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Stoichiometry

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Stoichiometry

Chapter 9

- In this chapter, we will revisit the mole concept and use it to relate quantities of reactants & products.

What is stoichiometry?

- Outside world connections – cooking, manufacturing, etc.
- Interpreting balanced chemical equations

What is stoichiometry?

- Chemical equation consists of reactants & products, coefficients are there to give # of particles involved (moles, molecules, formula units, atoms, etc.)
- Will later use coefficients from balanced equations to find actual amounts of substances (mass, volume, etc.)

French Toast

- 3 eggs, slightly beaten
- 1 c milk
- ½ tsp salt
- 6 slices of bread
- 2 Tbs sugar

French Toast

- The recipe makes 6 slices of French Toast. How would you make 12 slices?
- What is the ratio of eggs to bread? Did it change when the recipe was increased to make more servings?
- What would happen if you tried to make the recipe with only 1 egg?

Following a recipe is a lot like performing a chemical reaction. You have to have the right reactants and enough of each one or things will not turn out as planned.

Ratios are very important in balanced chemical equations.

- What is the ratio of H2 to O2 in the following equation?
- 2 H2 + O2 2 H2O

- Would the ratio change if more water was produced? Why?

How do I use stoichiometry?

- Follow the process for solving problems: qty of given moles of given moles of unknown qty of unknown

Mole-mole problems

- Converting from moles of one substance to moles of another substance requires a mole ratio from the balanced chemical equation

Mole-mole problems

Ex 1:

- A pink paint is produced by using 3 L of white & 1 L of red
- The paint ratio is ______
- If we only want 2 L of pink paint what do we need?
- Mix ___ L white & ___ L red

Mole-mole problems

Ex 2:

- NH4NO3 N2O + 2 H2O
- Suppose we wanted to know the number of moles of product created by 2.25 moles ammonium nitrate

Mole-mole problems

Ex 3:

- 2 HCl + Zn ZnCl2 + H2
- How many moles HCl needed to react with 5.70 moles Zn?

Mole-mole problems

Ex 4:

- N2O5 + H2O 2 HNO3
- How many moles HNO3 produced from 0.51 moles N2O5?

Mole-mole problems

Ex 5:

- Pb + 2 HCl PbCl2 + H2
- How many moles HCl needed to react with 0.36 moles Pb?

Verifying the law of conservation of matter

Balanced eqs “obey” the law:

- 2 H2 + O2 2 H2O
- 2 moles H2(@ 2.016 g/mole)= g
- 1 mole O2(@ 32.00 g/mole)= g
- 2 moles water(@ 18.016 g/mole) = g

4 types of problems:

1) Mass-mass – given mass of one substance & asked to find mass of another

- Process: (g of given) * (1 mole/molar mass of given) * (ratio of wanted to given) * (molar mass of wanted/1 mole)

4 types of problems:

1) Mass-mass

Example 1: How many grams of AgCl can be produced when 17.0 g AgNO3 reacts with NaCl?

4 types of problems:

1) Mass-mass

Example 2: How many grams of Cu2S can be produced from 9.90 g of CuCl reacting with H2S?

4 types of problems:

2) Mass-volume - given mass of one substance & asked to find volume of another

- Process: (g of given) * (1 mole/molar mass of given) * (ratio of wanted to given) * (molar volume/1 mole)

4 types of problems:

2) Mass-volume

Example 1: What volume of H2 can be produced (at STP) when 6.54 g Zn reacts with HCl?

4 types of problems:

2) Mass-volume

Example 2: What volume of ammonia can be produced from 14.01 g N2 reacting with H2?

4 types of problems:

3) Volume-mass - given volume of one substance & asked to find mass of another

- Process: (volume of given) * (1 mole/molar volume of given) * (ratio of wanted to given) * (mass/1 mole)

4 types of problems:

3) Volume-mass -

Example 1: How many grams of NaCl can be produced by reacting 112 mL Cl2 (@ STP) w/ Na?

4 types of problems:

3) Volume-mass -

Example 2: Bromine reacts with 5600 mL hydrogen to give what mass of HBr at STP?

4 types of problems:

4) Volume-volume - given volume of one substance & asked to find volume of another

- Process: (volume of given) * (1 mole/molar volume of given) * (ratio of wanted to given) * (molar volume/1 mole)

4 types of problems:

4) Volume-volume -

Example 1: How many L of O2 are needed to burn 1.00 L of methane (@ STP)?

4 types of problems:

4) Volume-volume -

Example 2: What volume of Br2 is produced if 75.2 L of Cl2 react with HBr?

Limiting reactant – substance that determines how much product may be formed (or how much reactant was available)

Limiting reactant –

- Identifying –
- Determine possible amount of desired substance (need to do 2 dimensional analysis problems)
- The limiting reactant is the substance that results in the least amount of desired substance

- Determine possible amount of desired substance (need to do 2 dimensional analysis problems)

Limiting reactant Problems

Ex. 12 Fe + 3 S Fe2S3

- Given 111.7 g Fe & 160.35 g S, how many grams of product can be formed?

Limiting reactant Problems

Ex. 2Cu + 2 AgNO3 Cu(NO3)2 + 2 Ag

- Given 3.5 g copper & 6.0 grams silver nitrate, how much silver can be formed?

Limiting reactant Problems

Ex. 3Zn + S ZnS

- Given 3.5 g each reactant, how much product formed?

- how much product is formed compared with expected results
% yield = (actual amount/expected amount) x 100

(units are in terms of %)

Ex. 1) You combined 4.3 liters of Cl2 with some Na. You were able to recover 15.9 g of NaCl. What was the percent yield?

- 2 Na + Cl2 2 NaCl

Ex. 2) 5.00 grams of Cu mixed with excess AgNO3. This gave 15.2 g of Ag. What was the percent yield?

- Cu + 2 AgNO3 Cu(NO3)2 + 2 Ag

Ex 3) If 320. grams of octane are burned in oxygen and 392 grams of water are produced. What was the percent yield?

- 2 C8H18 + 25 O2 16 CO2 + 18 H2O