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Stoichiometry

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Introduction to

Stoichiometry

Mr. Shumway

Chemistry 1

- "Stoichiometry" is derived from the Greek words
- στοιχεῖον (stoicheion, meaning element]) and
- μέτρον (metron, meaning measure.

- That’s why it’s a funny looking word

- Nevertheless,
- Stoichiometry is the science that deals with measuring the various elements in chemical reactions

- Write down all that you can about this reaction below

N2(g) + 3H2(g) 2 NH3(g)

- What do you look at when looking at a chemical equations? There is a lot of information in a chemical equation…
- Particles
- Moles
- Mass
- Volume

N2(g) + 3H2(g) 2 NH3(g)

Particles

- How many particles are there for each reactant?
- How many particles are there in the product?

- There is 1 molecule of Nitrogen reacting with 3 molecules of hydrogen gas
- Producing 2 molecules of ammonia

- What is the mole ratio for this reaction?

- The mole ratio for this reaction is 1:3:2

N2(g) + 3H2(g) 2 NH3(g)

H H

N

H H

N N

H H

H

H H

H H

N

H

N2(g) + 3H2(g) 2 NH3(g)

Moles

- How many moles are there for each reactant?
- How many moles are there in the product?

- There is 1 mol of Nitrogen reacting with 3 mol of hydrogen gas
- Producing 2 mol of ammonia

- What is the mole ratio for this reaction?

- The mole ratio for this reaction is 1:3:2

N2(g) + 3H2(g) 2 NH3(g)

Mass – Law of conservation of mass

- What is the mass of each reactant?
- What is the mass of each product?
- Ex
- 1 mol of N2 = 28.0 grams of N2
- 1 mol of H2 = 2.0 grams of H2 , therefore, 3 mol of H2 = 6.0 grams of H2
- 1 mol of NH3= 17 grams, therefore, 2 mol of NH3 = 34.0 grams
- The mass of the reactants = the mass of the products
28.0 + 6.0 = 34.0

N2(g) + 3H2(g) 2 NH3(g)

Volume

- 1 mol of a gas at STP = 22.4L
- Therefore, what are the volumes of each gas from the reactants?
- What is the volume of the product gas that is formed?
- Ex
- 22.4 L of N2 reacts with 67.2L of H2 (3mol x 22.4L)
- Forming 44.8L of ammonia

Let’s Look at this problem

2H2S(g) + 3O2(g) 2SO2(g) + 2H2O(g)

- Balance the following equation and write what down all the information that you can
- Number of particles
- Number of moles of reactants and products
- Mass of reactants and products
- Volumes of gases at STP (if applicable)

- C2H2(g) + O2(g) CO2(g) + H2O(g)
- Na(s) + H2O(l)NaOH(aq) + H2(g)
- CO(g) +O2(g) CO2(g)

- What do the coefficients tell you in a chemical reaction?
- What can the coefficients tell you?

- What do the coefficients tell you in a chemical reaction?
- The coefficients tell you the number of molecules/atoms there are in a chemical reaction.
- What can the coefficients tell you?
- They can tell you the mole ratios between each of the reactants and products

Mole to Mole Ratios

Stoichiometry

Mr. Shumway

Chemistry 1

We can use the information from the chemical

equation and use them to help us calculate

useful information

- Mole – Mole Calculations
- Mass – Mass Calculations
- Mass-mole and mole mass conversions

- Mole – Mole Calculations
- Knowing the amount of moles of reactants, we can calculate the number of moles of product
- Using the mole ratios of the chemical equation

- Mole – Mole Calculations
How many moles of ammonia are produced

when 0.60 mol of nitrogen reacts with

hydrogen?

N2(g) + 3H2(g) 2NH3(g)

This is what we want

This is what we have

How many moles of ammonia are produced

when 0.60 mol of nitrogen reacts with

hydrogen?

N2(g) + 3H2(g) 2NH3(g)

1

What is the ratio/relationship between these molecules?

How many moles of ammonia are produced

when 0.60 mol of nitrogen reacts with

hydrogen?

N2(g) + 3H2(g) 2NH3(g)

2 mol of NH3(g)

= 1.2 mol NH3

0.60 mol of Nitrogen

X

1 mol of N2(g)

Write down what you have first

Because we have the number of moles, we need a conversion factor that has mol of Nitrogen in it

“A mole ratio”

Let’s look at this problem on the board

- Calculate the number of moles of reactants required to make 7.24 mol of Ammonia
N2(g) + 3H2(g) 2NH3(g)

The formation of aluminum oxide from its constituent elements is represented by this equation

- How many moles of aluminum are needed to form 2.3 mol of Al2O3?
- How many moles of oxygen are required to react completely with 0.84 mol of Al?
- Calculate the number of moles of Al2O3 formed when 17.2 mol of O2 reacts with aluminum.

