Wednesday, February 9, 2011

LESSON 23 Energy & Percent Yield


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 rxns, enthalpy decreases
  • In endothermic rxns, enthalypy increases









CALORIMETRY
  •  To experimentally determine the heat released, we need to know 3 things 
  1. Temperature change (ΔT) 
  2. Mass (m) 
  3. Specific Heat Capaacity (CΝ = 4.19J / g °C) 
These are related by the equation:
ΔH = mCΔT 

Examples
Calculate the heat required to warm a cup of 500g of water (CΝ = 4.18J / g°C) from 20.0°C to 50.0 °C. 
ΔH = mCΔT 
= (500g)(4.18J / g°C) (50.0°C - 20.0°C) 
= 62 700 J = 6.3 x 10^4 J  = 62.7 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 x 100 
Examples
Production of Urea: 2NH3 + CO2  -> CO(NH2)2 + H2O 
If 32.1g of Urea are produced, determine theoretical yield of CO2? 
32.1g x  [mol/60g] = 0.535mol x [1/1] x [44.0g/mol] = 23.5 g 
What is the % yield of CO2 if 18.0g is produced? 
[18.0g/23.0g] x 100 =  78%
LIMITING REACTIONS
Usually one chemical gets used up before the other 
  • The chemical used up first is called the limiting reactant 
  • Once it's used up the reaction stops 
  • L.R. determines the quantity of products formed 
  • To determine L.R. assume one reactant is used up. Determine how much of this reactant is required. 
Find the L.R. when 2.8mol of H2 reacts with 1.25 mol of O2. 
2H2 + O2 -> 2H2O
2.8mol x [1/2] = 1.4mol of O2          O2 is L.R! 
1.24mol x [2/1] = 2.5mol of H2        O2 is L.R.! 


★ kim 


posted by chemistry11 @ 2:50 AM

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