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CBSE Class 11-science Answered

state hess's law. how does it follow the first law of thermodynamics.
Asked by paulnaveed202 | 17 Dec, 2020, 12:36: PM
Expert Answer
Hess's law states that theheat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps. This is also known as the law of constant heat summation.
 
For Example-

Hess's law can be applied to calculate enthalpies of reactions that are difficult to measure. It is very difficult to control the oxidation of graphite to give pure CO. However, enthalpy for the oxidation of graphite to CO2 can easily be measured. The application of Hess's law enables us to estimate the enthalpy of formation of CO.

C + O2 → CO2,         Syntax error from line 1 column 49 to line 1 column 73. Unexpected '<mstyle '.H° = -393 kJ/mol

CO + 1/2 O2 → CO2,         Syntax error from line 1 column 49 to line 1 column 73. Unexpected '<mstyle '.H° = -283 kJ/mol.
 
Subtracting the second equation from the first gives
 
C + 1/2 O2 → CO,         Syntax error from line 1 column 49 to line 1 column 73. Unexpected '<mstyle '.H° = -393 - (-283) = -110 kJ/mol

The equation shows the standard enthalpy of formation of CO = -110 kJ/mol.

Answered by Ravi | 18 Dec, 2020, 06:27: PM

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