Hydrogen gas for industrial
processes is usually produced by the steam reforming of methane, or CH4. In this reversible process, methane
reacts with steam to produce hydrogen and carbon monoxide, CO. The energies of selected bonds are
listed in the table. Give a balanced chemical equation
for this reaction.
The problem tells us that we have
methane reacting with steam and reforming hydrogen and carbon monoxide. So, our reactants are methane, CH4,
and steam or gaseous water, H2O. And they’re reacting to form
hydrogen gas, H2, and carbon monoxide, CO. Now that we’ve identified the
products and the reactants for this reaction, let’s balance this equation.
We have one carbon in the reactants
and one carbon in the products. We have six hydrogens in the
reactants, but only two hydrogens in the products. And we have one oxygen in both the
reactants and the products. Since everything is balanced on
each side except for the amount of hydrogen in the products’ side, H2 should have a
stoichiometric coefficient of three. So, now that our equation is
balanced, we’ll see that the chemical equation for this reaction is CH4 plus H2O
reacting to form 3H2 plus CO.
Calculate the total change in bond
energy for this reaction, per mole of hydrogen gas produced.
During a chemical reaction, the
bonds between atoms in the reactants are broken. It takes energy to break these
chemical bonds. We call this energy the bond
energy, and it’s often given per mole of bonds broken. When the products are formed from
the individual atoms, new bonds are made. When a new chemical bond is formed,
it releases energy. So, for example, it’ll take 432
kilojoules of energy to break one mole of H–H bonds. So, if a mole of H–H bonds were
formed, the energy that is released would be 432 kilojoules.
So, we’ll be able to find the
change in bond energy for this reaction by summing the bond energies of the bonds
that are broken during the reaction and subtracting the sum of the bond energies
that are formed during the reaction. The energy of the bonds that are
formed is subtracted because this energy is released. Since the reactants have their
bonds broken while the products have their bonds formed, we could also think of this
as the bond energy of the reactants minus the bond energy of the products.
Before we calculate the change in
bond energy for this reaction, it’d be helpful to draw structures for each molecule
in the reaction so we know what bonds we have. So, these are the structures for
each of our products and each of our reactants. So, now let’s find the change in
bond energy. Methane has four C–H bonds, which
each have a bond energy of 411 kilojoules per mole. Water has two O–H bonds, which each
have a bond energy of 495 kilojoules per mole. We have three hydrogens in this
reaction. Each one has an H–H bond which has
a bond energy of 432 kilojoules per mole. Finally, carbon monoxide has a C≡O
triple bond, which has a bond energy of 1072 kilojoules per mole. Summing that together, we get 194
kilojoules per mole.
The question asked for the bond
energy per mole of hydrogen gas produced. And we’ve calculated the total
change in bond energy according to our balanced chemical equation which has three
moles of hydrogen gas. So, to find the total change in
bond energy for this reaction per mole of hydrogen gas produced, we need to divide
the number we just found by three, which gives us 65 kilojoules per mole.