Energy and Percent Yield

• Enthalpy is the energy stored in chemical bonds
• The Symbol of Enthalpy is H, Units of Joules (J)
• Change in Enthalpy is ∆H
• In exothermic rnxs Enthalpy decreases
• in edothermic rnxs Enthalpy increases

Here are the definitions of exothermic and endothermic reactions, in case you forget:

 

EXOTHERMIC:

 

The term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light, electricity or sound.

 

 

 

ENDOTHERMIC:

 

The word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat.

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)

These are related by the equation:

 

∆H = mC∆T

Example:

Calculate the heat required to warm a cup of 400 g of water (C= 41.8J/g°C) from 20.0°C to 50.0°C.

 

∆H = mC∆T

 

∆H =(400g)(4.18J/g°C)(50.0°C-20.0°C)

 

=50160 J

 

 

 

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?

 

********THIS EQUATION IS VERY IMPORTANT*********

 

Actual / Theoretical x 100% = % yield

 

                      or

 

Actual amount of product / Expected amount of product x 100% = % yield

 

 

Example: A student makes a single displacement reaction that produces 2.755 grams of copper. He determines that 3.150 grams of copper should have been produced. Find the student's percentage yield.

 

 

                                      actual amount of product

percentage yield = ------------------------------------------- x 100

                                    expected amount of product

 

                                   2.755g

percentage yield = --------------- x 100

                                   3.150g

 

percentage yield = 87.46 %

 

 

 

 

 

 Try these percent yield questions!