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A Level Chemistry > Module 2 Section 1: Atoms and Reactions > Flashcards

Module 2 Section 1: Atoms and Reactions Flashcards

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1
Q

Definition of relative atomic mass

A

The relative atomic mass (Ar) is the weighed mean mass of an atom of an element compared to 1/12th of the mass of an atom of carbon12

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2
Q

Definition of relative isotopic mass

A

Relative isotopic mass is mass of an atom of an isotope, compared with 1/12th the mass of an atom of carbon 12

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3
Q

What does relative atomic mass look like as a number

A

Its an average so its not usually a whole number

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4
Q

What does relative isotopic mass look like as a number

A

Relative isotopic mass is usually a whole number

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5
Q

A natural sample of chlorine contains a mixture of Cl-35 (75%) and Cl-37 (25%), what is the relative isotopic masses and relative formula mass

A

Relative isotopic mass is 35 and 37
Its relative formula mass is 35.5: ( 0.75 x 35 ) + ( 0.25 x 37 = 35.5 )

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6
Q

What is mass spectra

A

Mass spectra are produced by mass spectrometers - devices which are used to find out what samples are made up of by measuring the masses of their components
Mass spectra can tell us the relative isotopic masses and abundances of different elements

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7
Q

What would a mass spectrum for Li6 and Li7 look like

A

100-|
| 92.4%
| |
80-| |
| |
| |
60-| |
| |
| |
40-| |
| |
| |
20-| |
| 7.6% |
|______|___|__________
5 6 7 8 9
Y axis: % abundance of lithium isotope
X axis: mass\charge

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8
Q

How to work out relative atomic mass from a spectra graph

A
  1. Multiply each relative isotopic mass by its relative isotopic abundance and add up the results
    2.Divide by the sum of the isotopic abundances.
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9
Q

What do the X and Y axis show on a mass spectra graph

A

X axis:
units are (relative isotopic/ molecular) mass/charge ratio
Since the charge on the ions is mostly +1, you can often assume the x - axis as the relative atomic mass

Y axis:
Gives abundance of ions, often as a percentage
For an element, the height of each peak gives the relative isotopic abundance

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10
Q

How is the amount of substance measured

A

Measured using a unit called the mole
The number of moles is given the symbol n
The number of particles in one mole is 6.02x10^23 ( avogadro’s constant )

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11
Q

Formula to work out the number of moles in terms of particles

A

Number of moles = number of particles you have/ number of particles in a mole
E.g. 1.5x10^24 carbon atoms, how many moles of carbon is this?
No. of moles = 1.5x10^24/ 6.02x10^23 = 2.49 moles

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12
Q

What is molar mass and it’s units

A

Molar mass is the mass of one mole
Molar mass is the same as the relative formula mass
Measured in g mol ^-1

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13
Q

Find the molar mass of CaCO3
Ca: 40.1
C: 12
O: 16

A

Relative formula mass of CaCO3 = 40.1 + 12 + ( 3 x 16 ) = 100.1
Molar mass is 100.1 g mol^1
1 mole of CaCO3 weighs 100.1g

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14
Q

Formula for number of moles

A

Number of moles: mass of substance/ molar mass

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15
Q

What is the mass of 1U / 1AMU (atomic mass unit) equivalent to?

A

The mass of a proton or neutron

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16
Q

How does a mass spectrometer work - (don’t learn but understand)?

A

1
A sample is placed in the mass spectrometer.
2
The sample is vaporised and then ionised to form positive ions.
3
The ions are accelerated. Heavier ions move more slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated.
4
The ions are detected on a mass spectrum as a mass-to-charge ratio m/z. Each ion reaching the detector adds to the signal, so the greater the abundance, the larger the signal.

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17
Q

What do titrations do

A

They allow you to find out exactly how much acid is needed to neutralise a quantity of alkali

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18
Q

Titration procedure

A

Measure out the volume of alkali using a pipette and put it in a flask, along with some indicator e.g. phenolphthalein
First, do a rough titration to see around where the alkali is neutralised and the indicator changing colour
To take a rough titration, take an initial reading to see how much acid is in the burette to start
Then add acid to the alkali - swirling the flask
Stop when the indicator shows a permanent colour change
Record final reading from the burette
Now do an accurate titration and run the acid to within 2cm3 of the end point, then add it dropwise and stop just as the solution changes colour permanently
Work out the amount of acid used to neutralise the alkali though the final reading minus the initial reading, this volume is known as the titre
Repeat the titre until the results are within 0.1cm3 of eachother (concordant) and calculate the mean titre
Wash out the conical flask between each titration to remove any acid or alkali left in

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19
Q

What are the two indicator changes

A

Methyl orange: turns yellow to orange when adding acid to alkali (alkali first)
Phenolphthalein: turns pink to colourless when adding acid to alkali (alkali first)

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20
Q

How to record volume accurately

A

Read the volume of the burette from the bottom of the meniscus
Take the readings to the nearest 0.05cm3

