}(g)+3{{H}_{2}}O(l)\text{ } \\\ & \Delta \text{H}_{f}^{\circ }=-1560kJmol{{e}^{-1}} \\\ \end{aligned}$	The standard enthalpy of formation of formation is;-$\begin{aligned} & {{H}_{2}}(g)+\dfrac{1}{2}{{O}_{2}}(g)\to {{H}_{2}}O(l)\text{ } \\\ & \Delta \text{H}_{f}^{\circ }=-286kJmol{{e}^{-1}} \\\ \end{aligned}$

So, it’s easy to distinguish the standard enthalpy of combustion and standard enthalpy of formation in the tabular form rather than in the paragraph form.

Note:
While calculating the standard enthalpy of combustion and standard enthalpy of formation ,each substance or compound must be in their standard states. Standard enthalpy of combustion is always negative ( because heat is released) and standard enthalpy of formation can either be positive or negative.