8.3 limiting reactants_2

8/13/2019 8.3 Limiting Reactants_2

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Chapter 8 Chemical

Quantities in Reactions

8.3

Limiting Reactants

Copyright 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings


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Limiting Reactant

A limiting reactantin a chemical reaction is the

substance that

Is used up first.

Stops the reaction.

Limits the amount of product that can form.


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Reacting Amounts

In a table setting, there is 1

plate, 1 fork, 1 knife, and

1 spoon.

How many table settings are

possible from 5 plates, 6 forks,

4 spoons, and 7 knives?

What is the limiting item?

Copyright 2008 by Pearson Education, Inc.

Publishing as Benjamin Cummings


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Reacting Amounts

Four table settings can be made.

Initially Use Left over

plates 5 4 1

forks 6 4 2

spoons 4 4 0

knives 7 4 3

The limiting item is the spoon.


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Example of Everyday Limiting

Reactant

How many peanut butter sandwiches could be

made from 8 slices bread and 1 jar of peanut

butter?

With 8 slices of bread, only 4 sandwiches

could be made. The bread is the limiting item.

Copyright 2008 by Pearson Education, Inc. publishing as Benjamin Cummings


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Example of Everyday Limiting

Reactant

How many peanut butter sandwiches could be

made from 8 slices bread and 1 tablespoon of

peanut butter?

With 1 tablespoon of peanut butter, only 1sandwich could be made. The peanut butter is

the limiting item.



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Limiting Reactants

When 4.00 mol H2is mixed with 2.00 mol Cl2,how

many moles of HCl can form?

H2(g) + Cl(g) 2HCl (g)

4.00 mol 2.00 mol ??? mol

Calculate the moles of product from each

reactant, H2and Cl2.

The limiting reactant is the one that produces

the smaller amount of product.


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Limiting Reactants Using

MolesHCl from H2

4.00 mol H2 x 2 mol HCl = 8.00 mol HCl

1 mol H2 (not possible)

HCl from Cl22.00 mol Cl2x 2 mol HCl = 4.00 mol HCl

1 mol Cl2 (smaller number)

The limiting reactant is Cl2because it is used up

first. Thus Cl2produces the smaller number of

moles of HCl.


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Checking Calculations

Initially H2

4.00 mol

Cl2

2.00 mol

2HCl

0 mol

Reacted/Formed

-2.00 mol -2.00 mol +4.00 mol

Left after

reaction

2.00 mol

Excess

0 mol

Limiting

4.00 mol


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Limiting Reactants Using Mass

If 4.80 mol Ca mixed with 2.00 mol N2, which is the

limiting reactant? 3Ca(s) + N2(g) Ca3N2(s)

Moles of Ca3H2from Ca

4.80 mol Ca x 1 mol Ca3N2= 1.60 mol Ca3N23 mol Ca (Ca used up)

Moles of Ca3H2from N2

2.00 mol N2x 1 mol Ca3N2 = 2.00 mol Ca3N21 mol N2 (not possible)

All Ca is used up when 1.60 mol Ca3N2forms. Thus,

Ca is the limiting reactant.


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Calculate the mass of water produced when

8.00 g H2 and 24.0 g O2react?

2H2(g) + O2(g) 2H2O(l)


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Calculate the grams of H2for each reactant.

H2:

8.00 g H2 x 1 mol H2 x 2 mol H2O x 18.02 g H2O

2.016 g H2 2 mol H2 1 mol H2O

= 71.5 g H2O (not possible)

O2:

24.0 g O2 x 1 mol O2 x 2 mol H2O x 18.02 g H2O

32.00 g O2 1 mol O2 1 mol H2O= 27.0 g H2O (smaller)

O2is the limiting reactant.

8.3 limiting reactants_2

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