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Chem C4-C6 > C5.1 - Monitoring Reactions > Flashcards

C5.1 - Monitoring Reactions Flashcards

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1
Q

how do you calculate the percentage yield?

A

(real yield / theoretical yield)x 100

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2
Q

what does calculating the percentage yield show?

A

the efficiency of a reaction

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3
Q

define the term ‘actual yield’

A

mass of product made in a chemical reaction

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4
Q

define the term ‘theoretical yield’

A

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

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5
Q

define the term ‘percentage yield’

A

proportion of the theoretical yield actually obtained during a reaction

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6
Q

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

A
  • reactants may burn in different ways than expected
  • some reactants present may not react
  • loss of some reactants
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7
Q

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

A

10.2 / 14.625

= 70%

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8
Q

define atom economy

A

a measure of the amount of reactants that become useful products

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9
Q

what is the equation to calculate atom economy?

A

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

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10
Q

how much is one cm3 in dm3?

A

1 cm3 = 0.001 dm3

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11
Q

how much is 1 dm3 in cm3?

A

1 dm3 = 1000cm3

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

what are the factors when choosing g a reaction pathway?

5

A
  • 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
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15
Q

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

A

it has a higher efficiency (less waste)

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16
Q

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

A
  • if the by-product is useful the atom economy is higher (can sell it for another use)
  • if not, the atom economy is lower
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17
Q

what makes the reaction pathway of a substance less desirable?

A

by products which are toxic, or of little use

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18
Q

what are titrations for?

A

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

19
Q

describe colour changes of methyl orange

A

yellow in alkali → red in acid

20
Q

describe the colour changes of phenolphthalein

A

pink in alkali → colourless in acid

21
Q

describe the process of how titrations are done

A

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

22
Q

what do indicators show?

A

the end point of titrations (changes colour)

23
Q

what is a burette?

A

a long graduated glass tube with a gap at the bottom

24
Q

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

A

volumetric pipette/ measuring cylinder

25
Q

what must be used to fill a pipette safely?

A

pipette filler (cone)

26
Q

what is a titre?

A

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

27
Q

describe two precautions needed to obtain a repeatable titre?

A
  • burette must be clamped vertically

- swirl the flask to mix its contents

28
Q

Define the term ‘limiting reactant’

A
  • the reactant that is completely used up

- therefore, it limits the amount of product

29
Q

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

A

smaller volume/ larger number of moles = higher concentration

larger volume/smaller number of moles = lower concentration

30
Q

describe the relationship between molar amounts of gases and their volumes

A
  • equal amounts of moles of gases occupy the same volume under the same conditions and pressure (RTP)
31
Q

how do you calculate the volume a gas takes up in room temperature (RTP - room temperature + pressure)

  • if you have the number of moles
A

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

32
Q

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

A

volume (cm^3) = mol x 24 000

33
Q

1cm3 = (in ml)

A

1 ml

34
Q

1dm3 = (in l)

A

1 litre

35
Q

Pb - why don’t you get 100% yield

A
  • side products form
  • reaction not going to completion.
  • losing products when separating it making the substance
36
Q

Pb - definition of atom economy

A

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.

37
Q

How do you read a Burette

A

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.

38
Q

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

A

Systematic error

39
Q

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

A

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.

40
Q

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

A

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.

41
Q

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

A

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

42
Q

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

A

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

43
Q

What is the molar gas volume in words

A

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

44
Q

What does standard conditions mean

A

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

Chem C4-C6 (19 decks)

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