Date post: | 19-Dec-2015 |
Category: |
Documents |
Upload: | winifred-bruce |
View: | 214 times |
Download: | 0 times |
BellworkNovember 5th
• Dig around in your brain and fill in the following…–1 mole = ___________________ atoms
–1 mole = ___________________ liters
–1 mole = ___________________ g H2O
–3 mole = ________________ molecules
CHEMISTRY-1 CHAPTER 9
Stoichiometry
Just What is a Chemical Ratio?• A mole ratio is like making a s’more.
• The “chemical reaction for a s’more would be…– 3 chocolate squares + 1 marshmallow + 2 graham crackers 1 s’more
• If you want to make 3 s’mores, how many marshmallows would you need?
• 3 s’mores = 3 marshmallows
• If you have 27 chocolate squares, how many smores can you make?
• 3 chocolate squares = 1 s’more so 27 chocolate squares = 9 s’mores
9.1 Stoichiometry – study of calculations of quantities in chemical reactions using
balanced chemical equations.
2Mg + O2 2MgO
2 moles Mg + 1mole O2 2 moles MgO
• The mole ratios can be obtained from the coefficients in the balanced chemical equation.
• What are the mole ratios in this problem?
• Mole ratios can be used as conversion factors to predict the amount of any reactant or product involved in a reaction if the amount of another reactant and/or product is known.
2Mg + O2 2MgO
Mole Ratio! What’s that mean?
Well, a stoichiometry problem gives you an amount of one chemical and asks you to solve for a different chemical.
To get from one type of chemical to another, a MOLE RATIO must be found between the two chemicals. You get the MOLE RATIO from the
BALANCED CHEMICAL EQUATION!
Moles of substance
given
Mass of substance
given
R.P. of substance
given
Volume of substance
given
Atoms, formula units, molecules
grams
L, mL
mol
MM1mol
22.4
L1m
ol
6.02 x 10 23
1mol
R.P. of substance wanted
Atoms, formula units, molecules
Mass of substance wanted
grams
Volume of substance wanted
L, mL
Moles of substance wanted
mol
22.4 L
1mol
MM 1mol
6.02
x 1
023
1mol
Coefficients in a balanced equation!
Always follow the road map…
Do not make up new paths!
A balanced chemical equation tells the quantity of reactants and products as well as what they are.
2 mol 1 mol 2 mol
*the coefficients are*
The MOLE RATIOMOLE RATIO is your mechanism of transition between the chemical that is your
starting given and the chemical you are solving for.
The MOLE RATIO is the bridge between the two
different chemicals
given
moles moles
? want(given) (want)
MoleRatio
9.2
• Lets get back to those s’mores. If you have the following reaction…
• 3 chocolate squares + 1 marshmallow + 2 graham crackers 1 s’more
• You have 325 chocolate squares and you want to make the maximum number of s’mores possible. Show a calculation for this reaction.
• 325 c.s. 1 s’more = 108.3333 or 108 s’mores• 3 c.s.
Mole - Mole Calculations – Mole ratios are used to calculate the number of moles of product from the given number of moles of reactant, or vice versa.
EXAMPLE
• Calculate the number of moles of Na2O that will be produced when 5.00 moles of Na completely react with oxygen gas.
4Na + O2 2Na2O
5.00 mol Na
4 mol Na
2 mol Na2O = 2.50 mol Na2O
mol-mol ratio
Mole-Mass Calculations:
• We use mole ratios from balanced chemical equations and the molar conversions from Ch. 7 to calculate amounts (grams) of substances needed or produced in chemical reactions.
EXAMPLE
• How many grams of KClO3 must decompose to produce KCl and 1.45 moles O2?
2KClO3 → 2KCl + 3O2
1.45 moles O2
3 mol O2
2 mol KClO3
1 mol KClO3
122.548 g KClO3=
118 g KClO3
MM
mol-mol ratio
Mass-Mass Calculations:
• Same as mole-mass calculations, with an additional step of converting mass to moles.
