If one mole rather than two moles of H₂O is produced from the following reaction, how much energy is produced? CH₄ gas plus 2O₂ gas react to form CO₂ gas plus 2H₂O gas plus 800 kilojoules. A) 100 kilojoules, B) 200 kilojoules, C) 400 kilojoules, D) 800 kilojoules, or E) 1200 kilojoules.
In the equation, we can see that on the product side, there is 800 kilojoules given out. It would take about 800 kilojoules to heat 2.5 litres of water from room temperature up to its boiling point. But what does it mean to have the energy as a term in the equation? Well, this is way too much energy for it to be released with one molecule of methane reacting with two molecules of oxygen. So, we can assume what we’re dealing with here are moles. One mole of CH₄ plus two moles of O₂ react to form one mole of CO₂ and two moles of H₂O, releasing 800 kilojoules of energy in the process.
So, our job is to work out how much energy is produced when only one mole of H₂O is produced from this reaction. At the moment, we’re getting 800 kilojoules of energy released per every two moles of H₂O. Which means the energy released per mole of H₂O is 400 kilojoules. If we multiply that by the number of moles of H₂O we’re producing, one mole, the energy released when we produce only one mole rather than two moles of H₂O is 400 kilojoules.