C5.1 - Monitoring Reactions

Question 1

how do you calculate the percentage yield?

Answer 1

(real yield / theoretical yield)x 100

Question 2

what does calculating the percentage yield show?

Answer 2

the efficiency of a reaction

Question 3

define the term ‘actual yield’

Answer 3

mass of product made in a chemical reaction

Question 4

define the term ‘theoretical yield’

Answer 4

maximum mass it is possible to make from a given mass of reactants (in perfect conditions)

Question 5

define the term ‘percentage yield’

Answer 5

proportion of the theoretical yield actually obtained during a reaction

Question 6

state 3 factors that prevent a percentage yield from reaching 100%

Answer 6

• reactants may burn in different ways than expected
• some reactants present may not react
• loss of some reactants

Question 7

5.75g of sodium is reacted with chlorine to produce sodium chloride. If the actual yield is 10.2g, calculate the percentage yield. (4 marks)

2Na + Cl2 -> 2NaCl

Answer 7

10.2 / 14.625

= 70%

Question 8

define atom economy

Answer 8

a measure of the amount of reactants that become useful products

Question 9

what is the equation to calculate atom economy?

Answer 9

(sum of Mr of desired product / sum of Mr of all products) x 100

Question 10

how much is one cm3 in dm3?

Answer 10

1 cm3 = 0.001 dm3

Question 11

how much is 1 dm3 in cm3?

Answer 11

1 dm3 = 1000cm3

Question 12

what is the equation to calculate concentration in g/dm3?

Answer 12

mass of solute (g) / volume of solution (dm3)

Question 13

what is the equation to calculate concentration in mol/dm3?

Answer 13

amount of solute (mol) / volume of solution (dm3)

Question 14

what are the factors when choosing g a reaction pathway?

5

Answer 14

• yield of the product
• atom economy of the product
• usefulness or otherwise of by-products
• rate of reaction
• equilibrium position, if it is a reversible reaction

Question 15

if the atom economy of a process is high, how does it affect the efficiency?

Answer 15

it has a higher efficiency (less waste)

Question 16

how does the by-product of a reaction affect the atom economy?

Answer 16

• if the by-product is useful the atom economy is higher (can sell it for another use)
• if not, the atom economy is lower

Question 17

what makes the reaction pathway of a substance less desirable?

Answer 17

by products which are toxic, or of little use

Question 18

what are titrations for?

Answer 18

to calculate the volume of acid/alkali needed to neutralise the acid/alkali

Question 19

describe colour changes of methyl orange

Answer 19

yellow in alkali → red in acid

Question 20

describe the colour changes of phenolphthalein

Answer 20

pink in alkali → colourless in acid

Question 21

describe the process of how titrations are done

Answer 21

1) add acid to a burette using a funnel, record the volume in the burette to start
2) add 25cm3 of alkali to a conical flask with volumetric pipette and pipette filler
3) and add some indicator
4) place conical flask on a white tile (see colour change clearly)
5) add acid to alkali slowly until you see a colour change completely
6) calculate how much acid has been added (TITRE)
7) repeat until you get concordant titres

Question 22

what do indicators show?

Answer 22

the end point of titrations (changes colour)

Question 23

what is a burette?

Answer 23

a long graduated glass tube with a gap at the bottom

Question 24

what is used to measure a alkali/acid when adding it to the conical flask?

Answer 24

volumetric pipette/ measuring cylinder

Question 25

what must be used to fill a pipette safely?

Answer 25

pipette filler (cone)

Question 26

what is a titre?

Answer 26

the difference between the final burette reading and the inital burette reading / the volume of acid added to the alkali

Question 27

describe two precautions needed to obtain a repeatable titre?

Answer 27

- burette must be clamped vertically | - swirl the flask to mix its contents

Question 28

Define the term ‘limiting reactant’

Answer 28

- the reactant that is completely used up | - therefore, it limits the amount of product

Question 29

how does the concentration of a solution relate to the mass of the solute and the volume of the solution?

Answer 29

smaller volume/ larger number of moles = higher concentration larger volume/smaller number of moles = lower concentration

Question 30

describe the relationship between molar amounts of gases and their volumes

Answer 30

- equal amounts of moles of gases occupy the same volume under the same conditions and pressure (RTP)

Question 31

how do you calculate the volume a gas takes up in room temperature (RTP - room temperature + pressure) - if you have the number of moles

Answer 31

volume of gas at RTP (dm^3) = mol x 24

Question 32

how do you calculate the volume of gas at RTP in cm^3

Answer 32

volume (cm^3) = mol x 24 000

Question 33

1cm3 = (in ml)

Answer 33

1 ml

Question 34

1dm3 = (in l)

Answer 34

1 litre

Question 35

Pb - why don’t you get 100% yield

Answer 35

- side products form - reaction not going to completion. - losing products when separating it making the substance

Question 36

Pb - definition of atom economy

Answer 36

A measure of how many atoms in the products, make up the desired product Atom economy is a measure of how many of the atoms in the reactants end up in the desired product.

Question 37

How do you read a Burette

Answer 37

Water forms a crescent-shape called a meniscus. The volume of a burette is designed be read from the bottom of the meniscus. If you are not at eye-level you will not get the correct reading.

Question 38

What type of error is caused by not reading off the bottom of a meniscus

Answer 38

Systematic error

Question 39

Explain why it is important to rinse the burette and volumetric pipettes with their solutions before using them in the experiment.

Answer 39

This removes any chemicals left behind that might react with the substances in this experiment. The solution used to rinse is considered contaminated and is discarded.

Question 40

Why is a “rough titration” performed before taking other results?

Answer 40

In order to know the approximate volume needed to reach the end point. It would be extremely time-consuming to add drop-wise from the very beginning. The rough result usually overshoots the end-point and so can’t be used.

Question 41

Why is it important a single indicator is used, rather than a mixture of indicators such as universal indicator?

Answer 41

A mixture of indicators would make it very difficult to see a clear colour change at the end point.

Question 42

Why is it important to only add a few drops of indicator to the conical flask?

Answer 42

You could alter the volume of solution, but more importantly: indicators may form precipitates that make the solution cloudy.

Question 43

What is the molar gas volume in words

Answer 43

1 mole of a gas takes up 24dm3 of volume under standard conditions.

Question 44

What does standard conditions mean

Answer 44

room temperature and pressure (25°C and 1 atmosphere of pressure).