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

• percentage yield of the product
• atom economy of the product
• usefulness of by-products
• rate of reaction
• equilibrium positions

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 an 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 (7)

Answer 21

1) add acid to a burette using a funnel, record the volume in the burette to start
2) add known volume of alkali to a conical flask
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 is a burette?

Answer 22

a long graduated glass tube with a gap at the bottom

Question 23

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

Answer 23

volumetric pipette/ measuring cylinder

Question 24

what must be used to fill a pipette safely?

Answer 24

pipette filler (the squeezy circle thing)

Question 25

what is a titre?

Answer 25

the difference between the final burette reading and the inital burette reading?

Question 26

describe two precautions needed to obtain a repeatable titre?

Answer 26

• burette must be clamped vertically
• flask must be swirled to mix contents

Question 27

Define the term ‘limiting reactant’

Answer 27

• the reactant that is completely used up

- therefore, it limits the amount of product

Question 28

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

Answer 28

smaller volume/ larger number of moles = higher concentration

larger volume/smaller number of moles = lower concentration

Question 29

describe the relationship between molar amounts of gases and their volumes

Answer 29

• equal amounts of moles of gases occupy the same volume under the same conditions and pressure (RTP)
• volume (dm^3) = amount of moles x 24

Question 30

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 30

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

Question 31

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

Answer 31

volume (cm^3) = mol x 24 000