Question

How much heat will be released when 12g of $ {H_2} $ reacts with 76 g of $ {O_2} $ according to the following equation?
 $ 2{H_2} + {O_2} \to 2{H_2}{{O \Delta H = - 571}}{\text{.6kJ}} $

Answer 1

Hint: This could be simply solved by applying the concepts of mole calculations. Also, we know that limiting reagent in a chemical reaction is a reactant that is totally consumed when the chemical reaction is completed, so here we need these concepts.

Formula used:
Here, we will use the number of moles formula:
 $ {\text{no of moles = }}\dfrac{{{\text{given mass}}}}{{{\text{molecular mass}}}} $

Complete step by step answer
We are given that 12g of $ {H_2} $ reacts with 76 g of $ {O_2} $ .
Given equation:
 $ 2{H_2} + {O_2} \to 2{H_2}{\text{O }} $
We are going to identify the moles of respective reactants.
Moles of dihydrogen= $ \dfrac{{12{\text{g}}}}{{2{\text{g}}{\text{.mo}}{{\text{l}}^{{\text{ - 1}}}}}} = 6{\text{mol}} $
Moles of dioxygen= $ \dfrac{{76{\text{g}}}}{{32{\text{g}}{\text{.mo}}{{\text{l}}^{{\text{ - 1}}}}}} = 2.38{\text{mol}} $
Given the stoichiometry, clearly, there is an insufficient molar quantity of dioxygen for complete combustion.
According to the given equation.
At most $ 4.75 $ mol hydrogen can react which will be = $ 2 \times 2.38{\text{mol}} $
And so, energy released can based on the molar quantity of dioxygen gas = $ 2.38 \times - 576.6{\text{kJ}}{\text{.mo}}{{\text{l}}^{{\text{ - 1}}}} = 1372.308{\text{kJ}} $
Dioxygen gas is the limiting reagent.
Thus, energy released can be based on the molar quantity of dioxygen gas = $ 1.3 \times {10^3}{\text{kJ}} $ .

Note:
It should always be kept in mind that limiting reagent is defined as the reactant in a chemical reaction that limits the amount of product that can be formed. The reaction will stop when all of the limiting reactant is consumed. Excess Reactant - The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant is completely consumed. In much the same way, a reactant in a chemical reaction can limit the amounts of products formed by the reaction. When this happens, we refer to the reactant as the limiting reactant (or limiting reagent).