EXAMPLE• When 24.0 g of Na are mixed with Cl2, are
52.0 g of NaCl produced? Explain.
24.0 g Na22.990 g Na
MM
1 mol Na
2 mol Na
Na + Cl2 → NaCl2 2
mol-mol ratio
2 mol NaCl
1 mol NaCl
MM
58.443 g NaCl
61.0 g NaCl
=
Are 52.0 g produced? Actually more than 52.0 g were produced.
• CaCO3, limestone, is heated to produce calcium oxide, CaO, and CO2. What mass of limestone is required to produce 156.0 g of CaO?
156.0 g CaO
56.077 g CaO
MM
1 mol CaO
1 mol CaO
CaCO3 (s) CaO (s) + CO2 (g)
mol-mol ratio
1 mol CaCO3
1 mol CaCO3
MM
100.086 g CaCO3
278.4 g CaCo3=
BellworkNovember 6th
• Magnesium metal reacts with oxygen in a synthesis reaction. Write and balance the equation, then fill in the chart concerning the particles and mass.
Total Particles of Reactants (look at coefficient)
Total Particles of Products
Total Moles of Reactants
Total Moles of Products
Total Mass of Reactants
Total mass of Products
BellworkNovember 7th
• A chemist combines 6.32g of C2H2 and 12.2g of oxygen. How many grams of carbon dioxide are produced?
A quick reminder…
• Let’s quickly remind ourselves of just how awesome a mole is!
• 1 mole = ________________particles
• 1 mole = ________________ liters
• 1 moles = _______________ grams
6.02 x 1023
22.4
The mass from the periodic table!
• At STP, how many grams of oxygen are needed to produce 19.8 liters SO3 according to the balanced equation below?
19.8 L SO3
22.4 L SO3
Volume @ STP
1 mol SO3
2 mol SO3
2SO2 (g) + O2 (g) 2SO3 (g)
mol-mol ratio
1 mol O2
1 mol O2
MM
31.998 g O2
14.1 g O2=
Combination Calculations:
BellworkTuesday, November 10th • A student drops a 3.40g piece of zinc
into hydrochloric acid. How many liters of hydrogen gas are produced if the reaction occurs at STP?
BellworkTuesday, November 10th • How many grams of aluminum sulfate
are produced if 23.33 g Al reacts with 74.44 g CuSO4?
9.3Reactants in Excess and Limiting Reactants
• Limiting Reactant - the reactant that is used up in a reaction. When you run out of a reactant, the reaction stops and no more product is formed.
• Excess Reactant - the reactant that is not used up in a chemical reaction.
• The limiting reactant determines the amount of product formed.
Important
Back to S’mores
• The s’mores can teach us one more lesson; the lesson of limiting reagents. Lets look at our balanced equation one more time.
• 3 chocolate squares + 1 marshmallow + 2 graham crackers 1 s’more
• If your friend is throwing a slumber party and has 137 chocolate squares and 47 marshmallows, how many total smores can be made.
• You will need to do the calculation with BOTH starting reactants.
• 137 choc.squares 1 s’more = 45.6 s’mores
• 3 choc.squares
• 47 marshmallows 1 s’more = 47 s’mores
• 1 marshmallow
• So, in this case, what LIMITS our production of s’mores?
• Chocolate squares! So chocolate squares are the limiting reagent!
Steps to Determining the Limiting Reactant
1. Find out how much could be produced from each amount of reactant.
2. Which ever produces less is limiting the amount produced. It is therefore the Limiting Reactant.
EXAMPLE• If 6.70 mol of Na reacts with 3.20 mol Cl2,
what is the limiting reagent? How many moles of NaCl will be produced?
Na + Cl2 → NaCl2 2
6.70 mol Na
3.20 mol Cl2
2 mol Na
2 mol NaCl
1 mol Cl2
2 mol NaCl
=
=
6.70 mol NaCl
6.40 mol NaCl
Which amount produced is less?
