February 4th, 2010: Energy & Percent Yield (Zac)

-Enthalpy is the energy stored in chemical bonds
-symbol of Enthalpy is H
-Change in enthalpy is ΔH
-In exothermic rxns enthalpy
-In endothermic rxns enthalpy increases










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 equation:
ΔH=mCΔT

Example:
Calculate the heat required to warm a cup of 400g of water (C=4.19J/g°C) from 20.0°C to 50.0 °C.
Δ  H = (400)(4.18)(30)
Δ  H = 50160 °C

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 x 100


Example
-In a double replacement reaction Silver Nitrate is reacted with Barium Sulphate.  If 2.0 mol of Silver nitrate react and 468g of silver sulphate are produced, determine the theoretical yield of Ag2SO4, and determine the percent yield.
 
First, balance the equation: 2AgNO3 + BaSO4 = Ag2SO4 + Ba(NO3)2
Then, determine the theoretical yield of Silver sulphate: 2.0 mol x 1/2 x 312g/1mol = 312g of Ag2SO4
then divide actual by theoretical and multiply by 100: 468/312 x 100: 150 % yield