Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

Stoichiometry

VIEWS: 78 PAGES: 26

									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.
     1.   Read problem twice.
     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
          units for the answer.
     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,
             check all of your work.
     6.      Make sure your answer is reasonable.
Problems with Amounts in
Moles
 Some problems involve data or answers in mole
  amounts instead of mass amounts.
 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
  measurement for the answer.
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.
Stoichiometry Roadmap
Another Roadmap
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
 MRE - Meal, Ready-to-Eat
 Each is a complete main dish within a
  pouch made of aluminum foil and plastic.
 Cannot be heated directly by burning
  trioxane(C3H6O3). Instead, the 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?
Answers
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

								
To top