Enthalpy of reaction | Thermodynamics | AP Chemistry | Khan Academy

The change in enthalpy for a chemical reaction, delta H, we could even write delta H of reaction in here, is equal to the heat transferred during a chemical reaction at constant pressure. So, delta H is equal to qp.

Let's say we are performing a chemical reaction, an aqueous solution under constant atmospheric pressure. The reactants and products of that chemical reaction make up the system, and everything else makes up the surroundings.

When heat flows from the surroundings to the system, the system or the reaction absorbs heat, and therefore, the change in enthalpy is positive for the reaction. This is called an endothermic reaction. If heat flows from the system to the surroundings, the reaction gave off energy. Therefore, the change in enthalpy for the reaction is negative, and this is called an exothermic reaction.

As an example of a reaction, let's look at the decomposition of hydrogen peroxide to form liquid water and oxygen gas. The change in the enthalpy for this reaction is equal to negative 196 kilojoules. The negative sign means the reaction is exothermic.

And for the units, sometimes you might see kilojoules. Sometimes you might see kilojoules per mole, and sometimes you might see kilojoules per mole of reaction. What kilojoules per mole of reaction is referring to is how the equation is written.

So, if we look at this balanced equation, there's a two as a coefficient in front of hydrogen peroxide, and therefore two moles of hydrogen peroxide are decomposing to form two moles of water and one mole of oxygen gas. So when two moles of hydrogen peroxide decompose, 196 kilojoules of energy are given off.

Next, let's calculate how much heat is released when 5.00 grams of hydrogen peroxide decomposes at a constant pressure. The first step is to find out how many moles of hydrogen peroxide that we have. So we take the mass of hydrogen peroxide, which is five grams, and we divide that by the molar mass of hydrogen peroxide, which is 34.0 grams per mole. Grams cancel out, and this gives us 0.147 moles of hydrogen peroxide.

Next, we take our negative 196 kilojoules per mole of reaction, and we're gonna multiply this by a conversion factor. When we look at the balanced equation for how it's written, there are two moles of hydrogen peroxide. So, for our conversion factor, for every one mole of reaction as it is written, there are two moles of hydrogen peroxide.

So, two moles of H2O2. Now the of reaction will cancel out, and this gives us negative 98.0 kilojoules per one mole of H2O2. So, two moles of hydrogen peroxide would give off 196 kilojoules of energy. And one mole of hydrogen peroxide would give off half that amount or 98.0 kilojoules of energy.

Next, we take our 0.147 moles of hydrogen peroxide. So, let me just go ahead and write this down here really quickly. So we have 0.147 moles of H2O2. And remember, we're trying to calculate the amount of heat that was released.

So, next we multiply that by negative 98.0 kilojoules per mole of H2O2, and moles of H2O2 will cancel out, and this gives us our final answer. So, the heat that was released when 5.00 grams of hydrogen peroxide decompose at constant pressure, this turns out to be equal to negative 14.4 kilojoules.