Download Amounts of Reactants and Products

More Stoichiometry!
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Chemical equations & balancing
Amounts of reactants and products
Limiting Reactants
Percent yield
Chemical Equations & Balancing Review
 Chemical change = reorganization of atoms Reactants  Products
o Bonds are broken and new ones form
 Chemical reactions are represented by chemical equations
Ex:
CH4 + O2  CO2 + H2O
REMEMBER:
 Matter (that means atoms) cannot be created or destroyed
 SO…All atoms present in the reactants must be accounted for in the
products.
 Which leads us to … BALANCING Equations
Information Conveyed by the Balanced Equation for Combustion of Methane
Reactants
CH4(g)+ 2O2(g)
1 molecule + 2 molecules
1 mole + 2 moles
6.022 x 1023 molecules +
2(6.022 x 1023 molecules)
16 g + 2(32 g)
80 g reactants
Products
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CO2 (g) + 2H2O (g)
1 molecule + 2 molecules
1 mole + 2 moles
6.022 x 1023 molecules +
2(6.022 x 1023 molecules)
44 g + 2(18 g)
80 g products
What a Chemical Reaction Means
Two types of information:
1. The nature of the reactants and products
2. The relative numbers of each (moles)
 Given as coefficients and subscripts
Equations also give the physical states of the reactants and products:
State
Symbol
(s)
Solid
(l)
Liquid
(g)
Gas
(aq)
Dissolved in Water (Aqueous solution)
Example:
Write each of the following in the form of a balanced chemical equation:
a) Hydrochloric acid (HCl) in aqueous solution is added to solid sodium
hydrogen carbonate (NaHCO3) to form carbon dioxide (CO2) gas,
liquid water (H2O), and sodium chloride (NaCl), which dissolves in
the water.
b) Liquid ethanol (C2H5OH) reacts with oxygen gas (O2) to produce
carbon dioxide gas (CO2) and water vapor (H2O).
Amounts of Reactants and Products
 Coefficients and subscripts represent the number of atoms or
molecules
 We can’t count the number of atoms directly, so we use their
masses
Example Problem:
What mass of oxygen (O2) will react with 96.1 g of propane (C3H8)?
1. Write a balanced equation:
2. Figure out how many moles of propane are present in 96.1 grams of
propane.
3. Remember that each mole of propane reacts with 5 moles of oxygen
gas, so we use a mole ratio to determine how much oxygen is
required for each mole of propane
4. Multiplying the number of moles of C3H8 by this factor gives the
number of moles of O2 required.
5. Since the original question asked for the mass of oxygen needed to
react with 96.1 grams of propane, moles of oxygen must be converted
to grams.
Steps to calculate the mass of reactants and products:
1. Write a balanced equation for the reaction (include physical state)
2. Convert the known mass of the reactant or product to moles of that
substance.
3. Use the balanced equation to set up the appropriate mole ratios.
4. Use the mole ratios to calculate the number of moles of the desired
reactant or product.
5. Convert from moles back to grams if required by the problem.
Sample Problems:
a) Solid lithium hydroxide (LiOH) is used in space vehicles to remove
exhaled carbon dioxide from the living environment by forming solid
lithium carbonate (Li2CO3) and liquid water. What mass of gaseous
carbon dioxide can be absorbed by 1.00 kg of lithium hydroxide?
(Hint: You will need to convert kilograms to grams before converting
it to moles)
b) Baking soda is often used as an antacid. It neutralizes excess
hydrochloric acid secreted by the stomach:
NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (aq)
Milk of magnesia, which is an aqueous suspension of magnesium
hydroxide, is also used as an antacid:
Mg(OH)2 (s) + 2HCl (aq)  2H2O (l) + MgCl2 (aq)
Which is the more effective antacid per gram, NaHCO3 or Mg(OH)2?
(Hint: determine the amount of HCl neutralized per gram of each antacid)
Limiting Reactants
 Limiting Reactant/Limiting Reagent = the reactant that limits the amount of
product that can be formed
 It is important to figure out which reactant is the limiting one so that you can
correctly calculate the products that will be formed.
Steps for Solving problems involving Limiting reactants:
1. Write and balance the equation for the reaction.
2. Convert known masses of reactants to moles.
3. Using moles of reactants (from step 2) and mole ratios, determine which
reactant is limiting.
4. Using the amount of the limiting reactant and mole ratios, calculate the
number of moles of the desired product.
5. Convert from moles of product to grams using its molar mass.
Sample Problem:
Nitrogen gas (N2) can be prepared by passing gaseous ammonia (NH3) over solid copper (II)
oxide (CuO) at high temperatures. The other products of the reaction are solid copper and
water vapor. How many grams of N2 are formed when 18.1g of NH3 is reacted with 90.4g
of CuO?
1. Write and balance the chemical equation for the reaction:
2. From the masses of the reactants, determine the moles of each using their
molar masses. (This will give you the amount of CuO available)
3. To determine which reactant is limiting, use the mole ratio between CuO and
NH3 (This will give you the amount of CuO that is needed)
4. Use the amount of limiting reactant and mole ratio to determine how much
of the N2 is formed.
5. Using the molar mass of N2 (28.02g), calculate the mass that is produced
Percent Yield
 Theoretical yield = the amount of product that is formed when one reactant
runs out
 Actual yield = how much product actually forms
 Percent yield = the comparison between theoretical and actual yield.
Example: If the reaction in the sample problem actually gave 6.63g of nitrogen
instead of the predicted 10.6g, the percent yield would be:
Sample Problem:
Methanol (CH3OH) can be produced by the reaction between carbon monoxide
(CO) and hydrogen gas (H2). Suppose that 68.5kg (68500 g) of CO (g) is reacted
with 8.60kg (8600g) of H2 (g).
a) Calculate the theoretical yield of methanol.
b) If 3.57 x 104 g of CH3OH is actually produced, what is the percent yield of
methanol?