Chem Unit 3 Lesson 3

Question 1

What is stoichiometry?

Answer 1

the study of the mass and amount
relationships between reactants and products in a chemical reaction

Question 2

What is the mole ratio?

Answer 2

The ratio of the amounts of the entities in a chemical reaction

Question 3

Mole ratio can be used to determine________?

Answer 3

the amount of substance (in moles) of any product or reactant in a
chemical reaction

Question 4

What is Mole ratio is based on?

Answer 4

the coefficients in a balanced
chemical reaction

Question 5

What are stoichiometric amounts?

Answer 5

An amount of reactant that is in the same proportion as the reactant coefficients in the balanced chemical
equation

Question 6

When stoichiometric amounts of reactants are available
for a chemical reaction, no reactants should_________?

Answer 6

remain when the reaction is complete (assuming the reaction goes to
completion)

Question 7

What is a Limiting reagent?

Answer 7

the reactant that is completely consumed in a chemical reaction

Question 8

What is an excess reagent?

Answer 8

the reactant that is still present after the reaction is complete

Question 9

Reason #1 of why we care about limiting reagents?

Answer 9

Reducing costs – companies will choose the most expensive reactant to be the limiting reactant, while the least expensive reactant as the excess
(ensuring that none of the expensive
reactant is wasted)

Question 10

Reason 2 on why we care about limiting reagents?

Answer 10

Reducing environmental impact – controlling the limiting and excess reagents can make manufacturing processes more environmentally
friendly

Question 11

Reason 3 on why we care about limiting reagents?

Answer 11

Environmental cleanup– neutralizing environmental pollutants can involve the use of limiting and excess reagents

Question 12

What is an example for reason 2?

Answer 12

Metal smelting (a way to obtain metals and metal oxides, but a large producer of sulfur dioxide waste,
which forms sulfuric acid with atmospheric water and falls as acid rain), is done with an excess of oxygen so all sulfur is converted to sulfur dioxide, which is collected and converted into sulfur acid to be used in industry (economical and much better for the environment)

Question 13

What is an example for reason 3?

Answer 13

water contaminated with toxic metal ions (from mining operations), can be precipitated as metal hydroxides by adding a large excess of a base like
sodium hydroxide
-The metal hydroxide precipitates are then filtered out!

Question 14

What is reason 4 that we care about limiting reagents?

Answer 14

Improving fuel efficiency– obtaining the most possible energy from our home furnaces, car engines, BBQ’s, burners, etc is important both economically and for
efficiency

Question 15

Example for reason 4?

Answer 15

Complete combustion releases more energy (per mole of fuel) than incomplete combustion – and to assure this, enough oxygen must be supplied (usually present in the air)

Question 16

What is theoretical yield?

Answer 16

the amount or mass of product
predicted by completing stoichiometric calculations using
a chemical equation (measured in grams)
This is the maximum possible yield

Question 17

What is actual yield?

Answer 17

the amount or mass of product actually collected by conducting an experiment or industrial
process (in real life)
-This amount cannot exceed theoretical yield, orsomething has gone very wrong
-This amount can be less than theoretical yield, indicating
errors during the experiment!

Question 18

Ideally, you want to have a percentage
yield that is______%

Answer 18

100%

Question 19

reason 1why actual yield is sometimes less than theoretical yield:

Answer 19

The nature of the reaction – sometimes converting reactants to products isn’t always possible

example: ammonia is an important chemical made from reacting hydrogen and nitrogen, but as the reaction proceeds, the ammonia produced begins to decompose back
into nitrogen and hydrogen!

Question 20

How can you avoid reason 1 of the a vs t yield problem?

Answer 20

To avoid this, chemists have learned to control temperature, pressure, and to use a catalyst to control the yield of ammonia (however, actual yield is never completely equal to theoretical yield)

Question 21

reason 2 why actual yield is sometimes less than theoretical yield:

Answer 21

Experimental procedure – losing small amounts of materials during a procedure is inevitable!

Ex:mass of reactants or products can be affected by spillage, splattering during heating, transferring, purification of product, faulty equipment, etc

Question 22

How can avoid the reason 2 of the a vs t problem?

Answer 22

To try to avoid this, chemists work on their skills in the lab, use better equipment (that is usually much
more expensive) and reducing the steps involved in the procedure

Question 23

reason 3 why actual yield is sometimes less than theoretical yield?

Answer 23

Impurities – within chemicals (due to contamination, age, improper storage, etc)

example: sodium hydroxide must be stored in an air-tight container, as it absorbs water from the atmosphere, whereas magnesium reacts with
oxygen in the air, to form an oxide layer making it difficult to react!

Question 24

reason 4 why actual yield is sometimes less than theoretical yield:

Answer 24

Competing side reactions – if a desired product is formed with similar structures to another product in
a reaction, it can be difficult to obtain the maximum amount of desired product

Question 25

How do we try to avoid reason 4 of the a vs t problem?

Answer 25

Chemists have to find ways to minimize the formation of the product that isn’t desired, and maximize the
yield of the product that is desired