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### Course: AP®︎/College Chemistry>Unit 6

Lesson 5: Introduction to enthalpy of reaction

# Enthalpy of reaction

The enthalpy change that accompanies a chemical reaction is referred to as the enthalpy of reaction and is abbreviated ΔH_rxn. The value of ΔH_rxn depends on how the balanced equation for the reaction is written and is typically given in units of kJ/mol-rxn. Created by Jay.

## Want to join the conversation?

• I always understood that to calculate the change in ▲H° for a rxn or if you wanted to calculate any change such as ▲S or ▲G or anything, you did products minus reactants. But I came across a formula for ▲H of reaction(not the standard one with the ° symbol) and it said that it was equal to Σ bond energy of bonds broken + Σbond energy of bonds formed. When do I know when to use the ▲H formula and when the ▲H° formula? Thanks!
• 1. standard enthalpy (with the little circle) is the enthalpy, but always under one atmosphere of pressure and 25 degrees C.
2. I believe both equations can work for both types of enthalpy. To figure out which formula to use, you just look at the data you're given. Technically I think you can do either formula if you're given all the information. They should both give you the same delta h value.
3. For the bonds broken and formed formula, it tends to give slightly less accurate results because the bond energy is affected by surrounding atoms.
I'm not perfect with chemistry, so apologies if I got something wrong!
• At why is 1/2 the conversion factor? The reactant has 2 moles and the product has 3 moles.
• When Jay mentions one mole of the reaction, he means the balanced chemical equation. And in the balanced chemical equation there are two moles of hydrogen peroxide. That's why the conversion factor is (1 mol of rxn/2 mol of H2O2).

Hope that helps.
• Is the enthalpy of reaction the same as enthalpy of combustion
(1 vote)
• Somewhat. Enthalpy of reaction is the enthalpy change associated with any chemical reaction. As such the value can be negative or positive donating the reacting as exothermic or endothermic respectively.

Enthalpy of combustion is the enthalpy change associated specifically with a combustion reaction. Since combustion reactions famously give off heat (making them exothermic), their enthalpies are usually large negative values.

So enthalpy of reaction would be the general term, and enthalpy of combustion would be a subset within enthalpy of reaction.

Hope that helps.
(1 vote)
(1 vote)
• Since we are observing the reaction for h2o2, and we are given the enthalpy change for the reaction given (which includes 2 mols of h2o2), we know we must convert it to reflect the 1 mol of h2o2 desired. There are 2 moles h2o2, we want the enthalpy change to be for 1 mol h2o2, so we divide by 2. Dr. Skipper demonstrated it in a way that is analogous to dimensional analysis since that is already familiar from stoichiometry and is more difficult to mess up.
(1 vote)
• Why was the conversion factor 1/2?
If there is -196 kj/mol, and there are two moles of h2o2, why isn't it multiplied by two?
(1 vote)
• First, keep in the mind that in chemistry capitalized and lowercase letters mean different things. Most of the units and symbols you used should be capitalized.

The enthalpy change of the reaction has units of kJ/mol rxn (or mole of reaction). Meaning that the reaction as a whole (using the balanced chemical equation) has an enthalpy change of -196 kJ of energy. So the reaction, as written, will release 196 kJ of energy.

We need the conversion at to convert kJ/mol rxn to kJ/mol of H2O2 if we want to know how much energy is released based on the amount of hydrogen peroxide we use.

Hope that helps.
(1 vote)
• Okay, so wouldn't just dividing by two be easier than solving those equations?
(1 vote)

## Video transcript

- [Instructor] 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 cancels 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 H202. And remember, we're trying to calculate, 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 H202, 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.