Energy and Percent Yield!

Enthalpy is the energy stored in chemical bonds. The symbol representing empalthy is H and change in empalthy is ΔH. Empalthy increases in endothermic reactions and decreases in exothermic reactions.
Exothermic Reaction



Endothermic Reaction



Calorimetry

To experimentally determine the heat released we need to know 3 things

1. Temperature change (ΔT)
2. Mass (m)
3. Specific Heat Capacity (C)

These are related by the question:
ΔH = mCΔT

Example:

Calculate the heat required to warm a cup of 800g of water (C= 8.36J/g°C) from 30.0° to 80.0°.

ΔH = mCΔT
= (800)(8.36)(50.0)
= 334 400J
= 3.34 x 10^5

Percent Yield

The theoretical yield of a reaction is the amount of products that should be formed. The actual amount depends on the experiment. So to get percent yield you have to put actual amount over the theoretical amount, or, put what you got over what you should have got.

Example:

1.98/2.00 = 99%

The percent yield is like a measure of success. It's how close the actual amount is to the predicted amount.

Example:

Find out the percent yield for the reaction between 4.78g of K and excess Se if 9.99g of K2Se is recovered

2K + Se -> K2Se

4.78g x 1mol/39.1 x 1/2 x 157.2/1mol = 9.61

9.99/9.61 = 104%

Lastly, here is a helpful video on determining percent yield!