Energy + Percent Yield

In today's class we learned how to calculate energy and also Percent Yield. Percent yield is basically the amount of products that should be formed in a reaction.

- Enthalpy is the energy stored in chemical bonds
- Symbol of Enthalpy is H
- units of Joules (J)
-Change in Enthalpy is ∆H
-In Exothermic reactions, enthalpy decreases
-In Endothermic reactions, enthalpy increases

Calorimetry:
-To experimentally determine the heat released we need to know 3 things:
1. Temperature change (∆T)
2. Mass (m)
3. Specific Heat Capacity (C)

They are all related by the equation:
∆H= mC∆T

e.g. Calculate the heat required to warm a cup of 700g of water (C= 4.18J/g°C) from 40.0° to 60.0°.
ΔH = mCΔT
= (700) (4.18) (20.0)
= 58 520 J / 1000
= 58.5 kJ


80.0 kJ of heat are added to a 700g glass of water initially at 40.0°C. Calculate the final temperature of the water (C= 4.18J/g°C).
ΔT = ΔH / Mc
= (80 000) / (700) (4.18)
= 27. 3°C
= 27. 3°C + 40.0°C
= 67.3 °C


Percent Yield
- The Theoretical yield of a reaction is the amount of products that should be formed
- Actual amount depends on the experiment
- The percent yield is like a measure of success
- How close is the actual amount to the predicted amount

Percent Yield = (Actual/ Theoretical) x100

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

2Na + O2 > Na2O2

3.24 x 1mol/23.0g x 1/2 x 78.0/1mol = 6.34g

% yield = 5.34/6.34 = 84%