Unit 6: Stoichiometry

How do manufacturers know how to make enough of their desired product?

Chocolate Chip Cookies

Cookie Recipe

1.5 c sugar

1 c. butter

3 eggs

2.25 c. flour

2 c. chocolate chips

makes 25 cookies

Using the following recipe, complete the questions.

1. If you have 6 c. of sugar, how many eggs will you need to use all of it?

2. If you have 8 c. flour, how many cookies (rounded to the nearest cookie) can you make?

Proportional Relationships

Stoichiometryis the measurement and calculation of the amounts of reactants and products in a chemical reaction (tells a chemist how much of an element or reactant to use, how much product

you can expect and specific amounts of substances)

Balanced chemical equations represent the relationship between the number of moles of reactants and the number of moles of products.

2 H2 + O2 2 H2 O

Balanced Equation: 2 H2 + O2 2 H2O moles 2 H2 1 O2 2 H2O grams 4 g 32 g 36 g

How many moles of O2 would you need to react if you had 4 moles of H2? ______How many grams of H2 would you need if you had 64 g of O2? _________How many grams of water would you be able to make with 8 g of H2? _______How many grams of O2 would be used if you had only 2 g of H2? _______

Based on Mole RatiosA conversion factor derived from the coefficients of a balanced equation

2 H2 + O2 2 H2 O

What is the molar ratio between H2 and O2?

What is the molar ratio between H2 and H2 O?

What is the molar ratio between H2 O and O2?

2 to 2

2 to 1

2 to 1

Example: 2K (s) + Br2 → 2KBr (s)

2 mol K 2 mol K 1 mol Br2

1 mol Br2 2 mol KBr 2 mol KBr

Balance the equation and write all possible mole ratios:

Al + CuSO4 → Al2(SO4)3 + Cu

In your notes, write down the molar relationships represented in the above equation.

What you derive from this chemical equation.

4Fe(s) + 3O2(g) → 2Fe2O3 (s)

Iron + oxygen → iron (III) oxide

4 atoms Fe + 3 molecules O2 → 2 formula units Fe2O3

4 mol Fe + 3 mol O2 → 2 mol Fe2O3

223.4 g Fe + 96.00 g O2 → 319.4 g Fe2O3

319.4 g reactants → 319.4 g products

4Fe(s) + 3O2(g) → 2Fe2O3 (s)

When doing stoichiometry problems…

1. Write the balanced equation.

2. Determine what starting value you are given and what unit your final answer should be in.

3. Set up the correct calculation.

Stoichiometry Road Map

How many moles of copper sulfate will react with 5.00 moles of aluminum?

1. Write balanced equation:

2. Calculation

1. Iron reacts with carbon dioxide to form iron (III) oxide and carbon monoxide. How many moles of carbon dioxide are needed to produce 2.2 moles of iron (III) oxide?

Balanced equation:

Calculation:

How many moles of carbon monoxide will also be produced?

Balanced equation:

Calculation:

When chlorine gas reacts with potassium iodide a single replacement reaction occurs. How many moles of potassium iodide will be needed to react with 2.45 moles of chlorine?

Balanced equation:

Calculation:

How many moles of iodine gas will be produced?

Balanced equation:

Calculation:

Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide gas. How many moles of sodium chloride will form from 0.65 moles of sodium carbonate?

Balanced equation:

Calculation:

Stoichiometry Road Map

How many grams of ferric oxide will be produced when 0.85 moles of iron reacts with oxygen in the air?

Balanced equation:

Calculation:

How many grams of aluminum will be needed to produce 2.94 moles of aluminum chloride if the aluminum reacts with an excess of hydrochloric acid?

Balanced equation:

Calculation:

How many moles of zinc sulfate are produces when 4.55 g of zinc reacts with an excess of sulfuric acid?

Balanced equation:

Calculation:

How many moles of phosphorus must be burned to produce 6.05 g of diphosphorus pentoxide?

Balanced equation:

Calculation:

Lithium nitride reacts with water to form ammonia, NH3, and lithium hydroxide. What mass of water is needed to react with 98.7 g of lithium nitride?

Balanced equation:

Calculation:

How many grams of carbon dioxide are produced when 3.785 g of methane are burned?

Balanced equation:

Calculation:

How many grams of silver nitrate are needed to react with 2.00 g of barium bromide in the double replacement reaction?

Balanced equation:

Calculation:

PERCENT YIELD

Percent YieldStoichiometric calculations are based on idealreactions.

Many reactions do not go to completion and not as much product is produced as expected.

We can express this amount as a percentage of what we expected to get out of a particular reaction.

Percent Yield

100yield ltheoretica

yield actualyield %

calculated on paper

measured in lab

1. Calculate the percent yield for the reaction

CO2 + H2O H2CO3

when 534 g of carbon dioxide react with an excess of water to produce 645 g of carbonic acid.

Actual yield = 645 g H2CO3

Theoretical yield = ? – how much should be produced?

g CO2 mol CO2mol H2CO3 g H2CO3

2. What is the % yield for the reaction

Cl2 + 2KBr 2KCl + Br2

in which 214 g of chlorine react with an excess of potassium bromide to produce 412 g of bromine?

LIMITING REACTANTS

Limiting Reactants

Available Ingredients–4 slices of bread–1 jar of peanut butter–1 jar of jelly

Limiting Reactant– bread

Excess Reactants– peanut butter and jelly

Limiting Reactant

Most of the time in chemistry we have more of one reactant than we need to completely use up the other reactant.

That reactant is said to be in excess (there is too much).

The other reactant limits how much product we get. Once it runs out, the reaction stops. This is called the limiting reactant.

Limiting Reactants

Limiting Reactant– first reactant used up in a reaction– determines the amount of product (limits

reaction)

Excess Reactant– added to ensure that the other reactant is

completely used up– cheaper & easier to recycle

1. If 4.7 moles of carbon reacts with 5.5 moles of oxygen to produce carbon monoxide, which is the limiting reactant?

2 C + O2 2 CO

Choose either reactant and calculate mol to mol for the other reactant:

mol C mol O2

4.7 mol C x 1_mol_O2 =

1 2 mol C

The limiting reactant is _______________.

2. What is the limiting reactant when 6.75 g of sodium reacts with 4.8 g of chlorine to produce sodium chloride?

Limiting reactant is _________________.

Photosynthesis equation:

6CO2 + 6H2O → C6H12O6 + 6O2

3. If a plant has 2.00g of CO2 and 2.00g of water at hand which is the limiting reactant?

4. What is the plants theoretical yield of glucose under these conditions? How much of each product can you produce if the reaction is 100% efficient? How much product can you produce if the reaction is only 87.6% efficient?

5. If the plant actually makes 1.11g of glucose what is its % yield of glucose?

6. How much of the excess reactant will be left unreacted?