8. Thermochemistry
Enthalpy of Formation
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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 74.87, negative 139, and negative 92.31 kilojoules per mole respectively. Alright. So step 0 is we check to see if the chemical reaction is balanced, and if not, then do the necessary steps to balance it. Here we we already have the equation balanced, so that's great. That means we can move on to step 1. Step 1 says, starting with the products, multiply the coefficients, so the numbers in red, of each substance with their enthalpy of formation value. Alright. So here we're gonna say that we have 1 mole of carbon tetrachloride and 4 moles of HCl. We said here that carbon tetrachloride has a value associated with negative 139 kilojoules per mole, and hydrochloric acid has negative 92.31. Here the moles will cancel out and we'll have kilojoules left. So this is negative 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 c l two, 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. C l two gas is the natural form of chlorine, so it has an enthalpy of 0. Methane is negative 74.87, and we have 1 mole, and here we have 4 moles. Moles cancel out and we'll be left with kilojoules. 4 times 0 is 0, so that doesn't matter. So this is just simply negative 74.8 7 kilojoules. Now that we have the amount for products and reactants, we go to step 3. We take both totals and place them into the standard heat of reaction formula to determine the standard enthalpy of reaction. So products was negative 925.24 kilojoules and reactants was negative 70 4.87 kilojoules. Delta h, standard enthalpy of reaction equals products minus reactants. When we plug that in that gives us negative 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.
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