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21
Q

How to make a standard solution

A

Using a mass balance, weigh out the mass of solid
Transfer solid to a beaker, use water to wash any solid left into the beaker
Add water until the solid completely dissolves and stir using a glass rod
Transfer solution to a volumetric flask
Rinse the beaker and glass rod with water, transferring this into the volumetric flask
Use water to fill flask up to graduation line
Use pipette to add final few drops but make sure not to overshoot the line
Put the lid on the flask and turn it over a few times to mix the solution

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22
Q

What is the theoretical yield

A

The mass of product that should be made in a reaction if no chemicals are lost in the process
This can be calculated using the masses of reactants and a balanced equation

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23
Q

What is the actual mass

A

This is the actual mass of product
This is always less than the theoretical yield
Some chemicals are always lost during transfers between containers

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24
Q

What is percentage yield

A

The actual amount of product you collect
It is written as a percentage of the theoretical yield
This can be worked out with the formula:
Percentage yield = actual yield/ theoretical yield x 100

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25
Q

What does the percentage yield tell you

A

Tells you how wasteful the process is
Based on how much of the product is lost due to the reactions not completing or losses during collection and purification

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26
Q

What effects percentage yield

A

The reaction may not have gone to completion
There may be other side reactions taking place
Purification of the product may result in loss of some product
The reaction may involved several steps with produce loss

27
Q

What is atom economy

A

A measure of the proportion of reactant atoms that become part of the desired product ( rather than by-products ) in the balanced chemical
Calculated using the formula:
% atom economy = molecular mass of desired product/ sum of the molecular masses of all products x 100

28
Q

Difference between addition reaction and substitution reaction

A

In an addition reactants combine to form a single product.
Atom economy for addition reactions is always 100% since no atoms are wasted

Substitution reaction is where some atoms from one reaction are swapped with atoms from another reactant
This reaction always results in at least two products - the desired product and at least one by product

29
Q

Why do companies choose reactions with high atom economies

A

Companies in chemical industry will choose to use reactions with high atom economy
High atom economy has environmental and economic benefits

30
Q

What does a low atom economy mean

A

Low atom economy means there’s lots of waste produced

31
Q

Why are reactions with low atom economies less sustainable

A

Many raw materials are in limited supply, so they must be used efficiently so they last as long as possible
Also, waste has to go somewhere - it’s better for the environment if less is produced

32
Q

Why are reactions at lower temperatures and pressures better for the environment

A

Reaction conditions with high energy demands cost a lot
e.g. high temperatures and pressures
This means that reactions at lower temperatures and pressures are better for the environment

33
Q

What type of materials are better for the environment

A

Raw materials that come from renewable sources
e.g. plants, enzymes are better than materials from non-renewable sources e.g. crude oil, coal

34
Q

What is the ideal gas equation

A

pV = nRT
p = pressure ( Pa )
V = volume ( m^3 )
n = moles
R = gas constant 8.314 J K^-1 mol^-1
T = temperature ( K )

35
Q

How to convert Celsius into Kelvin

A

K = C° + 273

36
Q

How to convert into m^3

A

1 cm3 = 1 x 10^-6 m3
1 dm3 = 1 x 10^-3 m3

37
Q

Why are reactions with a low atom economy worse

A

It costs money to separate the desired product from the waste products and more money to dispose of the waste product safely so they don’t harm the environment
Reactant chemicals are usually costly. It’s a waste of money if a high proportion of them end up as useless products

38
Q

What does the oxidation number tell you

A

Says how many electrons an atom has donated or accepted to form an ion, or to form a part of a compound

39
Q

What is the oxidation number of uncombined elements

A

Oxidation number of 0
This means they haven’t accepted or donated any electrons
E.g. Ag, Xe
Elements bonded to identical atoms will also have an oxidation number of 0
E.g. O2, H2

40
Q

What is the oxidation number of a monatomic ions

A

This is the same as it’s charge
E.g. oxidation number of a sodium ion is +1

41
Q

What is the oxidation number of molecular ions

A

The sum of the oxidation numbers is the same as the overall charge of the ions
Each constituent atom will have an oxidation number of its own, which adds up to the overall charge
E.g. SO4 2- ion:
Oxygen in this molecule has an oxidation of -2
4 oxygen atoms in the ion so the total charge from oxygens is -8
Overall charge is -2
So oxidation number of sulfur is +6, -8 + 6 = -2

42
Q

Oxidation number of a neutral component

A

The overall oxidation number is 0
If the compound is made up of more than one element, each element will have its own oxidation number
E.g. MgCl2:
Chlorine forms ions with a charge of -1
Oxidation number of each Cl is -1
Oxidation number of Mg ions is +2
Overall charge on MgCl2 is 0

43
Q

Key oxidation number rules to remember

A

Oxygen nearly always has an oxidation number of of -2, except in peroxides (O2 2-) where it’s -1, and molecular oxygen (O2) where it’s 0
Hydrogen always has an oxidation number of +1, except in metal hydrides (MHx, where M = metal) where it’s -1 and in molecule hydrogen (H2) where it’s 0
Oxygen has an oxidation number of +1 when bonded to F

