Here in this example question it says the reaction of methane with chlorine gas is illustrated by the reaction below. Here we need to calculate the enthalpy of reaction, the standard enthalpy of reaction. If the standard enthalpies of formation for methane, carbon tetrachloride and hydrochloric acid are negative 74.871, negative 139 and -92.31 kilojoules per mole respectively.
All right, so step zeros. We check to see if the chemical reaction is balanced and if not then do the necessary steps to balance it. Here we already have the equation balance, so that's great. That means we can move on to step one. Step one says, starting with the products, multiply the coefficients so the numbers in red of each substance with their enthalpy of formation value. So here we're going to say that we have one mole of carbon texture chloride and four moles of HCL. We said here that carbon tetrachloride has a value associated with -139 kilojoules per mole, and hydrochloric acid has -92.31. Here the moles will cancel out and we'll have kilojoules left. So this is -925.24 kilojoules.
Next we're going to go to the reactants. Also multiply their coefficients of each substance with their enthalpies of formation. Now realize here that for CL2, I didn't give us an enthalpy of formation because earlier we said that if an element is in its standard or natural state, its enthalpy of formation is equal to 0. CL2 gas is the natural form of chlorine, so it has an enthalpy of 0. Methane is -74.87 and we have one mole, and here we have formals. Moles cancel out and we'll be left with kilojoules. 4 * 0 is 0, so that doesn't matter. So this is just simply negative 74.87 kilojoules.
Now that we have the amount for products and reactants, we go to step three. We take both totals and place them into the standard heat of reaction formula to determine the standard enthalpy of reaction. So products was -925.24 kilojoules and reactants was -74.87 kilojoules. Delta H standard on enthalpy of reaction equals products minus reactants. When we plug that in, that gives us -850.37 kilojoules as our answer.
So yet this is another way that we're able to calculate the standard enthalpy of reaction. In this case, we did it through the use of the enthalpies of formation for each of the substances within the chemical reaction.