Stoichiometry Chemistry Chapter 12 " That which you persist in doing becomes easy to do - not that the nature of the thing has changed, but your power.

StoichiometryStoichiometryChemistryChemistry

Chapter 12Chapter 12

"That which you persist in doing becomes easy to do - not that the nature of the thing has changed, but your power and ability to do has increased." -- H.J. Grant

Chemistry & BakingChemistry & Baking

Chemistry is a lot like baking. What do Chemistry is a lot like baking. What do you need to bake chocolate chip cookies?you need to bake chocolate chip cookies?– RecipeRecipe

If you need more cookies, what are you If you need more cookies, what are you going to do to the recipe?going to do to the recipe?

In this example what are the reactants & In this example what are the reactants & the products?the products?A A balancedbalanced chemical equation provides chemical equation provides the same kind of quantitative information the same kind of quantitative information

that a recipe does!that a recipe does!

Using Balanced Chemical Using Balanced Chemical EquationsEquations

Chemist use Chemist use balancedbalanced chemical chemical equations as a basis to equations as a basis to calculate how calculate how much reactant is needed or product much reactant is needed or product is formed in a reactionis formed in a reaction..– If you know the quantity of one substance in a If you know the quantity of one substance in a

reaction, you can calculate the quantity of any reaction, you can calculate the quantity of any other substance consumed or createdother substance consumed or created..

The calculation of chemical quantities in The calculation of chemical quantities in chemical reactions is known as chemical reactions is known as stoichiometrystoichiometry..

Interpreting Chemical EquationsInterpreting Chemical Equations

A A balancedbalanced chemical equation can be chemical equation can be interpreted in terms of different quantities, such as:, such as:– # of atoms# of atoms - # and types of atoms - # and types of atoms– # of molecules# of molecules - Coefficients of a - Coefficients of a balancedbalanced

equation indicate the relative # of moleculesequation indicate the relative # of molecules– MolesMoles – Coefficients of a – Coefficients of a balancedbalanced

equation indicate the relative # of molesequation indicate the relative # of molesMassMass – can be calculated using mole / mass – can be calculated using mole / mass

relationship; law of conservation of mass.relationship; law of conservation of mass.VolumeVolume – can be calculate using mole / volume – can be calculate using mole / volume

relationship.relationship.

From From BalancedBalanced Equations to Moles Equations to Moles

A A balancedbalanced equation is essential equation is essential for for all calculations involving amounts of all calculations involving amounts of reactants & products.reactants & products.

NN22((gg) + 3H) + 3H22((gg) --) -- 2NH 2NH33((gg)) The most important interpretation of The most important interpretation of

this equation is that this equation is that 1 mol of nitrogen 1 mol of nitrogen reacts with 3 mol of hydrogen to reacts with 3 mol of hydrogen to form 2 mol of ammoniaform 2 mol of ammonia..– Based on this interpretation you can write Based on this interpretation you can write ratiosratios that relate moles of reactants to that relate moles of reactants to moles of product.moles of product.

Mole RatiosMole Ratios

A A mole ratio is a conversion factor derived from the coefficients of a derived from the coefficients of a balancedbalanced chemical equation chemical equation

NN22((gg) + 3H) + 3H22((gg) --) -- 2NH 2NH33((gg))

1 mol N2

3 mol of H2

2 mol of NH3

1 mol of N2

3 mol of H2

2 mol of NH3

Using Mole RatiosUsing Mole Ratios

In chemical calculations, mole ratios In chemical calculations, mole ratios are used to convert between are used to convert between – molesmoles of reactant and of reactant and molesmoles of product of product– molesmoles of reactants of reactants– molesmoles of products. of products.

Mole ratio is a conversion Mole ratio is a conversion factorfactor

Steps to SolveSteps to SolveStoichiometric CalculationsStoichiometric Calculations

In a typical stoichiometric problem:1. Balance the equation2. Convert given quantity to moles (if

needed)3. Then the mole ratio is used to convert

from moles of given to moles of wanted

4. Finally, the wanted moles are converted to specified unit of measurement, as required by the problem.

More on Cooking & ChemistryMore on Cooking & Chemistry

When baking, after getting your recipe, When baking, after getting your recipe, what do you do?what do you do?– Check ingredientsCheck ingredients

What happens if the cookie recipe calls What happens if the cookie recipe calls for 4 eggs and you only have 2?for 4 eggs and you only have 2?– Your recipe will be Your recipe will be limitedlimited by the fact that by the fact that

you only have 2 eggs; the eggs are the you only have 2 eggs; the eggs are the limiting ingredientslimiting ingredients..

A chemist often faces similar A chemist often faces similar situations.situations.

Limiting & Excess ReagentsLimiting & Excess Reagents

In a chemical reaction, an insufficient In a chemical reaction, an insufficient quantity of any of the reactants will quantity of any of the reactants will limit limit

the amount of product that formsthe amount of product that forms.. Limiting reagentsLimiting reagents – – the reagent that the reagent that

determines the amount of product that is determines the amount of product that is formed by a reaction. (formed by a reaction. (makes the leastmakes the least))– The reaction occurs until the limiting reagent The reaction occurs until the limiting reagent

is used up!is used up! Excess reagentExcess reagent – the reactant that is – the reactant that is

not used up.not used up.

What do we have to do next???

Now that we have moles, what next??Now that we have moles, what next??

Knows• Limiting reagent is Cu at 1.26 mol (from previous problem)

• 1 mol of Cu2S has a molar mass of • 159.1g

Next ???

What is mole ratio of Cu to Cu2S??

Now we have everything we need to Now we have everything we need to calculate how much product will be formed. calculate how much product will be formed.

So what’s next ??So what’s next ??

We should have started with 80.0g of Cu, however, we had already converted the 80.0g to moles (in previous problem).

Percent YieldPercent Yield

When When an equation is used to calculatean equation is used to calculate the the amount of product that will be formed during a amount of product that will be formed during a reaction, the calculated value represents the reaction, the calculated value represents the theoretical yieldtheoretical yield or the or the max product that could be produced from given amounts of reactants. from given amounts of reactants.

The amount of product actually formed during The amount of product actually formed during the reaction (in lab) is called the the reaction (in lab) is called the actual yieldactual yield..

The The percent yieldpercent yield is a ratio of the actual to is a ratio of the actual to theoretical yield expressed as a percent.theoretical yield expressed as a percent.

Percent Yield Percent Yield (cont)(cont)

The percent yield is a measure of the efficiency of a reaction carried out in the laboratory.

So how are we going to solve it??

What is mole ratio of CaCO3 to CaO ??

We have everything we need so let’s solve it !

Now let’s calculate the percent yield.Now let’s calculate the percent yield.

We have what we need so let’s solve it !