# Stoichiometry

Document Sample

```					Tonya Donahoo
4th Hour
Introduction
 Reactions can be used to make new
products, to break down reactants, or to
provide a source of energy. The
predictions made only relate to the
identities and relative amounts of the
products and reactants. It is very useful
to predict exactly how much mass of a
substance will be involved in a reaction.
Such predictions are a part of chemistry
known as stoichiometry.
What is Stoichiometry?
 Mass and quantity relationships among
reactants and products in a chemical
reaction.
Three-Step Method (Step 1)
1.    List what you know.
2.   Organize the information from the problem statement in a
list or table.
3.   Identify what you are asked to find, and write down the
4.   For all substances you will be working with, write
formulas, and determine the molar masses.
5.   If there is a reaction, write an equation for it, making
sure that it is balanced so you will have the correct mole
ratios.
6.   List any conversion factors that you might need, such as
molar masses, mole ratios, and unit conversions.
Three-Step Method (Step 2)
1.        Set up the problem
1.     Analyze what needs to be done to get the answer. See if there is
any information not in the problem that you need for the answer.
2.     Identify which value given can be used as a starting point. Write it
on left side of a sheet of paper. On the right side, write an equals
sign and a question mark with the units of the answer. Fill in the
conversion factors necessary to convert from what is given in the
problem to what is sought in the answer.
3.     Nearly all stoichiometry problems require the amounts of
substances to be in moles, so use the molar masses from Step 1 to
convert the amounts into moles, if necessary.
4.     If you need to change from amount of one substance to a different
substance, use mole ratios derived from the balanced chemical
equation. Remember that the mole ratio may not always be 1:1.
5.     Be sure to convert the data into appropriate units.
6.     When you have finished writing down your plan with all of the
conversion factors, check to see if the units cancel each other. If
they all cancel to give you the units you need for the answer, the
setup is correct.
Three-Step Method (Step 3)
1.        Estimate and calculate.
1.      First, estimate your answer. One way to do this is to round off the
numbers in the problem setup and make a quick calculation.
Another way is to compare conversion factors in the setup and
decide whether the answer should be bigger or smaller than the
beginning value.
2.      Then, begin your calculations by working through the problem
setup you made in Step 2.
3.      When you have finished your calculations, remember that you sill
don’t have the correct answer. You must round off and make sure
that the answer has the correct number of significant digits.
4.      Always report the answer with the correct units, not just as a
number.
5.      Compare your answer with the estimate. If they are not close,
Problems with Amounts in
Moles
 Some problems involve data or answers in mole
 These can be solved with an approach similar to
that used to solve the problems having both data
and answers in mass, but with a shortcut.
 There are fewer steps because one or both
molar mass conversions are unnecessary.
 If both the answer and the given data are in
moles, the only conversion factor necessary to
solve the problem is the mole ratio.
Using Density with
Stoichiometry
 The key to solving any stoichiometry
problem is to always calculate in moles.
 Once the number of moles is determined,
conversion factors can be used to convert
to the mass in grams.
 Similarly, once the mass is known, the
density of a substance can be used to
convert from mass to volume.
Density and Stoichiometry
 Density is defined as the mass of a
substance per unit volume, expressed as
D=m/V.
 Once again, the key is to use the density
value to set up a conversion factor that
will cancel the units in the measurement
you have and leave the units of the
Calculating Number of Atoms
 Just as molar mass, density, and mole
ratios can be used as conversion factors
in problems, Avogadro’s number, 6.022 x
1023, can be used to calculate the number
of atoms or formula units participating in
a reaction.
Summary
 Be sure to use the Three-Step Method
when solving a stoichiometry problem.
 Use conversion factors when necessary.
 Use Avogadro’s number to convert moles
to atoms or formula units.
How can stoichiometry be used?
 Air bags in cars
 Car engines
 Hot meals for soldiers
Stoichiometry in Air Bags
 Air bags are designed to protect
occupants in a car from injuries during a
high-speed front-end collision.
 When inflated, they gently slow down the
occupants of a car so they do not strike
the steering wheel, windshield, or
instrument panel as hard as they would
without the air bag.
Stoichiometry in Air Bags
 Front-end collision transfers energy to a crash
sensor that signals the firing of the ignitor, which
is similar to a small blasting cap.
 The ignitor provides heat energy to start a
reaction in a mixture called the gas generate,
which forms a gaseous product.
 The ignitor also raises the temperature and
pressure within the reaction chamber, so the
reaction occurs at a rate fast enough to fill the
bag before the occupant strikes it.
 Reaction chamber releases the gas into the bag
while a high-efficiency filter keeps the reactants
and solid products away from the occupant.
Gas Generate Equation
 Decomposition reaction:
 2NaN3(s) → 2Na(s) + 3N2(g)
 Single-Displacement Exothermic reaction
that makes bag fill faster:
 6Na(s) +Fe2O3(s) → 3Na2O(s) + 2Fe(s)
 Na2O(s) + 2CO2(g) + H2O(g) → 2NaHCO3(s)
Stoichiometry and Car Engines
 Every time you drive a car, you use
stoichiometry to control how fast the car
moves.
 Gasoline + air → carbon dioxide +water
+ energy
Stoichiometry controls
pollution?
 Automobile manufacturers use
stoichiometry to predict when
adjustments will be necessary to keep
exhaust emissions within legal limits.
 They must be sure that the stoichiometric
concerns are being met without raising
costs too much.
More Hot Food
 FRH - Flameless Ration Heater
 Each is a plastic sleeve containing a
paperboard-covered pad with holes in it.
Within the pad are metal particles
embedded in a polymeric matrix.
 The metal is 10% iron alloy, 90%
magnesium.
 Mass: 20.0g, 8.1g magnesium
 Uses only 45mL of water
FRH
 Water reacts with magnesium in an
exothermic single-displacement
reaction:
 Mg(s) + 2H2O(l) → Mg(OH)2(s)+ H2(g)
353kJ
 Similar heater in campers. Its uses a
synthesis reaction:
 CaO(s) + H2O(l) → Ca(OH)2(s)+64.4kJ
Hot Food for Soldiers
 Each is a complete main dish within a
pouch made of aluminum foil and plastic.
 Cannot be heated directly by burning
trioxane heats a metal cup of water
containing the MRE pouch.
Summary
 Stoichiometry is used when designing air
bags for cars.
 Stoichiometry has played a major part in
constructing better ways of feeding
soldiers that are in need of quick and
easy meals.
Quiz
1. Stoichiometry is mass and quantity
relationships among _________ and
________ in a chemical reaction.
2.   The key to solving stoichiometry
problems is to always calculate in ____?
3.   How is stoichiometry used?
4.   What kinds of reactions are used when
an air bag inflates?
5.   Which meal uses trioxane? The MRE or
FRH?
1. Reactants, products
2. Moles
3. Air bags, car engines, soldier’s meals.
4. Decomposition, single-displacement.
5. MRE
Bibliography
 Haber Process. Isua. 22 May 2009.
http://www.lsua.us/chem1001/sampletest/01M6fAns.
htm
 Compounds in Gasoline. Elmhurst. 22 May 2009.
http://www.elmhurst.edu/~chm/vchembook/515gasoli
necpdQUIZ.html

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 78 posted: 4/18/2010 language: English pages: 26
How are you planning on using Docstoc?