Unit 6: Stoichiometry in Chemical Reaction

Question 1

Stoichiometry is

Answer 1

relationship between reactants and products in a chemical reaction

Question 2

The coefficients in the equation give… (2)

Answer 2

the ratio of one chemical to another chemical used in the reaction
- they represent the mole ratio of the reactants and products in the reaction

Question 3

In the molar ratio of a chemical reaction, does the left side have to be equal to the right side (2)

Answer 3

No, the left side is not equal to the right side
- the sum of the masses of “reactants” however must equal the total mass of the “products”

Question 4

The law of Conservation Mass (2)

Answer 4

• during a chemical reaction, atoms are neither created or destroyed. The # of atoms remains constant
• This means the mas must remain constant as well

Question 5

A stoichiometric amount is (2)

Answer 5

the predicted amount of a reactant, relative to another reactant, that will react according to the balanced chemical equation
- when stoichiometric amounts of the reactions are available for a chemical reaction, no reactants should remain when the reaction is complete, assuming that the reaction proceeds to completion.

Question 6

Percentage Purity

Answer 6

• In reality, it is virtually impossible to obtain a chemical in a pure form.
• Even the purest chemicals purchased from chemical supply companies contain impurities
• The purity of a chemical is commonly indicated as the %purity
  EQUATION
  %purity (by mass)= (mass of pure chemical/ mass of impure sample) x100%

Question 7

The Excess Reagent is

Answer 7

the substance that we have more than enough. Some of the excess reagent is left over after the reaction

Question 8

The Limiting Reagent is

Answer 8

the substance that runs out first. When it runs out the reaction stops.
- The limiting reagent dictates how much of the products will be made

Question 9

How to determine the limiting reagent in a chemical reaction (4)

Answer 9

• we divide the mole of each reactant coefficient
• whichever gives a smaller # is the limiting reagent. The other one is the excess reagent.

Question 10

Percentage (%) Yield (2)

Answer 10

%yield= (actual yield/theoretical yield) x100%
- it’s the percentage of how much of the expected yield was actually obtained from the reaction

Question 11

Theoretical Yield (4)

Answer 11

AKA Maximum Yield
- It’s the maximum amount of product that can be made from a given amount of reactant
- It’s obtained from stoichiometry calculations
- it’s what you expect

Question 12

Actual Yield (4)

Answer 12

AKA Experimental Yield
- the amount of product actually obtained (lab data)
- It is usually less than the theoretical yield
- It is what you actually get

Question 13

What factors make actual mass less than the theoretical mass. List them

Answer 13

1) Competing Reactions
2) Incomplete Reaction
3) Reactant Purity
4) Collection Techniques
5) Experimental Error

Question 14

Competing Reactions (2)

Answer 14

• the reactants may undergo different reactions other than the desired one, producing undesirable products. –> essentially when u only need 1 of the 2 products made by a reaction
• the desired products may further react to produce other substances

Question 15

Incomplete reaction

Answer 15

• a reaction may not proceed to completion. Not all of the available reactants are converted into products

Question 16

Reactant Purity

Answer 16

Reactants may have contaminants. The mass of the reactant is less than the measured mass due to the presence of contaminants (not 100% pure)

Question 17

Collection Techniques

Answer 17

• spillage, splattering, losses during isolation & purification of product
• usually unavoidable but can be reduced by improving lab skills or having fewer steps

Question 18

Experimental Error

Answer 18

• Error due to precision of instruments or to experimental design.
• Ex. you forgot to take a measurement, the balance isn’t accurate.