Question

- Water can be produced in laboratory by the
following reaction
2H2 + O2 + 2H2O
If Hf is the heat of formation of H20 and Hr is
the heat of this reaction, then
a. Hf=Hr
b. Hf=Hr divided by 2
C. Hr = Hf divided by 2
d. Hf = 2Hr
Please explain the answer

Answer 1

Given :

• Heat of formation of H2O is Hf

• Heat of reaction of 2H2 + O2 ➝ 2H2O is Hr

To find :

• Relationship between Hf & Hr

Solution :

• Heat of formation is defined as heat change when 1 mole of substance is formed.

For H2O

H2 + (1/2) O2 ➝ H2O ∆H = Hf. (1)

• Heat of reaction is defined as heat change when given number of mole of reactant completely undergo reaction to form given number of mole of products

For above reaction,

2H2 + O2 ➝ 2H2O ∆H = Hr (2)

Multiplying reaction (1) by 2 we get,

2H2 + O2 ➝ 2H2O ∆H = 2×Hf (3)

Comparing reaction (2) & (3)

we find that both reaction are same , therefore there heat change should be same

➠ Hr = 2Hf

➠ Hf = (1/2) Hr

ANSWER :

Option B) Hf = Hr divided by 2