Percent Yield and Energy in Stoichiometry

Energy

• Enthalpy is the energy stored in chemical bonds
• Symbol of Enthalpy is H

                - units of joules (J) 

• Change in Enthalpy is ΔH 
• In exothermic reactions, enthalpy decreases
• In endothermic reactions, enthalpy increases

          

Calorimetry

• to experimentally determine the heat release we need to know 3 things

1. Tempurature change (ΔT)
2.  Mass (m)
3. Specific heat capacity (C)

• These are all related by the equation: ΔH = mCΔT

Example:

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

 ΔH = mCΔT

        =(400g)(4.18J)(30°C)

        =50160 J

        =50.02 kJ

Percent Yield

•  The theoretical yield of a reaction is the amount of products that should be formed.
• The 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?

%Yield = Actual/Theoretical

 Example:

The production of Urea: 2NH3 + CO2 → CO(NO2)2 + H2O

If 47.7g of Urea is produced, determine the theoretical yield of CO2.

What is the percent yield of CO2 if 12.0g is produced.

47.7g (mol/60g)(1/1)(44g/mol) = 35g

12.0/35 = 34%

-Benedict Suratos