Hello, fatimasaboor123.
I believe that :
- An endothermic reaction is a reaction where heat is taken in.
- An exothermic reaction is a reaction where heat is given out.
Whether heat is taken in or given out, can be easily identified.
This is how:
- If the product side has a lower bond energy than the reactant side, it can be cognized that heat has been given out.
- If it's the other way round, i.e. If the product side has a higher bond energy than the reactant side, it can be cognized that heat has been taken in.
This can be generalized as the formula: Enthalpy Change = Heat/Bond energy on product side - Heat/Bond energy on reactant side.
Note: The reactant side is the LHS of the reaction equation. The product side is the RHS of the equation.
In respect to our formula:
- The answer will be positive if the Heat/Bond energy on the product side is higher, meaning "heat is taken in", meaning a endothermic reaction.
- The answer will be negative if the Heat/Bond energy on the product side is lower, meaning "heat is given out", meaning a exothermic reaction.
Let us use this knowledge by solving a problem:
The equation is
H2 + F2 :rarrow: HF
The bond energies for this equations are:
- F - F bond: 155 kilojoules per mole
- H - H bond: 432 kilojoules per mole
- F - H bond: 565 kilojoules per mole
Step 1: Let's balance the equation:
H2 + F2 :rarrow: HF
First, let's write down the no. of atoms of each element in both the LHS and RHS:
Let's add a coefficient of 2 to the HF molecule.
H2 + F2 :rarrow: 2HF
Again, let's write down the no. of atoms of each element in both the LHS and RHS:
The equation has been balanced.
Step 2: Let's replace the formulas of atoms with the bond structures.
H - H + F - F :rarrow: H - F H - F
Step 3: Let's calculate the bond energies for the LHS and RHS:
H - H + F - F :rarrow: H - F H - F
There is one H - H bond and one F - F bond on the LHS. There are two H - F bonds on RHS.
- F - F bond: 155 kilojoules per mole
- H - H bond: 432 kilojoules per mole
- F - H bond: 565 kilojoules per mole
So, the bond energy on the LHS will be 155 + 432 kilojoules per mole (kJ/mol), which will be 587 kJ/mol.
And, the bond energy on the RHS will be 565 * 2 kJ/mol, which will be 1130 kJ/mol.
So we have the bond energies:
- LHS: 587 kJ/mol
- RHS: 1130 kJ/mol
Step 4: Use the formula.
Enthalpy Change = Heat/Bond energy on product side - Heat/Bond energy on reactant side.
When we use the equation:
Enthalpy change = 1130 kJ/mol - 587 kJ/mol = 543 kJ/mol.
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As said earlier, the answer will be positive if the Heat/Bond energy on the product side is higher, meaning "heat is taken in", meaning a endothermic reaction.
So, this reaction is a endothermic reaction i.e. it has taken in heat.
Thank you for reading this article cum blog. I hope this is informative. Here are the sources I used for gathering information.
https://www.bbc.co.uk/bitesize/guides/zwfr2nb/revision/5 (https://www.bbc.co.uk/bitesize/guides/zwfr2nb/revision/5)
https://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html (https://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html)
Thank you. :)