Ch. 9 Notes -- Stoichiometry

1
Ch. 9 Notes -- Stoichiometry

• Stoichiometry refers to the calculations of
  chemical quantities from __________________
  chemical equations.
• Interpreting Everyday Equations
• 2 guards 2 forwards 1 center ? 1
  basketball team
• ___ ___ ___ ? __________
• Practice Problems
• 1) How many guards does it take to make 7 teams?
  ______
• 2) How many forwards are there in 8 teams? ______
• 3) If you have 8 centers, 17 forwards and 14
  guards, how many teams can be made? ______
  What do you run out of first? _________

balanced
2G
2F
C
G2F2C
14
16
7
guards
2

• Interpreting Chemical Equations
• ___N2 (g) ___H2 (g) ? ___NH3 (g)
• The first thing that must be done is to
  ______________ the equation!
• Here are the kinds of information you can get
  from the equation
• ____ mole N2 ____ moles H2 ? ____ moles
  NH3
• ____ molecule N2 ____ molecules H2 ? ____
  molecules NH3
• ____ liter N2 ____ liters H2 ? ____ liters
  NH3
• ____ grams N2 ____ grams H2 ? ____ grams
  NH3

3
2
1
balance
1
3
2
1
3
2
1
3
2
28.0
6.0
34.0
To be discussed in later chapters
3
Stoichemertry

• Mole-Mole Conversions
• mole conversion factor comes from the
  coefficients of the balanced chemical equation.
• Step 1 Write down the equation (if not already
  given)
• Step 2 Predict the products (if not already
  given)
• Step 3 BALANCE the equation (if not already
  done so)
• Step 4 Write down the given.
• Step5 Set up a conversion factor to change from
  moles to moles.

4

• Practice Problems N2 (g) 3H2 (g) ?
  2NH3 (g)
• How many moles of ammonia can be made from 7
  moles of nitrogen reacting with an excess of
  hydrogen?
• 2) How many moles of hydrogen are required to
  completely react with 8 moles of nitrogen to
  produce ammonia?
• 3) How many moles of hydrogen are needed to react
  with an excess of nitrogen to make 10 moles of
  ammonia?

2 mol NH3
7 mol N2

14 moles of NH3
x
1 mol N2
3 mol H2
8 mol N2
x

24 moles of H2
1 mol N2
3 mol H2
10 mol NH3

15 moles of H2
x
2 mol NH3
5
Other Conversion Problems

• Mass-Mass (grams to moles to moles to grams)
• Step 1 Write down the equation (if not already
  given)
• Step 2 Predict the products (if not already
  given)
• Step 3 BALANCE the equation (if not already
  done so)
• Step 4Write down the given and convert from
  grams to moles.
• Step5 Convert from moles of the given to moles
  of the unknown using a mole conversion factor.
• Step 6 Convert from moles of the unknown to
  grams.

6
N2 (g) 3H2 (g) ? 2NH3 (g) Practice
Problem How many grams of ammonia can be made
from reacting 39.0 grams of nitrogen with an
excess of hydrogen?
2 mol NH3
1 mol N2
17.0 g NH3
39.0 g N2
47.4 g NH3
x
x
x

28.0 g N2
1 mol N2
1 mol NH3
7
Limiting Reagent (or Limiting Reactant)
runs out

• The limiting reagent is the reactant that
  ___________ _____ first.
• The reactant that is in abundance is called the
  ___________ reagent.

excess
8
Calculations Step 1 Do a mole-mole conversion
for one of the substances. This answer is how
much of it you need. Step 2 Compare your answer
to how much reactant was given. Do you have
enough? If not, this reactant is your limiting
reagent! Practice Problems N2 (g) 3H2
(g) ? 2NH3 (g) 1) If 2.7 moles of nitrogen
reacts with 6.3 moles of hydrogen, which will you
run out of first? 2) If 3.9 moles of nitrogen
reacts with 12.1 moles of hydrogen, what is the
limiting reagent?
Were you given enough H2? ______ Limiting Reagent
______
3 mol H2
No ! (6.3lt8.1)
2.7 mol N2
8.1 moles of H2 are needed for the reaction.
x

1 mol N2
H2
Were you given enough H2? ______ Limiting Reagent
______
3 mol H2
Yes ! (12.1gt11.7)
3.9 mol N2
11.7 moles of H2 are needed for the reaction.
x

1 mol N2
N2
9
Excess Reagent (or Excess Reactant)

• How many moles of excess reagent do you have?
• Step 1 Do a mole-mole conversion starting with
  the limiting reagent as the given.
• The answer you get is how much of the excess
  reagent you need to completely react with the
  limiting reagent.
• Sometimes you get lucky and you already did this
  conversion from the previous problem!
• Step 2 Subtract this answer from the amount
  given in the original problem, and that is how
  many moles of excess reagent there are.

10
Excess Reagent (or Excess Reactant)
Practice Problem Find the number of moles of
excess reagent from the previous practice
problems.
1 mol N2
6.3 mol H2
2.1 moles of N2 are needed for the reaction.
x

3 mol H2
2.7 moles of N2 were originally given, so the
excess will be
2.7 moles given - 2.1 moles needed 0.6 moles of
N2 excess

• For the second practice problem, we already
  started with the limiting reagent, so all you
  have to do is subtract

12.1 moles given -11.7 moles needed 0.4 moles of
H2 excess
11
Percent Yield

• Percent Yield is a ratio that tells us how
  ________________ a chemical reaction is.
• The higher the yield, the more efficient the
  reaction is.
• Actual or experimental Yield
• Theoretical or ideal Yield
• The ___________ yield is the amount you
  experimentally get when you run the reaction in a
  lab.
• The _______________ yield is the amount you are
  ideally supposed to get if everything goes
  perfectly. You can calculate this amount using
  stoichiometry!

efficient
x 100
Yield
actual
theoretical
12
Percent Yield
Practice Problem 2H2 (g) O2 (g) ?
2H2O (g) 1) A student reacts 40 grams of
hydrogen with an excess of oxygen and produces
300 grams of water. Find the yield for this
reaction. Step 1 Do a mass-mass conversion
starting with the given reactant and converting
to the product, (in this example, the water.)
The answer you get is how much water you
theoretically should have produced. Step 2
The other value in the question, (300 grams), is
what you actually produced. Divide them to get
your yield!
2 mol H2O
1 mol H2
18.0 g H2O
40 g H2
360 g H2O
x
x
x

2.0 g H2
2 mol H2
1 mol H2O
300
Yield
x 100
83.3
360