Balancing Equations and Stoichiometry

Chemical Equations

Terms:(s) = solid(l) = liquid(g) = gas

D = heat(aq) = aqueous

solution

Balancing Equations• * Use coefficients to balance equations!

• Step 1: Balance metals first.

• Step 2: If possible, consider poly-atomic ions as a group. If “OH” is present on one side and H2O is present on the other side, break up water into H and OH.

Balancing Equations

• Step 3: Balance other elements

Step 4: Balance H’s and O’s last.

• Step 4: Double-check.

Sample Problem

• Balance the reaction:• Cu + AgNO3 Ag +

Cu(NO3)2

• Ca(OH)2 + H3PO4 H2O +

Ca3(PO4)2

Stoichiometric Calculations

• Given the reaction:

• C3H8 + 5O2 CO2 +

4H2O

• Info: molar ratios

Problem

• C3H8 + O2 CO2 +

4H2O

• If 25 grams of C3H8 is used, how much O2 is needed?

Before anything else….

• I. Find the molar masses of each compound in the reaction.

• II. Calculate the mole of the compound given using the equation:

mole = gram / molar mass

Solution

• 1. Balance equation.• 2. Get molar ratios from

balanced equation.• 3. Find actual moles using

given masses.

Solution (cont.)

• 4. Re-adjust moles.• 5. Convert moles to grams

if required.

Steps in Stoichiometry• 1. Get the molar masses of each cpd in

the equation.

• 2. If grams are given, convert grams to moles using the equation: mole = gram/molar mass

• 3. Balance the equation.

• 4. If only 1 mass is given (Case I), there is no limiting reagent. Re-adjust each mole using the molar ratios from the balanced equation.

• 5. If more than 1 mass is given, there is a LIMITING REAGENT! Base all actual moles of needed reactant and desired product on the Limiting Reagent (not on the Excess)! (Case II)

• 6. Convert all moles to grams using the equation: gram = mole x molar mass

• 7. Grams of Reactants must equal grams of products.

Limiting and Excess Reagents

• Limiting reagent = limits the amt. of product that can form

• Excess Reagent = reagent that is over and above what is needed

Case II Stoichiometry

• Has a limiting and excess reagent

• Case II applies when there are 2 or more given masses or moles

• 5. Convert moles to grams, if needed.Gram = mole x molar

mass

• 6. Calculate % Yield and % Error, if needed.

Determining the Limiting Reagent

• To determine the limiting reagent, divide all calculated moles by the coefficients in the balanced reaction. The smallest value is the Limiting Reagent.

• Please note: Do not use these values for the rest of your calculations. This is only for the IDENTIFICATION of the Limiting Reagent!

Yields

• Theoretical Yield –the amount of product formed when the limiting reagent is totally consumed

Yield

• Actual Yield - often given as percent yield% Yield = actual yield X 100

• theoretical yield