Stoichiometry

The study of quantities of substances in chemical reactions

Interpreting Chemical EquationsN2 + 3 H2 2 NH3

Particles:1 molecule of Nitrogen reacts with 3 moleculesof Hydrogen to produce 2 molecules of Ammonia (NH3).

Moles: 1 mole of Nitrogen reacts with 3 moles of Hydrogen to produce 2 moles of Ammonia (NH3).

The important thing to notice is that the coefficients determine the ratio of each reactant

to each product.

MassBalanced reactions must obey the law of conservation of mass.

Using the mole idea you can determine the mass of species in this equation:

used

1 mole N2 = 28 g3 mole H2 = 6 gproduced

2 moles NH3 = 34g

Volume@STP conditions

22.4 L of N2 reacts with 67.2 L of H2 to produce

44.8L of NH3

Notice that volume of gases is NOT conserved

22.4 + 67.2 ≠ 44.8

Interpreting Chemical EquationsN2 + 3 H2 2 NH3

Which of the following are conserved in a chemical reaction?

A. Volume of GasB. MolesC. MassD. # of Particles

C.Mass

The rearrangement of atoms during the reaction can change the # of everything else. But mass is conserved.

Mole-Mole CalculationsN2 + 3 H2 2 NH3 Relating moles of

reactants to moles of products.

We do not always use full moles of reactants, or want full moles of products.

We more often deal with partial moles.

If this reaction started with 2 moles of N2, how many moles of NH3 could be produced?

1:2 --- 2:4It’s a simple ratio

For more complicated ratios use this guide

Mole-Mole CalculationsN2 + 3 H2 2 NH3 So if we start with .75

moles of N2, how much ammonia is produced?

How many moles of H2must be used?

If we want to produce 3.18 moles of ammonia, how many moles of hydrogen do we need to use?

Known moles x

Coefficient of what you want to find out

Coefficient of what you know

4Al + 3 O2 2 Al2O3

Write three mole ratios from this equation

How many moles of Aluminum metal are needed to produce 3.7 moles of Aluminum Oxide?

How many moles of O2 are required to react completely with 14.8 moles of Al?

Calculate the moles of Al2O3 formed when 0.78n of O2 react with Al.

Mass-Mass CalculationsN2 + 3 H2 2 NH3

Our tools do not directly measure moles.

The balances measure…Mass (g)

So we often must convert to moles from mass, and mass from moles.

Solving mass-mass problems:

1. Convert the given quantity to MOLES.(Divide mass by molar mass)

2. Calculate unknown moles by multiplying by a mole ratio. (unknown/known)

3. Convert the answer in # 2 to mass.(Multiply by molar mass)

Mass-Mass CalculationsN2 + 3 H2 2 NH3

Solving mass-mass problems:

1. Convert the given quantity to MOLES.(Divide mass by molar mass)

2. Calculate unknown moles by multiplying by a mole ratio. (unknown/known)

3. Convert the answer in # 2 to mass.(Multiply by molar mass)

If 8.5 grams of Nitrogen are reacted, what mass of ammonia can be produced?

If 16.4 grams of Hydrogen are reacted, how many grams of Nitrogen are needed to fully react?

K2O + H2O 2 KOH

1. Calculate the mass of KOH produced when 4.80 g of K2O is reacted.

2. How many grams of water are needed for this reaction?

3. If 50.0 g of KOH is produced, what is the mass of Water used?

Solving mass-mass problems:

1. Convert the given quantity to MOLES.(Divide mass by gfm)

2. Calculate unknown moles by multiplying by a mole ratio.(unknown/known)

3. Convert the answer in # 2 to mass.(Multiply by gfm)

2 Al + 3 Cl2 2 AlCl3

If 70.9 g of Chlorine Gas are used, how many moles of Aluminum Chloride can be produced?

Solving mass-mass problems:

1. Convert the given quantity to MOLES.(Divide mass by molar mass)

2. Calculate unknown moles by multiplying by a mole ratio.(unknown/known)

What is the mass of the AlCl3 ?

3. Convert the answer in # 2 to mass.(Multiply by molar mass)

Other legs of the mole wheel

If given a mass of a reactant

you can find # of particles produced

If given the volume (@STP) of a reactant

you can find the mass produced

If given the # of molecules of a product

you can find # molecules of reactant

2 H2O2 2 H2O + O2

Starting with 29.2 g of hydrogen peroxide

How many molecules of Oxygen are produced?

What is the volume of the Oxygen gas produced? (assume STP conditions)

What mass of water is produced?

MnO2

Limiting Reagent analogy

IF you have:

40 slices of turkey

28 slices of cheese

6 heads of lettuce

1 mole of tomatoes

Jars of mayo and mustard

2 slices of bread

How many sandwiches can you make?

What limits your production of sandwiches?

Bread is your “limiting reagent”

Limiting Reagent analogy

Your reaction is your recipe Seldom do you have perfect amounts of each

ingredient.

The limiting reagent (reactant) is what determines the amount that is produced.

