Reactions and Mass

Question 1

What is relative formula mass?

Answer 1

The relative mass of a molecule calculated by adding the atomic masses of the atoms present

Question 2

Reacting mass equation

Answer 2

Mass= Mr x moles

Question 3

Exothermic reaction

Answer 3

A reaction that releases energy in the form of heat, occurs when making bonds

Question 4

Endothermic reaction

Answer 4

A reaction that absorbs energy in the form of heat, occurs when breaking bonds

Question 5

Give two examples of an endothermic reaction

Answer 5

Cooking food and photosynthesis

Question 6

What is Ea?

Answer 6

Activation Energy, the minimum amount of energy needed for particles to react

Question 7

What is ∆H?

Answer 7

Total energy change in a reaction

Question 8

How to calculate Mr?

Answer 8

Add the relative atomic masses of each element and times them by the quantity if needed, e.g (O2 would be 2x16).

Question 9

How to calculate moles, mass and Mr?

Answer 9

moles= mass/mr, mass= mr x moles, mr= mass/moles

Question 10

How to find percentage composition?

Answer 10

Find the Mr for all elements, then add them to find a total. use the Mr of the element asked and put it over the total mr. Divide and times by 100 to find the percentage.

Question 11

Example for percentage composition

Answer 11

What is the percentage of oxygen in magnesium oxide (MgO)?
Mr of mg is 24
Mr of O is 16 - element asked for
16+24=40 - total Mr
16/40=40%

Question 12

How to do limiting reactant equations

Answer 12

Draw a table with the reactants and products down the side and the headings; mass, Mr, moles, ratio and limiting. Then add the masses given in the question into the table. Using a periodic table, find the Mr and divide the mass by the Mr to get the moles. Find the ratio. Keeping the ratio in mind, use the moles to find which reactant is limiting. Whichever reactant is limiting, the moles in that reactant will be the same amount of moles in the product. Use the mr of the product and times it by the moles to find the mass of the product.

Question 13

What does the law of conservation of mass state?

Answer 13

No atoms are lost or made during a chemical reaction so the mass of the products is equal to the mass of the reactants

Question 14

What is Avogadro’s number?

Answer 14

6.02 x 10^23

Question 15

What is another name for this number?

Answer 15

One mole, it is the same number of atoms present in 12 grams of carbon

Question 16

What is a limiting reactant?

Answer 16

A limiting reactant means no matter how much excess reactants are added, if no more of a limiting reactant is added no more product is made.

Question 17

Why are limiting reactants useful?

Answer 17

They are useful so exact measurements do not need to be taken

Question 18

How do people in the chemical industry ensure there is as little waste as possible in reactions?

Answer 18

In order to limit economic loss and increase profit, chemists consider the percentage yield of a reaction and the atom economy of a reaction

Question 19

What is percentage yield?

Answer 19

Percentage yield is a measure of how effective an industrial process is in producing a desired product.

Question 20

How is percentage yield calculated?

Answer 20

Actual yield/ theoretical yield= %

Question 21

Why is percentage yield rarely 100%

Answer 21

You may lose some product during separation/ purification, the desired product may be very reactive or the reaction may be reversible

Question 22

What is atom economy?

Answer 22

The efficiency of a reaction based on how many atoms are used- the aim is to minimise waste while maximising the use of atoms in a reaction

Question 23

What is the equation for atom economy?

Answer 23

(Mr of desired products/ total Mr of reactants) x 100

Question 24

Lithium reaction with air

Answer 24

Soft but firm silver metal turns black quite quickly

Question 25

Lithium reaction with water

Answer 25

Effervescence as hydrogen is produced, floats and moves until dissolved

Question 26

Sodium reaction with air

Answer 26

Softer, lighter (colour) silver metal turns light grey and dull quicker

Question 27

Sodium reaction with water

Answer 27

Combusts with an orange flame, melts into a sphere, moved quicker until dissolved

Question 28

Potassium reaction with air

Answer 28

Even lighter (colour) and softer silver metal turns shiny to dull the fastest

Question 29

Potassium reaction with water

Answer 29

Combusts with a lilac flame, reacts the fastest

Question 30

How do we measure the rate of reaction?

Answer 30

By measuring the amount of product formed/ time or the amount of reactant used up/ time

Question 31

Give 5 factors that affect the rate of reaction

Answer 31

Change in temperature, change in the concentration of the reactant, if the reactant is solid then a change in the surface area, if the reactant is gas then a change in the pressure and adding a catalyst

Question 32

The relationship between collisions and rate of reaction

Answer 32

The more frequent and successful the collisions are, the greater the rate of reaction

Question 33

What is turbidity?

Answer 33

A change in the clearness or opaqueness of the liquid