What is the molar enthalpy of combustion of magnesium? This was the lab question that I was asked to find after performing a lab based on Hess' law. I can't find anywhere in my notes how to calculate this, and the text book doesn't explain their steps properly, and I have different information than the book does. I was wondering how to go about doing this? I am wondering, is the enthalpy change the same thing? I know how to calculate the enthalpy change, where you break up the question into parts finding molar mass, and then using delta H= mct/n but I am not sure if this is the same thing.

Question

What is the molar enthalpy of combustion of magnesium? This was the lab question that I was asked to find after performing a lab based on Hess' law. I can't find anywhere in my notes how to calculate this, and the text book doesn't explain their steps properly, and I have different information than the book does. I was wondering how to go about doing this? I am wondering, is the enthalpy change the same thing? I know how to calculate the enthalpy change, where you break up the question into parts finding molar mass, and then using delta H= mct/n but I am not sure if this is the same thing.

Thanks.

Answer 1

If you can do enthalpy change for the experiment (the combustion), that will be for so many grams of Mg, then multiply by molar mass Mg to get molar enthalpy of combustion.

Answer 2

To calculate the molar enthalpy of combustion of magnesium, you can use the principles of Hess's law and the enthalpy change during the reaction.

The molar enthalpy of combustion refers to the enthalpy change that occurs when one mole of magnesium burns completely in oxygen to form magnesium oxide. It represents the heat energy released during this reaction.

Here are the step-by-step instructions to calculate the molar enthalpy of combustion:

1. Determine the thermochemical equations: In this case, you need to find the chemical equation representing the combustion of magnesium. It will specify that magnesium reacts with oxygen to form magnesium oxide.

2. Collect data: Determine the enthalpy changes (ΔH) for the other chemical reactions involved in Hess's law. These reactions should include the combustion of magnesium in oxygen and some reference reactions whose enthalpy changes are known.

3. Apply Hess's law: According to Hess's law, the overall enthalpy change of a reaction is equal to the sum of the enthalpy changes of the individual reactions that make up the overall reaction. Use the known enthalpy changes to construct an equation that cancels out the spectator species, leaving only the desired reaction.

4. Manipulate equations: Adjust the coefficients of the chemical equations so that they sum up to the desired overall equation. This should reflect the stoichiometric ratio between the reactants and products in the balanced chemical equation for the combustion of magnesium.

5. Calculate the molar enthalpy: Once you have the desired equation, the enthalpy change of that equation will give you the molar enthalpy of combustion of magnesium.

Note: The enthalpy change and molar enthalpy are related but not the same. The enthalpy change (ΔH) represents the heat exchange during a specific reaction, whereas the molar enthalpy is the enthalpy change per mole of substance involved in the reaction.

Remember, having accurate data and precise measurements in your lab experiment is crucial for obtaining reliable results.