February 2nd - Energy & Percent Yield

Today, we were introduced to enthalpy. Enthalpy is the energy stored in chemical bonds. The symbol for enthalpy is H and the units for enthalpy are Joules (J). Change in enthalpy is represented by ∆H. The symbol delta, as you know, represents a change in something.

In exothermic reactions, enthalpy decreases. This is because energy is released, so less energy is stored in the chemical bonds. In endothermic reactions, enthalpy increases. Energy is absorbed, increasing the amount of energy in the chemical bonds.

Just look at this wicked exothermic reaction!

Calorimetry is used to experimentally determine the heat released in a reaction. To perform calorimetric calculations, we need to know three things:

• Temperature change (∆T)
• Mass (m)
• Specific Heat Capacity (C)

These are all related by the equation ∆H = mC∆T.

Here are a few examples we can do:

Ex.) Calculate the heat required to warm a cup of 400g of water (C = 4.18 J/g ^oC) from 20.0 ^oC to 50.0 ^oC.

All we have to do for this one is plug in the numbers. We start with the formula ∆H = mC∆T.

∆H = mC∆T

∆H = (400)(4.18)(50.0-20.0)

∆H = 50160 = 50.2 kJ

Easy!

We also learned about percent yield. Percent yield is the theoretical yield of a reaction. In other words, it is the amount of product that should be formed. However, the actual amount depends on the experiment. It can be calculated using:

actual (what we found) 

theoretical (what we should’ve found)

Then, multiply everything by 100 to change the decimal into a percentage of the theoretical amount.

Ex.) Determine the percent yield for the reaction between 3.74g of Na and excess O₂ if 5.34g of Na₂O₂ is recovered.

The first step, like always, is to draw a free body diagram write a balanced equation:

2Na   +   O₂   à   Na₂O₂

Then, using what we know about molar mass conversions, we change the amount of sodium we have into our theoretical amount of Na₂O₂:

3.74g   x   mol/23.0g   x   (1/2)   x   78.0g/mol   =   6.34g of Na₂O₂

We’ve just found out the theoretical yield. The actually amount was the amount given in the question. Now, to calculate the percent yield, we use our formula:

(5.34/6.34)   x   100   =   84.2%

We have 82.4% of what we should have had. This means we have quite a bit of error somewhere in our experiment!

Getting perfect percent yield is quite difficult!

So fun, right? To learn more about percent error and enthalpy, visit your local library. You can also watch these videos:

Posted by Michael.