5.3 Measuring Energy Changes

Energy is absorbed when bonds are broken, and released when bonds are formed. You can then use bond enthalpy values to calculate ∆H in a reaction. 

*Bond enthalpy values are averages taken across a range of environments

∆H= ∑ Bonds Broken - ∑ Bonds Made

Example:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

 4 C-H Bonds

 2 O=O Bonds

Which yields C(g) + 4H(g) + 4O(g)

This then forms:

2 C=O Bonds

4 O-H Bonds

So

∆H= [4(C-H) + 2(O=O)] - [2(C=O) + 4(O-H)]

Wednesday 11/2/15 - Hess' Law

In class we did further review of Hess' Law, especially practicing how to use it to solve problems. For example, we had to practice reading a problem and drawing the Hess' Cycle involving various other equations to try and solve the ∆H for the original equation. We also had to read about the laws of thermodynamics, which are useful when learning about enthalpy.

Laws of Thermodynamics:

1. Conservation of energy (energy is neither created nor destroying, merely transferred)
2. Entropy will always increase
3. Reaching absolute zero in temperature (which would also be zero mass and zero motion) is impossible

Additional Review:

∆H for reaction is the same, no matter which route it takes, as long as conditions are the same

∆Hc: ∆H for complete combustion of one mole of a substance with oxygen
∆Hf: ∆H for forming one mole of substance from elements in standard state

Standard conditions: 298K (or just fixed temp.), 1 atm (or 100 KPa), 1M solutions

Bond enthalpy: ∆H to break bonds

Today, we began to review for the quiz on 5.2 that will be this Wednesday.

To review, we went over the Heat of Combustion for Magnesium Challenge, whereas from the results of a theoretical experiment, we processed the proposed data as to see what the Heat of Combustion for Magnesium was.

With help from each other we solved the task, though answers differed.

Hess' Law

3rd February 2015

In today's class we carried out a lab to demonstrate Hess' Law and accordingly increase our understanding of said law. The aim of the lab was to determine the enthalpy change of a particular reaction. However, since the actual reaction is immeasurable, we needed to combust two separate compounds in order to determine the  total enthalpy change of the reaction.



Hess' Law states that the route taken doesn't impact the final enthalpy change between reactants and products. Therefore, utilizing two separate reactions, we were able to find the enthalpy change of a single reaction.

We also continued to work on our lab reports in class. The aim of the lab was to determine the relationship between the enthalpy change of combustion of alcohols and their carbon chain length.