Transcript Stoichiometry
Stoichiometry
Chapter 12
What is stoichiometry?
The study of quantitative relationships between amounts used and products formed by a chemical reaction Based on the law of conservation of mass Chemical bonds in reactants break and new chemical bonds form to produce products, but the amount of matter present at the end of the reaction is the same as was present in the beginning Mass of reactants = mass of products
Mass of reactants products = Mass of
Example:
CH 4
Mole Mass
CH 4 + 2O 2
CO 2 + 2H 2 0
2O 2
CO 2
2H 2 0
Mole Ratios
Mole Ratio- a ratio between the number of moles of any substances in a BALANCED chemical equation Example:
CH 4
+ 2O 2
1 mol
CH 4
: 2 mole
CO O 2 2 + 2H 2 0
2 mol
H 2 0
: 2 mol
O 2
Your Turn Determine all possible mole ratios for the following chemical equation: 3Fe (s) + 4 H 2 0 (l) Fe 3 O 4(s) + 4 H 2(g)
3Fe
(s)
+ 4 H
2
0
(l)
Fe
3
O
4(s)
+ 4 H
2(g)
Practice Problems
Write a balanced chemical equation for each reaction and determine the possible mole ratios.
1.
2.
3.
Nitrogen reacts with hydrogen to produce ammonia (NH 3 ) Hydrogen peroxide (H 2 O 2 ) decomposes to produce water and oxygen.
Pieces of zinc react with a phosphoric acid solution to produce solid zinc phosphate and hydrogen gas.
N 2(g) + 3H 2(g) 2 NH 3(g) 2H 2 O 2 2H 2 O + O 2 3Zn (s) + 2 H 3 PO 4(aq) Zn 3 (PO 4 ) 2(aq) + 3 H 2 (g)
Stoichiometric Calculations
Mole Mole Calculations
Plan to solve:
1.
Balance Equation 2.
3.
4.
Identify
given
quantity (in mol) Conversion factor (mole ratio) Calculate
unknown
quantity (in mol) Example: The elements lithium and oxygen react explosively to form lithium oxide, Li 2 O. How many moles of lithium oxide will form if 2 mol of lithium react?
Mole Mass Calculations
Plan of solve:
1.
Balance Equation 2.
3.
4.
5.
Identify
given
quantity (in mol) Conversion factor (mole ratio) Molar mass of
unknown
Calculate
unknown
quantity (in g/mol) quantity (in grams) What mass, in grams, of glucose when 3.00 mol of water is produced reacts with carbon dioxide?
__CO 2 (g) + __H 2 O (l) __C 6 H 12 O 6 (s) + __O 2(g)
Mass Mole Calculations
Plan of attack:
1.
2.
Balance Equation Identify
given
quantity (in grams) 3.
4.
5.
Molar mass of
given
quantity (in g/mol) Conversion factor (mole ratio) Calculate
unknown
quantity (in mol) The first step in the industrial manufacture of nitric acid is the catalytic oxidation of ammonia. The reaction is run using 824g NH 3 and excess oxygen. How many moles of NO __NH 3(g) + __O 2 (g) __NO (g) are formed? + __H 2 O (g)
Mass Mass Calculations
Plan of attack:
1.
2.
3.
4.
5.
6.
Balance Equation Identify
given
quantity (in grams) Molar mass of
given
quantity (in g/mol) Conversion factor (mole ratio) Molar mass of unknown quantity (in g/mol) Calculate
unknown
quantity (in grams) How many grams of SnF 2 reaction of 30.00g of HF are produced from the with Sn?
Practice Problem
Sulfuric acid is formed when sulfur dioxide reacts with oxygen and water. Write the balanced chemical equation for the reaction. If 12.5 mol SO H 2 SO 4 2 reacts, how many moles of can be produced? If 2.50 g SO 2 react with excess oxygen and water, how many grams of H 2 SO 4 are produced?
2SO
2(g)
+ O
2(g)
+ 2H
2
O
(l)
2H
2
SO
4(aq)
Limiting Reactants
Why do reactions stop?
How many complete cars can you make?
Limiting Reactant - The reactant in a chemical reaction that limits the amount of product that can be formed . The reaction will stop when all of the limiting reactant is consumed. Excess Reactant - The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant is completely consumed . The excess reactant remains because there is nothing with which it can react.
The “Have and Need” Method
How can you determine which reactant is limited? Masses of both reactants are given Determine the grams or moles of both reactants given (“Have”) Determine the grams or moles “needed” of each reactant by relating them
Practice Problem
When 36.0 g of H 2 O is mixed with 167g of Fe, which is the limiting reactant? What mass in grams of black iron oxide is produced?
What mass in grams of excess reactant remains when the reaction is completed?
3 Fe (s) + 4 H 2 O (g) Fe 3 O 4 (s) + 4 H 2 (g) Determine the grams of both reactants given
(“Have”)
Determine the grams “needed” of each reactant by relating them with their “Have and Need”
3
Fe
(s)
+
4
H
2
O
(g)
Fe
3
O
4 (s)
+
4
H
2 (g) 36.0 g H 2 0 167 g Fe Use the limiting reactant to calculate the product mass
Addition Practice 6 Na
(s)
+ Fe
2
O
3(s)
3Na
2
O
(s)
+ 2 Fe
(s) If 100.0g Na determine and 100.0g Fe 2 O 3 are used in the reaction, The limiting reactant The reactant in excess The mass of solid iron produced The mass of excess reactant that remains after the reaction is complete
6 Na
(s)
+ Fe 100.0g Na
2
O
3(s)
3Na
2 (s) and
100.0g Fe
2
O
3
+ 2 Fe
(s)
Percent Yield
The theoretical yield is the maximum amount of product from a given amount of reactant The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment The percent yield actual yield to the theoretical yield expressed as a percent of product is the ratio of the % Yield = actual yield (from an experiment) theoretical yield (from calculation) x 100
Example
If 75.0g of CO reacts to produce 68.4g of methanol (CH 3 OH), what is the percent yield of CH 3 OH?
CO (g) + 2H 2 (g) CH 3 OH (l)
2
Al
(s)
+
3
CuSO
4(aq) 3
Cu
(s)
+ Al
2
(SO
4
)
3 (aq) Aluminum reacts with an aqueous solution containing excess copper (II) sulfate. If 1.85 g Al reacts and the percentage yield of Cu is 56.6%, what mass of Cu is produced?
Use the data to determine the percent yield of the following reaction.
2Mg (s) + O 2 (g) 2MgO (s) (Oxygen is in excess) Reaction Data Mass of crucible Mass of cruicble + Mg Mass of Mg Mass of Crucible + MgO Mass of MgO 35.67 g 38.06 g 39.15 g