The other ingredients are considered to be excess reagents Cheese, turkey, tomatoes…

Limiting Reagent Definition

Limiting Reactant: (a.k.a. Limiting Reagent)

The substance that controls the quantity that can form In a chemical reaction.

Excess Reactant: (a.k.a. excess reagent)

The substance that is not used up completely in a reaction.

Determining Limiting Reagent2 Na + Cl2 2 NaCl

Starting with 6.7n of Na, and 3.2n of Cl2

How many moles of NaCl can be produced?

A. Determine limiting reagent

B. Use limiting reagent to determine amount of product formed.

1. Start with a known amount of one reactant.

2. Use the mole ratio to determine the amount of product.

3. Determine how much the other reactant would produce.

4. The reactant producing the LEAST is limiting.

2 Na + Cl2 2 NaClStarting with 6.70n of Na, and 3.20n of Cl2How many moles of NaCl can be produced?

If 6.7n Na are used

6.7 x ½ = 3.35n Cl2 needed

Compare the 3.35n Cl2 needed,to 3.20n Cl2 you have.

We have less Cl2 than we need so Chlorine is the limiting reagent.

Use the limiting reagent to determine the moles of NaCl produced.

1. Start with a known amount of one reactant.

2. Use the mole ratio to determine the amount of product.

3. Determine how much the other reactant would produce.

4. The reactant producing the LEAST is limiting.

2 Na + Cl2 2 NaClStarting with 6.70n of Na, and 3.20n of Cl2How many moles of NaCl can be produced?

We have determined the 3.2n of Cl2 to be limiting.

Now it becomes a regular mole-mole problem.

3.2n Cl2 x 2/1 =

6.4n NaCl

You could then find the mass of NaCl produced.

What determines the amount of product?

The limiting reagent!

16 Cu + S8 8 Cu2S

Starting with 1.25n Cu and .78n of S

determine the moles of Copper (I) Sulfide made.

What mass of Cu2S is made?

99.4 g of Copper sulfide

0.625 n of Cu2S

MassMoles

Mg + 2 HCl MgCl2 + H2

How many grams of Hydrogen can be produced when 6.00 g of HCl is reacted with 5.00 g of Mg?

1. Convert each reactant to moles.2. Determine limiting reagent by multiplying by the

mole ratio.3. Use limiting reagent to determine moles of product.4. Multiply moles of product by GFM to find mass

Find the volume of this Hydrogen @ STP

Massmoles

2 H2 + O2 2 H2O

Given 40grams of Hydrogen and 90 grams of Oxygen, determine the limiting reagent.

Determine the mass of water that can be produced.

How much excess reagent remains?

Mass Moles

Percent Yield

Why don’t you always get 100% on your chemistry tests?

There are many possible reasons You went skiing during the important lectures.

You worked on your AP history homework instead of studying for chem.

Rippet is not as good as he thinks he is.

% Yield

In Chemical Reactions we don’t always get 100% of the products our equations predict.

Remember the Magnesium Burning Lab? 2 Mg + O2 2 MgO

Theoretically all the Mg should have converted to MgO.

In reality, some things went wrong.

% Yield

An equation predicts the THEORETICAL YIELD

When a reaction is carried out in the lab we get

ACTUAL YIELD

2 Mg + O2 2 MgO

We reacted 2.5g of Mg, what mass of MgO could be produced?

Assume there is unlimited O2 from the air so Mg is the limiting reagent.

0.103 n of Mg x 2/2 = 0.103n MgO

We only had 3.61 grams produced…So we do NOT have 100% yield!

% Yield

Actual (lab results)

Theoretical (equation prediction)X 100 %

Multiply by 100, then add the “%” sign

3.61 / 4.15 = .869

.869 x 100 +”%”

= 86.9% Yield

CaCO3 CaO + CO2

What is the % Yield if 24.8 g of CaCO3 is heated to give 13.1g of CaO?

1. Convert g CaCO3 to moles of CaCO3 .Only 1 reactant, so it’s the limiting reagent.

Mole ratio is 1:1

2. Convert moles of CaO to grams CaO.

3. Use % Yield Formula: actual x 100%

theoretical

2 Al + 3 CuSO4 Al2(SO4)3 + 3 Cu

What is the % yield when 4.65g of Copper is made when 1.87g of Al reacts with excess Copper Sulfate.

1. Excess means there is plenty, the other reagent is limiting.

2. Convert grams Al to moles Al.

3. Find moles of Cu expected (multiply by mole ratio)

4. Convert moles Cu to grams Cu.1. This is the Theoretical amount of Copper

5. Use the % Yield Formula.

Which balloon will have the greatest volume of gas?NaHCO3 + HC2H3O2 NaC2H3O2 + CO2 + H2O

Reaction Mass of NaHCO3

Moles of Acetic Acid

1 0.50g 0.02n

2 1.00g 0.02n

3 1.50g 0.02n

4 2.00g 0.02n

5 3.00g 0.02n