Stoichiometry Notes

 In every chemical reaction, the mass and number of atoms are always conserved.  This is the law of conservation of mass  The mass of the reactants = the mass of the product

 The atoms in a chemical reaction are exactly the same atoms but are in a different combination after the reaction.  2H 2 +O 2 = 2H 2 O  There are still the same hydrogen and oxygen atoms on both sides of the equation.

Solving stoichiometry problems  The first step to almost all stoichiometry problems is converting to moles.  Usually we convert the amount of substances in grams to the amount of substance in moles

Mole-Mole Ratio  Allows us to convert from the amount of one substance in an equation to the amount of another substance in the same equation  2H 2 + O 2 = 2H 2 O  If given the amount of Hydrogen, we can determine how much water will form using the mole to mole ratio.

Mole-Mole Ratio  This is determined by the coefficient in front the substance from the balanced equation.  2H 2 + O 2 = 2H 2 O  For every 2 moles of Hydrogen we can produce 2 moles of H 2 O

Limiting Reactants  If 2 eggs and ¼ cup of oil are needed for one bag of brownie mix, how many bags of brownie mix do you need to use up 8 eggs?  How many bags of brownie mix to use 2 cups of oil?

Limiting Reactant  Which ever reactant (brownie mix, oil, eggs) allows you to make the least amount of product, is your limiting reactant.  This is what limits you from making more

Example  Using the situation from the previous slide, which of the following is your limiting reactant:  Reactants  12 eggs  3 cups of oil  10 bags of brownies

 If each bag of brownies makes one tray of brownies, how many trays of brownies can you make?

% Yield  % Yield = Actual Yield__ x 100% Theoretical Yield Actual Yield = The amount of product from the reaction Theoretical Yield = The maximum amount of product that you could have produced

Actual Yield vs Theoretical Yield  These can vary for multiple reasons:  Reactions don’t always go to completion  Impure substances (using tap water instead of distilled water)  Loss or spilling during lab  Competing side reactions

Review  What does stoichiometry allow us to do?  What is the first step in almost an stoichiometry problem?  What is the limiting reactant and how do you find it?  What is % yield?  Why are percent yield and actual yield usually different values?