Use the following information to answer the next question.
The formation of water is accompanied by the release of a huge amount of energy, 572 kJ to be precise.This is an example of an exothermic reaction, a reaction that produces energy. It is also an example of combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen.
2H2 + O2 → 2H2O + Energy
If 24 g of hydrogen is reacted with 24 g of oxygen to form water, then the amount of energy released in the reaction will be
- A )
- B )
- C )
- D )
Use the following information to answer the next question.
| The formation of water is accompanied by the release of a huge amount of energy, 572 kJ to be precise.This is an example of an exothermic reaction, a reaction that produces energy. It is also an example of combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen. | 2H2 + O2 → 2H2O + Energy |
If 24 g of hydrogen is reacted with 24 g of oxygen to form water, then the amount of energy released in the reaction will be
Hide Solution
The given reaction is:
2H2 + O2 → 2H2O…….. (1)
4 g+32 g2 × 18 = 36 g
2 moles1 mole
I.e., 2 moles of H2 react with 1 mole of oxygen.
24 g of H2 = 24/2 = 12 moles of H2 ---- --- ----- ---- ----- ------ ---->24/2 because it is dihydrogen.
24 g of O2 = 24/16 = 1.5 moles of O2 ---- ---- ------ ------- ---- ---->THISSTEP should be 24/32 = 0.75 moles since it is dioxygen.
Hence, hydrogen is present in excess, and some part of it will remain unreacted at the end of the reaction. Therefore, oxygen acts as a limiting reagent.
According to equation (1),
1 mole of O2 gives 2 moles of water where 1 144 kJ (572 × 2) of energy is released.
∴ 1.5 (24 g) moles of oxygen will give 3 moles of water, --- --- --- --- --- -- ->0.75 moles give 1.5 moles of water
i.e., energy released = 572 × 3 = 1 716 kJ -- --- --- --- --- -- --- --- --- --- ->Energy released = 572*0.75 = 429 J
The correct answer is C.
There were 2 mistakes actually, one is the no. of moles. Secondly, you've written that 1 mole of O2 gives 1144 kJof energy. By fact and as per the question, this is 572 J for 2 mol. of H2O .
The first mistake, as pointed out by you is correct. The number of moles of oxygen should be 24 / 32 = 0.75 as the molar mass of oxygen gas is 32 g. However, the answer to this question will be option (C) 1716 kJ. This is because energy is always determined in terms of kJmol-1 that is per mole of the product formed. The reaction between hydrogen and oxygen leading to the formation of 1 mole of water is as follows
H2 + 1/2 O2 → H2O (a)
the energy of 572 kJ is associated with the above reaction. Also note that in the above reaction, 0.5 moles or 16g of oxygen is required. So,
enthalpy change for the reaction with 0.5 moles of O2 → 572 kJmol-1
enthalpy change for the reaction with 1 mole of O2 → (572 / 0.5) kJmol-1
enthalpy change for the reaction with 0.75 moles of O2 → [(572 X 0.75) / 0.5] kJmol-1 = 858 kJmol-1
This would give the energy released per mole of water. However, we have to calculate the energy change for the reaction
2H2 + O2 → 2H2O
in which 2 moles of water are being formed. The above equation can be obtained by multiplying equation (a) with 2. Therefore, we will multiply equation (a) with 2 and the energy change associated with this using the given quantities (24 g H2 and 24 g O2) i.e. 858 kJmol-1.
Thus Δ H = 2 X 858 = 1716 kJmol-1.