44
Q

How can Roman numerals tell you oxidation numbers

A

If an element can have multiple oxidation numbers, or isn’t in its ‘normal’ oxidation state, its oxidation number can be shown by using Roman numerals, e.g. (I) = +1, (II) = +2
The Roman numerals are written after the name of the element they correspond to

45
Q

What do ions with names ending in -ate contain

A

Contain oxygen and another element
E.g. sulfate contains sulfur and oxygen

Sometimes the other element in the ion can exist with different oxidation numbers
They will form different -ate ions
In this case the oxidation number is attached as a Roman numeral after the name of the -are compound
E.g. sulfate(VI) ions, the sulfur has oxidation number +6

46
Q

How to work out the formula of iron(III) sulfate

A

Has an oxidation number of +3
Formula for a sulfate ion is SO4 2- and it has an overall charge of -2
Overall charge of compound is 0, so you need to find a ratio of Fe 3+: SO4 2- to make the overall charge 0

(+3x2) + (-2x3) = 6 + -6 = 0
Ratio is 2 : 3
Formula is Fe2(SO4)3

47
Q

How to find the systematic name for ClO2 -

A

Formula contains chlorine and oxygen so it’s a chlorate
Oxygen usually exists with an oxidation number of -2
2 oxygens, so this will make a total charge from oxygen -2 x 2 = -4
The overall charge on the molecule is -1, so chlorine must have an oxidation number of +3, since -4 + 3 = -1
Systematic name is chlorate(III)

48
Q

What is oxidation and reduction

A

Oxidation is a loss of electrons
Reduction is a gain of electrons

49
Q

What are oxidising and reducing agents

A

An oxidising agent accepts electrons and gets reduced
A reducing agent donates electrons and gets oxidised

50
Q

How do oxidation numbers change with redox

A

The oxidation number for an atom will increase by 1 for each electron lost
The oxidation number will decrease by 1 for each electron gained

51
Q

How to work out whether something has been oxidised or reduced

A

Assign each element an oxidation number before the reaction, and after the reaction
If the oxidised number has increased, then the element has lost electrons and has been oxidised
If the oxidised number has decreased, then the element has gain electrons and has been reduced

52
Q

How do metals and non-metals act in redox reactions

A

When metals for compounds, they generally donate electrons to form positive ions
Meaning they usually have positive oxidation numbers

When non-metals form compounds, they generally gain electrons
Meaning they usually have negative oxidation numbers

53
Q

What happens to metals when they react with acids

A

They are oxidised to product a salt and hydrogen gas
This is a redox reaction:
The metal atoms are oxidised, losing electrons to form positive metal ions (in salts)
The hydrogen ions in solution are reduced, gaining electrons and forming hydrogen molecules

54
Q

How to work out percentage uncertainty

A

Margin of error/ quantity measured x 100
Margin of error may have to be multiplied by 2 as sometimes you are measuring change (taking two readings)
E.g. temperature change, mass change, change in volume

55
Q

How to construct a half equation with an example IO3- -> I2

A

Write out and balance equation: 2IO3- -> I2
Find out difference in oxidation numbers (2IO3-: +5 and I2: 0)
IO3- has gained 10e-
Equation is 2IO3- + 10e- -> I2
6 oxygen on reactants side so add 6H2O to balance the products out
2IO3- + 10e- -> I2 + 6H2O
12H+ ions on products side so add 12H+ on reactant side
Full equation: 2IO3- + 10e- + 12H+ -> I2 + 6H2O

56
Q

How to combine half equations of:
MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O
2Cl- -> Cl2 + 2e-

A

MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O
2Cl- -> Cl2 + 2e-
5 electrons on MnO4- and 2 electrons 2Cl-
Multiply MnO4- by 2 and multiply 2Cl- by 5
(Get rid of electrons when combining)
2MnO4- + 16H+ + 10e- -> 2Mn2+ + 8H2O
10Cl- -> 5Cl2 + 10e-
2MnO4- + 16H+ + 10Cl- -> 2Mn2+ + 8H2O + 5Cl2

57
Q

What is disproportionation ?

A

A redox reaction where the same element is oxidized and reduced
Tends to be chlorine

58
Q

How to work out “how many atoms in” questions

A

Find moles of substance (mass/Mr)
Multiply by 6.02x10^23

59
Q

Assumptions for an ideal gas

A

Random motion
Elastic collisions
Negligible size
No intermolecular forces

60
Q

Definition of an acid

A

Proton (H+) donors

61
Q

What are alkalis

A

Soluble bases
Release OH- ions in water

62
Q

What is water of crystallisation

A

These are water molecules that are incorporated into the lattice
A solid salt containing water of crystallisation is hydrated

63
Q

What do you rinse burette with before titration and why

A

Rinse with acid/ titrant before titration
This rinsing ensures that there is no residual water to dilute the titrant when the burette is filled

64
Q

What do you rinse conical flask with before titration and why

A

The conical flask is to be rinsed with water prior to use.
Use distilled water for rinsing because any dilution of the solution in the conical flask will not affect the number of moles of the substance

A Level Chemistry (15 decks)

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