Hess's Law Simplified: Calculating Delta H

When working with chemical equations, we can manipulate them in three main ways.First, we can reverse a reaction. When we do this, we change the sign of delta H from negative to positive or vice versa.Notice how the products and reactants switch places, and the delta H value changes from negative 484 to positive 484 kilojoules.Second, we can multiply an equation by a coefficient. When we do this, we multiply delta H by the same number.Here, when we multiply the equation by 2, delta H changes from negative 242 to negative 484 kilojoules.Finally, we can combine equations by adding them together. When we do this, we also add their delta H values.In this example, combining these reactions gives us a new equation with a delta H of negative 414 kilojoules, which is the sum of negative 394 and negative 20 kilojoules.These manipulation rules are essential tools that allow us to calculate unknown enthalpy changes using known reactions.