C5 - monitoring and controlling chemical reactions

Question 1

what is mole?

Answer 1

a mole is an amount of substance which contains 6.02x10^23 elementary entities

Question 2

what are three equations which involve moles?

Answer 2

• m = n x Mr
• no. of particles = n x avogadros constant
• n = c x (v/1000)

Question 3

what are some different ways to measure the volume of gas given off during a reaction?

Answer 3

• inverted measuring cylinder
• burette
• gas syringe
• pan-top balance (mass loss)

Question 4

how many cm^3 are in 1 dm^3?

Answer 4

1000

Question 5

what volume does 1 mole of any gas occupy at room temperature and pressure?

Answer 5

24dm^3

Question 6

what is the molar volume?

Answer 6

the volume that 1 mole of any gas occupies at room temperature and pressure (24dm^3)

Question 7

how can number of moles be calculated with volume?

Answer 7

number of moles = volume/24

Question 8

what type of reaction is CaCO3 -> CaO + CO2?

Answer 8

thermal decomposition reaction

Question 9

why wouldn’t a reaction make 100% of the theoretical yield?

Answer 9

• system may be at equilibrium (reversible)
• may have impure starting reactants
• may lose reactants and/or products during manufacturing process e.g. during filtration
• side reactions may occur

Question 10

how is percentage yield calculated?

Answer 10

(actual yield/theoretical yield) x 100

Question 11

what is atom economy?

Answer 11

• a way of measuring how many atoms from the reactants are used to make the products

Question 12

what does 100% atom economy mean?

Answer 12

all the atoms in the reactants are used in forming the products e.g. the Haber Process

Question 13

what is high atom economy better for?

Answer 13

profits and the environment

Question 14

what’s bad about reactions with low atom economies?

Answer 14

• they use up resources very quickly
• make lots of waste materials that have to be disposed of
• tend to be unsustainable

Question 15

why are low atom economy reactions usually not profitable?

Answer 15

• raw materials can be expensive to buy
• waste products can be expensive to remove and dispose of responsibly

Question 16

what is the atom economy if there’s only 1 product?

Answer 16

100%

Question 17

how is atom economy calculated?

Answer 17

(Mr of useful product/Mr of all products) x 100

Question 18

what is a solute?

Answer 18

a substance that dissolves in a solvent

Question 19

what is a solvent?

Answer 19

a substance that can dissolve a solute to form a solution

Question 20

what is a solution?

Answer 20

a mixture formed when one substance dissolves into another

Question 21

what is concentration?

Answer 21

a measure of how much solute is in a given volume of solution; it can be measured in g/dm^3 or mol/dm^3

Question 22

how do you convert g/dm^3 to mol/dm^3?

Answer 22

divide by Mr

Question 23

how do you convert mol/dm^3 to g/dm^3?

Answer 23

multiply by Mr

Question 24

how do you convert cm^3 to dm^3?

Answer 24

divide by 1000

Question 25

how do you convert dm^3 to cm^3?

Answer 25

multiply by 1000

Question 26

how many litres/ml in 1 dm^3?

Answer 26

1 dm^3 = 1 litre = 1000ml

Question 27

what is the difference between strength and concentration in acids?

Answer 27

• concentration tells us how many moles of acid there are per litre
• strength tells us how much it ionises

Question 28

how are pH and H ^+ concentration related?

Answer 28

• as hydrogen ion concentration increases by a factor of 10, the pH value of a solution decreases by a factor of 1

Question 29

what is titration?

Answer 29

a titration is a practical in which we measure exactly what volume of two solutions will react together i.e. measuring the volume of acid needed to neutralise a measure of volume of alkali

Question 30

what is the endpoint?

Answer 30

• the point where a reaction finishes (when the indicator changes colour)

Question 31

how is the endpoint observed?

Answer 31

using a suitable indicator (for acid-base titrations, this is where the pH reaches 7/neutral)

Question 32

what is a standard solution?

Answer 32

any solution that you know the concentration of

Question 33

what are titrations used to find out?

Answer 33

concentrations

Question 34

how is a titration carried out?

Answer 34

1. using a pipette, measure out a set volume of the alkali into a flask; add a few drops of an indicator (usually phenolphthalein or methyl orange)
2. fill a burette with a standard solution (a known concentration) of acid
3. use the burette to add the acid to the alkali a bit at a time; swirl the flask regularly; go slowly (a drop at a time) when the alkali’s almost neutralised
4. the indicator changes colour when all the alkali has been neutralised (this is called the endpoint)
5. record the volume of acid used to neutralise the alkali
   6 repeat process a few times, ensuring very similar results each time and calculate mean

Question 35

what colour is phenolphthalein in alkalis and acids?

Answer 35

• pink in alkalis
• colourless in acids

Question 36

what colour is methyl orange in alkalis and acids?

Answer 36

• yellow in alkalis
• red in acids

Question 37

why is universal indicator not used in titrations?

Answer 37

it changes colour gradually and we want a single colour change

Question 38

where should you keep the burette while you fill it?

Answer 38

eye level/below eye level
(don’t want to be looking up if any acid spills)

Question 39

how do you work out when the alkali is neutralised in a titration?

Answer 39

• do a rough titration first
• note approx. amount of acid needed, then go slowly as you get near the amount on the next run

Question 40

what factors need to be considered in industry?

Answer 40

• atom economy
• percentage yield
• rate of reaction
• whether reaction is reversible