Question

Consider the following reaction and answer
${H_2} + C{l_2} \to 2HCl;\Delta H = - 184kJ$
What is the amount of energy absorbed when 0.365 kg of $HCl$ is formed?
(A) -920 kJ
(B) -1840 kJ
(C) -184 kJ
(D) -92 kJ

Answer 1

Firstly we will find the number of moles of $HCl$ from the given quantity in question. Now we know from the equation that for producing 2 moles of $HCl$ we need -184 kJ of energy. So using binary method or simple cross multiplication method, we can get the amount of energy for a given quantity of $HCl$ when we find moles from a given mass.

Complete Step by step answer: The given equation is:

${H_2} + C{l_2} \to 2HCl;\Delta H = - 184kJ$

From the above equation, using stoichiometry, we can say that one mole of hydrogen gas combines with one mole of chlorine gas to produce two moles of $HCl$ gas and in this process -184 kJ of energy is involved.
Now, in given question, we have been given 0.365 kg of $HCl$
We need to convert this into moles.
$n = \dfrac{m}{M}$
Here n= moles (we have to find out), m= mass of $HCl$ = 0.365 kg = 365 gm (given in question),
M= Molar mass of $HCl$ = Atomic Mass of H + Atomic Mass of Cl = 1 + 35.5 = 36.5
Now, we substitute these values in above equation to find out moles, n:
$n = \dfrac{{365}}{{36.5}} = 10 moles$
Now, we know that there are 10 moles of $HCl$ and we know we need -184 kJ energy for 2 moles $HCl$ to be produced. Therefore using cross multiplication or unitary method, we can write:
$\therefore {\text{Required energy = }}\dfrac{{10 \times ( - 184)}}{2}$
Hence on simplification, we get:
${\text{Required Energy}} = - 920kJ$

Thus the correct option is (A) -920 kJ.

Note: Be careful and convert kg into grams as for calculation of moles we need mass in grams. Now we can get moles easily using the formula of moles. Remember the concept of stoichiometry and the way in which they react.
See to it that we don’t convert units of energy from Kilojoules to joules, or any other unit. Take care of cross multiplication.
Remember atomic mass of elements to get molar mass of molecule, to calculate moles of $HCl$ formed.