Can somebody help me how to remember 'when should we do reactant - product and product - reactant' in various enthalpies?? (Thermodynamics)?

Question

Answer

Only for enthalpy of formation,deltaS & deltaG it is product minus reactant and for remaining it is reactant minus product...

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Don't know exactly as i have studied this long before.. what i can say is just see the trend or relate it to energy.. like if energy is consumed.. this happens.. or whatever is easier for you.. thanks,

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Understanding the Reactant-Product and Product-Reactant Conventions in Thermodynamics

In thermodynamics, it is important to determine the direction of a reaction and the corresponding sign of the enthalpy change. This can be done by following the reactant-product and product-reactant conventions. These conventions help us remember the orientation of the reaction and the associated enthalpies.

Reactant-Product Convention:
The reactant-product convention is used when determining the enthalpy change for a reaction. It states that the enthalpy change (ΔH) is positive when the reaction proceeds from reactants to products.

Product-Reactant Convention:
The product-reactant convention is used when determining the enthalpy change for the reverse reaction. It states that the enthalpy change (ΔH) is negative when the reaction proceeds from products to reactants.

Remembering the Conventions:
To remember these conventions, you can use the following mnemonic devices:

1. 'Rise Over Run':
Think of the reactant-product convention as a rising line on a graph. The reaction starts from the reactants (the starting point, or 'rise') and goes up towards the products (the ending point, or 'run'). This upward movement signifies a positive enthalpy change.

2. 'Reverse the Arrow':
For the product-reactant convention, imagine reversing the arrow in the reaction equation. By mentally flipping the arrow, you are reversing the direction of the reaction. This reversal corresponds to a negative enthalpy change.

Example:
Consider the combustion of methane:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Using the reactant-product convention, the enthalpy change (ΔH) for this reaction would be positive, as the reaction proceeds from the reactants (CH4 and O2) to the products (CO2 and H2O).

However, if we consider the reverse reaction, which is the decomposition of water:

CO2(g) + 2H2O(g) → CH4(g) + 2O2(g)

Using the product-reactant convention, the enthalpy change (ΔH) for this reverse reaction would be negative, as the reaction proceeds from the products (CO2 and H2O) to the reactants (CH4 and O2).

Conclusion:
Understanding the reactant-product and product-reactant conventions allows us to determine the direction and sign of the enthalpy change for a reaction. By using mnemonic devices like 'Rise Over Run' and 'Reverse the Arrow,' you can easily remember when to apply these conventions in various enthalpies.

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