- Design a step-by-step method that will help you make conversions between moles to moles in a chemical reaction.

Mole to Mass Calculations

Stoichiometry

Mr. Shumway

Chemistry 1

- Review
- You can calculate the number of moles of product through the mole ratios of the chemical equation.
- Following through with step by step dimensional analysis

- You can calculate the number of moles of product through the mole ratios of the chemical equation.
- If we can calculate the number of moles, we can also calculate the mass of the product and/or the reactants.

A + X AX

Steps to Follow

- It is important that if you are given grams to first convert to moles because you can use the mole ratio from the chemical equation
- From moles of X to moles of AX (using the mole ratio)
- Convert moles of AX to grams of AX

- Calculate the number of grams of NH3 produced by the reaction of 5.40g of hydrogen with nitrogen
N2(g) + 3H2(g) 2NH3(g)

- Step 1: calculate the number of moles by the mass given
N2(g) + 3H2(g) 2NH3(g)

1 mol of H2(g)

= 2.7 mol H2

5.40 g of H2 gas

X

2.0 g of H2(g)

Write down what you have first

Because we have the grams of Hydrogen, we need a conversion factor that has grams/mol with the proper molecules. Using the molar mass of the molecule.

- Step 2: Convert from moles to moles
N2(g) + 3H2(g) 2NH3(g)

2 mol of NH3(g)

= 1.8 mol NH3(g)

2.70 mol of H2 gas

X

3 mol of H2(g)

Write down what you got from the last problem

Because we have the moles of Hydrogen, we need a conversion factor that will give us the number of moles of ammonia gas, using the mole ratio in the chemical reaction.

Essentially, we’re converting from moles given to moles desired

- Step 3: Convert from moles to grams
N2(g) + 3H2(g) 2NH3(g)

17.0 g of NH3(g)

= 30.6 g NH3(g)

1.80 mol of NH3(g)

X

1 mol of NH3(g)

Write down what you got from the last problem

Because we have the moles of ammonia, we need a conversion factor that will give us the grams of ammonia gas, using the molar mass of the molecule.

The combustion of acetylene gas is represented

by this equation.

2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g)

- How many grams of oxygen are required to burn 13.0 g of C2H2?
- How many grams of CO2 and grams of H2O are produced when 13.0g of C2H2 reacts with the oxygen required to burn 13.0g of C2H2?

Acetylene gas, C2H2, is produced by adding water to calcium carbide, CaC2

CaC2(s) + 2H2O(l) C2H2 + Ca(OH)2(aq)

- How many grams of acetylene are produced by adding water to 5.00g of CaC2?
- How many moles of CaC2 are needed to react completely with 98.0 g of H2O?

- Construct a flow chart/map of how to solve the problems you’ve encountered thus far.

- Get into your table groups.
- Each group will have a different problem to work on, you will have 20 min to solve the problem. Everyone in the group MUST KNOW HOW TO SOLVE THE PROBLEM
- One random person will be chosen to come to the board and do the problem and explain their work, this will determine the groups grade.

- What is a limiting reagent that you described in your mini lab?

Limiting Reagent

Stoichiometry

Mr. Shumway

Chemistry 1

- Limiting reagent
- Limits or determines the amount of product that can be formed in a reaction

- Excess reagent
- Is more than enough to react with a limiting reagent

Ex. Problem

Sodium Chloride is prepared by the reaction of sodium metal with chlorine gas. What will occur when 6.70 mol of Na reacts with 3.20 mol of Cl2?

What is the limiting reagent?

How many moles of NaCl are produced?

How much excess reagent remains unreacted?

- Step 1- What do we know?
- We know/have
- 6.70 mol of Na
- 3.20 mol of Cl2

- We know/have
- Step 2 – What do we want to know?
- We want to calculate three things
- What is the limiting reagent?
- How many moles of NaCl are produced
- How much excess reagent remains unreactedd

- We want to calculate three things

- Step 3 – Write down a BALANCED chemical reaction
- Step 4- convert what you know into moles if not already

- Step 5 – Look at the mole ratio and calculate the amount of substance needed for a complete reaction
- Use this information to determine the limiting reagent and excess reagent and label them.

