In a chemical reaction, delta H represents the sum of the heats of formation, commonly measured in kilojoules per mol (kJ/mol), of the products minus the sum of those of the reactants. The letter H in this form is equal to a thermodynamic quantity called enthalpy, representing the total heat content of a system. Enthalpy, measured in joules (J), is equal to the system's internal energy plus the product of the pressure and the volume. The Greek letter delta looks like a triangle and is used in chemical equations to represent change. Calculating delta H involves balancing the reaction, adding the heats of formation and finding the difference between the heats of formation of the products and those of the reactants. This method assumes constant pressure within the system.
Balance the chemical reaction by making sure that you have the same number of atoms of each molecule on the reactant and product sides of the equation. In a simple example in which water and carbon react to form carbon monoxide and hydrogen gas, the balanced equation looks like this: H2O + C -> CO + H2. Notice that there are the same number of hydrogen, oxygen and carbon atoms on the left (reactant) and right (product) sides of the equation.
Look up the heats of formation for the compounds in your equation. There are heats of formation reference tables in most chemistry books, and this information can also be found through a simple online search. The heat of formation for liquid H2O is –285.83 kJ/mol and for CO is –110.53 kJ/mol, and the heats of formation for the elements H2 and C are both 0 kJ/mol. If you have a reaction with more than one molecule of a given compound, multiply the heat of formation value by the number of molecules of that particular compound in your reaction.
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Add together the heats of formation for the reactants, H2O + C, which is –285.83 kJ/mol + 0 kJ/mol = –285.83 kJ/mol.
Add together the heats of formation for the products, CO + H2, which is –110.53 kJ/mol + 0 kJ/mol = –110.53 kJ/mol.
Subtract the sum of the heats of formation of the reactants from that of the products to determine delta H: delta H = –110.53 kJ/mol – (–285.83 kJ/mol) = 175.3 kJ.