Unit 3 Test Review - Quantities in Chemical Reactions

Question 1

What is Precision?

Answer 1

The closeness of two or more measurements to one another.

Question 2

What is Accuracy?

Answer 2

The closeness of a measured value to a standard or known value.

Question 3

What is the abbreviation for metric conversions? what do you do when converting small values to larger ones, and vice versa?

Answer 3

King Henry Died By Drinking Chocolate Milk.

CONVERSIONS:
Smaller - Larger = Division by factors of 10
Larger - Smaller = Multiply by factors of 10

Question 4

What is scientific notation?

Answer 4

The notation is used to express very large or small numbers.

Question 5

What are the rules for sig figs when multiplying and dividing vs adding and subtracting?

Answer 5

Multiplying and dividing the answer should be the least amount of sig figs from the two.

Adding and subtracting is the smallest decimal count.

Question 6

Where does the word mole come from?

Answer 6

German word for molecule (mdekul)

Question 7

What is the mole?

Answer 7

Chemical counting unit
Since atoms and too small and numerous to count, the mole is used to determine how many particles are present in a measured chemical. It represents the amount of particles in a sample of matter.

Question 8

How do you determine the unit of a chemical?

Answer 8

Depending on the chemical the particles of a mole could be:
- atoms ( applies to elements )
- molecules (applies to molecular compounds )
- formula units (applies to ionic compounds)

Question 9

How is the mole expressed?

Answer 9

Numerical Quantity
Expressed in units of Mol
Expressed in mathematical formulas as ‘n’ variable.

Question 10

What does 1 mole represent?

Answer 10

Avagadro’s constant
Expressed in mathematical formulas as ‘NA’
It is a constant and always represents 6.02 x 10^23

Question 11

Why is the mole reasonable?

Answer 11

It deals with extremely small particles which is important in the world of chemistry, but it is unreasonable in the microscopic world.

Question 12

What do the variables represent in a Molar Quantity / Particle Count question?

Answer 12

Nx = nxNA

Nx = number of particles, units match particle in question.

nx = molar quantity, units of mol

NA = Avagadro’s constant, 6.02 x 10^23
units are “particle/mol”

Question 13

Does every chemical formula have an embedded particle ratio?

Answer 13

Yes

Question 14

What is molar mass?

Answer 14

The mass of one mole of a pure chemical substance.

Question 15

Can the particle count/mass of an element change?

Answer 15

No

Question 16

What is the mathematical formula symbol for molar mass?

Answer 16

Mx

Question 17

What is the unit for molar mass, and why?

Answer 17

g/mol
to determine the molar mass, you must add together the atomic masses of each atom present in the chemical
one mole of an atom of an element has a mass in grams equivalent to the element’s average atomic mass

Question 18

What is the formula for mass, and molar quantity?

Answer 18

mx = nxMx

m = mass units (g)

n = molar quantity (mol)

M = molar mass units (g/mol)

Question 19

What is the relationship between mass and particle count?

Answer 19

One mole of any chemical can obtained by measuring the molar mass of the chemical
The molar mass also contains 6.02 x 10^23 particles of that chemical

The variable that bridges the gap between mass and number of particles is the mole as the mole is common between both formulas

Question 20

What is stoichiometry?

Answer 20

Greek derivation: “stoicheion” = element
“metron” = measure

It is the study of the relationship between the relative quantities of reactants and products involved in a chemical reaction.

The quantities are particle and mole proportions (never mass)

Question 21

What is a mole ratio? How is it determined in Stoichiometry?

Answer 21

Used to calculate mass relationships between the chemicals participating in a chemical reaction
Determined from the coefficients of a balanced chemical equation (first step of solving stoichiometric problem)

Question 22

What is a limiting reagent?

Answer 22

The reactant that stops a chemical reaction
The reactant that is completely consumed in a chemical reaction
The reactant determines the amount of product that can form.

Question 23

What is an excess reagent?

Answer 23

The reactant of a chemical reaction that still has mass remaining after the reaction has stopped.

Question 24

What do masses of a reaction prove?

Answer 24

The masses of the reactant determine how long a reaction will run and how much product is made.
If a reactant is used up, the reaction will stop no matter how much mass is left of the other reactant.

Question 25

What will happen if the masses are equal to the proportions of the mole ratio?

Answer 25

Theoretically, both reactants will be completely consumed when the reaction stops

Question 26

Why have a limiting reagent?

Answer 26

• In the real world, they can reduce costs as in a lab setting making the most expensive chemical a limiting reagent will allow all of it to be used up, and let the less expensive chemical be the excess
• Reduces environmental impact, if a chemical reaction is undergoing and a toxic chemical is an LR, less of it will be put in the atmosphere vs if it was an excess reagent
• They also improve fuel efficiency

Question 27

What is the yield of a reaction?

Answer 27

• Refers to the quantity of products formed, using stoichiometric calculations a chemist is able to calculate the maximum amount of a product that can be formed from a chemical reaction given a set mass for each reactant.
• This maximum calculated value is the mass of the product that should be produced from a chemical reaction (in lab setting, this is difficult to achieve this maximun amount)

Question 28

What is theoretical yield?

Answer 28

Calculated mass of product that is determined by stoichiometric calculations.

Question 29

What is an actual yield?

Answer 29

• Measured mass of product produced when the chemical reaction is carried out in a lab setting, the actual yield can be lower, higher, or rarely equal to the theoretical yield.
• If the actual yield does not match the theoretical yield, it can be resorted back to possible experimental error.

Question 30

What is the percent yield?

Answer 30

Indicates how efficient chemical reaction is, important for cost analysis in industry.

Question 31

What is the percent yield formula?

Answer 31

% yield = actual yield / theoretical yield x 100

Question 32

What factors impact percent yield?

Answer 32

• Errors in procedure: mass lost before measured (spilt, miscounted), makes percent yield less than true theoretical yield
• Competing side reactions: If equipment is not cleaned properly in a lab setting, and a chemical reaction is undergone another one could occur and compete with the main one being analyzed, thus impacting mass and theoretical yield calculations
• Impurities; The failure to take chemical impurities into account can cause errors in masses of reactant when they overchange into products (tight containers not closed properly)