This is the amount produced, and determines which is the limiting reactant
Cl2 is the limiting reactant
• To determine how much excess reactant is left after a reaction, subtract how much of the excess reactant reacted from how much excess reactant you started with.
EXAMPLE• When 0.500 mole of aluminum reacts with
0.720 mole of iodine, to form aluminum iodide, AlI3(s), what mass of AlI3 is produced? How much of the excess reactant will remain?
I will separate this problem into two different steps.
1.Find the limiting reactant and amount produced (as in previous example)
2.Find amount in excess (left over)
1. When 0.500 mole of aluminum reacts with 0.720 mole of iodine, to form aluminum iodide, AlI3(s), what mass of AlI3 is produced?
Al + I2 → AlI32 23
0.500 mol Al
0.720 mol I2
2 mol Al
3 mol l2
2 mol AlI3
2 mol AlI3
1 mol AlI3
407.7 g AlI3 = 204 g AlI3
1 mol AlI3
407.7 g AlI3 = 196 g AlI3
amount produced
Limiting Reactant? I2
2. How much of the excess reactant will remain?
First…take the Limiting Reactant and the amount with which you began……then convert to find the amount that reacted with it…
0.720 mol I2
3 mol l2
2 mol Al
…then subtract the amount used from the amount given (starting amount)
= 0.480 mol Al
0.500 mol Al 0.480 mol Al- = 0.020 mol Al
this is the excess (left over)
Al + I2 → AlI32 23
given in the problem
BellworkWednesday, November 11th
• 5.9 L of carbon dioxide is combined with 8.4 g MgO in a synthesis reaction to form magnesium carbonate. How many grams of magnesium carbonate is created in this reaction?
Percent Yield
• Theoretical Yield – The maximum amount of product that could be formed from given amounts of reactants. (What you should get) (You calculate this using dimensional analysis)
• Actual Yield - amount of product actually obtained in the reaction (What you really get) (You either get this in a lab, or are given it in a problem)
• Percent Yield describes how much product was actually made in the lab versus the amount that theoretically could be made.
Percent yield tells you how close you were to the 100% mark.
• Reactions do not always work perfectly. Experimental error (spills, contamination) often means that the amount of product made in the lab does not match the ideal amount that could have been made.
% yield = Actual Yield x 100
Theoretical Yield
Amount you actually obtain in an experiment
Amount you should obtain in an experiment (this we will calculate)
EXAMPLE
12.5 g of copper are reacted with an excess of chlorine gas, then 25.4 g of copper(II) chloride are obtained. Calculate the theoretical yield and the percent yield.
First write the balanced equation, then find the
theoretical yield (amount you “should get”)
12.5 g Cu
63.546gCu
1 mol Cu
First write the balanced equation, then find the theoretical yield (amount you “should get”)
Cu + Cl2 → CuCl2
1 mol Cu
1 mol CuCl21 mol CuCl2
134.452 g CuCl2
MM mol-mol ratio MM= 26.4 g CuCl2
this is the theoretical yield
Now do the % yield calculation
% yield = Actual Yield x 100
Theoretical Yield
% yield = 25.4 g CuCl2
26.4 g CuCl2=X 100
from calculation
given in problem
96.2 %
When I was a sophomore in college, we had a lab where we were supposed to isolate caffeine from tea leaves. Most of my caffeine was washed down the drain in a freak accident. Although I should have had 5.0 g of caffeine, I only ended up with 0.040 g of caffeine and a bad grade on the lab. What was my percent yield?
0.040 g
5.0 gX 100 = 0.80 %
What is the theoretical yield if 5.50 grams of hydrogen react with nitrogen to form ammonia?
H2 + N2 NH33 2
5.50 g H2
2.016 g H2
1 mole H2
3 moles H2
2 moles NH317.031 g NH3
1 mol NH3
= 31.2 grams NH3 = Theoretical Yield!!!!
Only 20.4 grams of ammonia is actually produced in the lab. What is the percent yield?
20.4 g NH3
31.2 g NH3
X 100 = 65.4 % yield