Start with what we know

Required amount of Chlorine

Mole Ratio

- 1 mol of Cl2

- = 3.35 mol of Cl2

- 6.70 mol of Na x

- 2 mol of Na

Knowing that we have 6.70 mol of Na and knowing the mole ratio between sodium and chlorine, we can calculate the minimum amount of chlorine we need for this reaction to take place

Using the mole ratio, we can see that 3.35 mol of Cl2 is needed to react with 6.70 mol of Na

Because we do not have enough Cl2,

Cl2 is the limiting reagent.

How much Chlorine did the problem say we had?

Can someone explain why?

3.20 mol of Cl2!

Mole Ratio

Because Cl2 is the limiting reagent, the amount of product is determined by the amount of Cl2 we have.

Total amount of product formed

Therefore, the number of moles of Cl2 is going to be used to calculate the maximum amount of product that can be formed.

HOW MUCH EXCESS DO WE HAVE? ( Na?)

Mole Ratio

The amount of excess reagent remaining is the difference between the given amount (in the beginning of the problem, a value of 6.70 mol of Na) and the amount of sodium needed to react with the limiting reagent.

Amount of Na used up in the reaction

6.70 mol Na – 6.40 mol Na = 0.30 mol Na in excess

Therefore, we need to now calculate how much Na would be used up in the reaction with Cl2

Copper reacts with sulfur to form copper (I)

sulfide.

What is the maximum number of grams of Cu2S that can be formed when 1.87 mol of Cu reacts with 2.19 mol of S?

Hydrogen gas can be produced in the laboratory by the reaction of magnesium metal with hydrochloric acid.

How many grams of hydrogen can be produced when 1.45 mol of HCl is added to 2.31mol of Mg?

Assuming STP, what is the volume of this hydrogen?

Write down a systematic approach to dealing

with Limiting reagent type problems. How

would you solve this type of problem, step-by

step?

- What is a limiting reagent? How can you know which reactant is the limiting reagent?
- What is an excess reagents? How can you know which reactant is the excess reagent?

Percent Yield

Stoichiometry

Mr. Shumway

Chemistry 1

Theoretical Yield

- When an equation is used to calculate the amount of product that will form during a reaction, the value is theoretical.
Actual Yield

- The amount of product that forms when the reaction is carried out in the laboratory
Percent Yield

- Is the ratio of the actual yield to the theoretical yield. It measure the efficiency of the reaction

What is the percent yield of this reaction if

24.8g of CaCO3 is heated to give 13.1g of CaO?

Looking at this problem, let’s identify the important information

This problem tells us the actual yield, 13.1g of CaO

What do we want?

We have the actual yield, now we need the theoretical.

And we have all we need to solve for the theoretical yield.

Converting to moles using molar mass of CaCO3

Mole ratio, looking at the coefficients

Converting to grams using molar mass of CaO

Given

24.8 g CaCO3

This is our theoretical yield.

Now we can plug this into our equation

What is the percent yield if 3.74g of copper is produced when 1.87g of aluminum is reacted with an excess of copper (II) sulfate?

Energy Changes in Chemical Reactions

Stoichiometry

Mr. Shumway

Chemistry 1

Exothermic Reactions

- Releases energy in the form of heat in a chemical reaction
Endothermic Reactions

- Energy is absorbed in a chemical reaction
Thermochemical Equation

- An equation that includes the amount of heat produced or absorbed by a chemical reaction.

Exothermic Reaction

“Releases Energy”

Or gives off energy

Where would the energy be in the chemical reaction?

The products or reactants?

The Energy would be in the products

Endothermic Reaction

“Absorbs Energy”

Or requires energy

Where would the energy be in the chemical reaction?

The products or reactants?

This equation is called a thermochemical equation

The Energy would be in the reactants

We can use the thermochemical equation to

help us calculate the energy required for a

chemical equation.

Think of this as another conversion factor to

add

Using the equation just given, calculate the

energy required to decompose 5.20 mol of

CaCO3 (in Kilojoules)

1

5.20 molCaCO3

= 915 KJ

Enthalpy

Is the amount of heat that a substance has at a given temperature and pressure.

Is symbolized by “H”

If an exothermic reaction is taking place then the reaction has a “-ΔH”

If an endothermic reaction is taking place then the reaction has a “+ΔH”

Standard Heat of Formation

is where the “ΔH” for a reaction in which one mole of a compound is formed from its elements of that compound.

The air pollutant sulfur trioxide reacts with water in the atmosphere to produce sulfuric acid and heat.

How much heat is released when 583 g of SO3(g) reacts with water?

Carbon dioxide can be decomposed into carbon

monoxide and oxygen by the absorption of

heat.

How many molecules of CO2(g) can be decomposed by the addition of 22.2KJ of heat energy?