Hess Law – Calculation for Methane formation

Calculation for Methane formation using Hess Law - Chemistry

METHANE GAS – CH4

What is the enthalpy change when methane is formed using carbon graphite and hydrogen?

When a chemical reaction takes place, heat is either given off or absorbed. If given off, it is called an exothermic reaction. If absorbed, it is called endothermic reaction.

In this chapter we want to find out about the heat change when METHANE (a gas) is formed using carbon graphite and hydrogen. Instead of using the word heat change, you can call it ENTHALPY change.

We use HESS LAW to determine the enthalpy change. At this stage we don’t know if the process is exothermic or endothermic and depending on whether the enthalpy change is negative or positive, it will tell us what whether the process is exothermic or endothermic.

 

  carbon graphite + hydrogen = methane

hess-law-methane-equation

Given the enthalpy change of the following combustion processes:
mehtane-formation-indi

You may be wondering why you’re given the values of enthalpy change for the three processes

Carbon combustion
Hydrogen combustion &
Methane combustion

This is because you can derive enthalpy change for methane formation by adding those three combustion together using Hess Law.

Watch a video – Enthalpy change for methane formation

Here’s a 2.5 minutes long video showing you how to calculate the enthalpy change for methane formation using Hess Law. It’s a really long calculation and don’t get discouraged. If you don’t get it the first time, watch the video again.

When you’ve watched the video, you can recap what you’ve learnt by following the rest of this page.

See a video example given by http://www.khanacademy.org

hess-law-methane-equationhess-law-enthalphy-change-question

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Life would be so easy if we could measure the methane forming process in the laboratory. However, a large problem exists — the enthalpy change for forming methane from carbon and hydrogen cannot be measured in the laboratory because the process is extremely slow to produce any meaningful results. So, Hess’s law comes in very useful because if you can use another set of processes to go from A to B, you can borrow the calculations to find out the enthalpy or heat change. As we explained earlier— it does not matter what route or how many steps, the overall enthalpy change is the same!

cowIn the methane forming process, you can use other reactions to calculate the enthalpy change. Instead you can conduct three different combustion processes which you’ll later learn how these equations combined together can produce the exact wanted methane equation — provided they have elements that make up the methane — moo!

  • combustion of carbon
  • combustion of hydrogen
  • combustion of methane

By recording the enthalpy change for these three processes, you can then solve for enthalpy change of methane by juggling around with these three equations because Hess’s law states it doesn’t matter what steps or how many steps as long as you are going from A to B under the same pressure.

So let’s try and work out the solution slowly and don’t worry if you get lost but keep going back and forth this page till you can understand.

In Hess’ law problems, we need to organize the chemical reactions so that the total effect yields the reaction needed. In order to do that, the following rules must be followed:

  • A reaction or process can be reversed.
  • If the reaction is reversed, the sign for the enthalpy ∆H (the values) must also be reversed.
  • The reaction can be multiplied by a constant.
  • The enthalpy ∆H must also be multiplied by the same constant if reaction is multiplied by a constant.

Methane formation – see a diagrammatic view of the calculation

How to caluclate the enthalpy change for methane formation using Hess Law

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