12 stoichiometry for students

8/4/2019 12 Stoichiometry for Students

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Calculations from

Chemical Equations:Stoichiometry
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Objectives: Define stoichiometry and describe itsimportance

Relate stoichiometry to balanced chemical

equations Identify and solve different types ofstoichiometry problems

Determine the limiting reactant of a chemicalreaction

Calculate the amount of product formed in achemical reaction when reactants are present

in nonstoichiometric proportions


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Definition

is the calculation of quantitative(measurable) relationships of the

reactants and products in chemicalreactions (chemicals); that exists inchemical formulas and chemical

reactionsstoy-key-ah-meh-tree


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Definition

STOICHIOMETRY of a reaction dealswith the combining proportions of the

elements or compounds involved inchemical reactions.

It tells us the relationship of the moles

and weights of the reactant to oneanother and to the products.


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Etymology

Stoichiometry derived from the Greekwords

(stoikheion, meaningelement)

(metron, meaning measure)


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Have you ever used hydrogenperoxide to sterilize a minorcut?


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The fuel of some rocket is amixture of hydrazine N2H4 andhydrogen peroxide H2O2.


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N2H4 + 2 H2O2 N2 + 4 H2O


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1 mol N2H4 + mol 2 H2O21 mol N2 + 4 moles H2O

How you read this?

1 mole of hydrazine reacts with 2 moles

of hydrogen peroxide to produce 1 moleof nitrogen and 4 moles of water


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Consider a reaction:

2C2H2 + 5O2 4CO2+ 2 H2O

2moles 5moles 4moles 2moles


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Explanation

This explains that for a reaction toundergo, 2 moles of C2H2 should react

to 5 moles of O2 to produce 4 moles ofCO2 and 2 moles of H2O.

This is mole relationship.

It is very important that equationshould be balanced.


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Mole-Mole Relationship

Let us consider the reaction in whichnitrous oxide N2O is produced by

decomposition of ammonium nitrateNH4NO3.

NH4NO3 N2O + 2 H2O


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Suppose you want to find outhow many moles of N2O and

H2O are produced from 2.25

moles of NH4NO3.

Soln: Determine the molar ratios

indicated by the equation


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NH4NO3 N2O + 2 H2O

Molar ratios:1:1 NH4NO3 to N2O1:2 NH4NO3 to H2O


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H2O are produced from 2.25moles of NH4NO3.2.25 mol NH4NO3 X 1 mol N2O =4.50 mol N2O

1 mol NH4NO3


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H2O are produced from 2.25moles of NH4NO3.2.25 mol NH4NO3 X 2 mol H2O =2.25 mol H2O

1 mol NH4NO3


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The decomposition of 2.25moles of ammonium nitrateproduces 2.25 moles ofnitrous oxide and 4.50 molesof water.


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If we are given 10 moles of C2H2, howmany moles of CO2 would be produced?


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Consider a reaction:

2C2H2 + 5O2 4CO2+ 2 H2O

2moles 5moles 4moles 2moles


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How many moles of carbon dioxide willbe produced by the complete

combustion of 2.0 mole of glucose(C6H12O6)?


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PRACTICE PROBLEM:

1. Magnesium burns in oxygen to producemagnesium oxide. How many moles ofoxygen are needed to burns 0.52 mole ofmagnesium?

2. How many moles of Al (NO3)3 will beproduced when 0.75 mol AgNO3 reactsaccording to the following equation: 3 AgNO3+ Al Al (NO3)3 + 3 Ag?

ANSWERS :1. 0.52 mol O22. 0.25 mol Al (NO3)3


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Verifying the Law of Conservation of Matter

2 H2 + O2 2 H2O

Note: The Mass of the reactant and productis found by multiplying the molar mass ofthe substance by the number of moles ofthat substance in the balanced equation.The molar mass of H2 is 2 g/mol, of O2 is 32

g/mol, and of H2O is 18 g/mol.


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2 mol H2 (2 g H2 /1 mol H2 ) = 4 g+ 1 mol O2 ( 32 g O2 /1 mol O2 ) = 32 g

36 g2 mol H2O (18 g H2O/ 1 mol H2O) = 36 g


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QUANTITY OFGIVEN

STEPS IN SOLVING STOICHOIMETRYPROBLEMS

MOLES OF GIVEN

Convert to moles

MOLES OF

UNKNOWN

QUANTITY OFUNKNOWN

Convert todesired units


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MASS to MASS PROBLEMS

What mass of water is produced from 1.5grams of glucose?C6H12O6 + 6 O2 6 CO2 + 6 H2O

Step 1: Convert the given mass to molesusing the molar mass of glucose, which is180 g/mol

1.5 g C6H12O6 X 1 mol C6H12O6 = 0.0083 mol180 g C6H12O6 C6H12O6


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MASS OF GIVEN

STEPS IN SOLVING MASS TO MASSPROBLEMS

MOLES OF GIVEN

Convert to molesusing the molarmass of given

MOLES OF

UNKNOWN

MASS OFUNKNOWN

Convert massusing molar massof unknown

Find the molar ratio


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STEP 2: Determine the number of moles of waterthat are produced

0.0083 mol C6H12O6 X 6 mol H2O = 0.050 mol1 mol C6H12O6 H2O

STEP 3: Convert the number of moles of H2O intograms using molar mass. This time you need touse the molar mass of water, which is 18 g/mol

0.050 mol H2O X 18 g H2O = 0.90 g H2O1 mol H2O

Thus, 1.5 grams of glucose produce 0.90 g ofwater.


