When solving Hess law problems,do we need to use all the equations given to get the Htotal or just choose the relevant equations that are related to the final equation required?
Let's say there r 5 equations given,but the relevant equations are 3 only ,so we can neglect the rest 2 equations rite? Thank you
Yes, you're right. Choose what you need and ignore the others.
When solving Hess's law problems, it is important to use only the relevant equations that are related to the final equation you are trying to find. Hess's law states that the overall enthalpy change for a chemical reaction is the sum of the enthalpy changes of the individual reactions that make up the overall reaction. Therefore, you need to select the equations that can help you construct a pathway from the given reactants to the desired product.
If there are multiple equations given, it is important to evaluate which equations will allow you to construct the desired pathway. You should choose the equations that have the same substances on both sides as the desired reaction, or equations that allow you to create or eliminate common intermediates.
If there are equations that do not have any direct correlation or connection to the desired reaction or intermediate species involved, then you can exclude them from your calculations. However, it is crucial to double-check your choices to ensure you have included all the relevant equations needed for constructing the pathway.
So, in your example, if there are five equations given but only three of them are relevant to constructing the pathway for the desired reaction, you can ignore the remaining two equations.
Remember to balance the equations appropriately and consider the stoichiometric coefficients when adding or subtracting the reactions to cancel out any species that appear in both the reactant and product sides of the equations. By summing up the enthalpy changes of the relevant equations, you can determine the overall enthalpy change (Htotal) for the desired reaction.