Question Video: Bond Energy Changes in the Steam Reforming of Methane Chemistry

Hydrogen gas for industrial processes is usually produced by the steam reforming of methane (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. a) Give a balanced chemical equation for this reaction. b) Calculate the total change in bond energy for this reaction, per mole of hydrogen gas produced.

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Video Transcript

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.

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