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ONE STEP WAY1.5 g C6H12O6 X 1 mol C6H12O6 X 6 mol H2O

180 g C6H12O6 1 mol C6H12O6

X 18 g H2O = 0.90 g H2O1 mol H2O


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Mass-Mass Relationship

Recall that from the weight and MW ofa compound, we can determine the

number of moles, n, [n = (W/MW)].We could therefore establish a weight-to-weight relationship of each

compound involved a balanced chemicalreaction.


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Sample Problem

Based on the balanced equation:

4NH3(g) + 5O2 4NO(g) + 6H2O(g)

How many grams of NH3 will be required toproduce 200g of H2O?


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Sample Problem

What is the weight in grams of H3PO4 isproduced from 250g of P4O10?

P4O10 + 6H2O 4H3PO4


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MASS- VOLUME PROBLEMS

When the car is involved in a collision, amotion sensor sets off a spark thatcauses sodium azide to decompose

explosively. The result is the productionof nitrogen gas, which quickly inflates thebag. The chemical equation for this

reaction is:2 NaN3 2 Na + 3 N2


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The ratio between moles of NaN3 and

moles of N2 is 2:3.

STEP 3 Convert the number of moles ofN2 to volume. The molar volume of a gas

at STP is 22.4 L/mol.


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125 g NaN3 X 1 mol NaN3 x 3 mol N2 x 22.4 L N265.0 g NaN3 2 mol NaN3 1 mol N2

= 64.6 L N2 ( STP )


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MASS OF GIVEN

STEPS IN SOLVING MASS TO VOLUMEPROBLEMS

MOLES OF GIVEN

Convert to molesusing the molarmass of given

MOLES OF

UNKNOWN

VOLUME OF GASEOUSUNKNOWN

Convert to volumeusing molar volume

of a gas

Find the molar ratio

Theoretical Versus Actual


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Theoretical Versus ActualYields

Previous stoichiometric calculations areassumed that all the reactants involved

in the reaction were converted toproducts.

This seldom happens in actual practice

because many reactions do not go tocompletion.

Theoretical Versus Actual


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Theoretical Versus ActualYields

They end up in an equilibrium state inwhich appreciable amounts of reactants

and products are left.At this state, the products are theactual yields


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Theoretical yield

Theoretical yield is the amount ofproduct obtained if the reaction goes to

completion, i.e., all reactants areconverted to products.


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Percent yield = actual yieldtheoretical yield

100


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Percent Yield Determination

Silver bromide was prepared byreacting 200 grams of magnesium

bromide and an adequate amount ofsilver nitrate. Calculate the percent yieldif 375 grams of silver bromide wasobtained from the reaction.


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Practice Exercise

The reaction of copper sulfate, and phosphine toform copper phosphide is given by:

3CuSO4 + 2PH3 Cu3P2 + 3H2SO4

When enough amount of PH3 was added to 100

grams of CuSO4, 40 grams of Cu3P2 wereobtained. Calculate the theoretical yield of Cu3P2and the percentage yield (MW CuSO4, = 160; MWCu3P2 =254)


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Limiting And Excess Reactants

Generally, the reactants are not broughttogether in the ratio in which they

react.Almost always, one of the reactants isneeded to follow all the reactants to

combine.


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Limiting And Excess Reactants

This reactants reacts completely and thepossible extent of a reaction depends highlyon the amount of this reactant.

This reactant is called limiting reactant.

Other reactants are in excess because acertain amount of them will be left unreacted

after a complete reaction.


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Guidelines

Select a reactant to be used as initialbasis for calculation.

Compute the theoretical weights of theother reactants using the basis.

Compare the theoretical weight to the

given actual weight of the reactant.


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Guidelines

If the theoretical weight is greater thanthe actual weight, the reactant is

already the limiting reactant.If not, i.e., the theoretical weight is lessthan the actual weight, the said

reactant is an excess reactant.


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Sample Problem

How many moles of HCl can beproduced by reacting 4.0 moles of

hydrogen and 3.5 moles of chlorine.Which compound is the limitingreactant?


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Sample Problem

Fluorine reacts with iron to produce iron (III)fluoride. If 5 g of fluorine is added to 10 g ofiron:

a. Determine the limiting and excess reactants

b. Calculate weight of iron (III) fluorideobtained from reaction.

c. How much of the excess will be leftunreacted?

12 stoichiometry for students

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