AQA A level Chemistry

Atom

• The smallest particle of matter

• Made up of protons, neutrons and electrons

Element

Element is only made up of one sort of atom. They are all listed in the periodic table

Molecule

Two or more atoms covalently bonded together. The bonding can be either ionic or covalent

When ionic substances and acids dissolve in water…

They separate into their positive and negative ions. The ions become hydrated - they interact with water molecules rather than each other

Compound

• Has two or more different types of atom bonded together

• Bonding can be either ionic or covalent

Mixture

Mixture has two or more different elements and/or compounds in the same space and can be separated into the individual components.

Acid

Substance that donates H+ ions

Base

A base is a substance that accepts H+ ions

Alkali

An alkali is a base that releases hydroxide ions in aqueous solution

Salt

Formed when the hydrogen ions from an acid are replaced by a metal or ammonium ion

How has the current model of the atom developed over time?

• Dalton - elements are made of tiny particles called atoms

• Thomson - atoms have negative particles called electrons + the plum pudding model where electrons are scattered through a cloud of positive charge

• Rutherford - the gold foil experiment. alpha particles were fired at a thin sheet of gold foil, most went straight through but some were deflected at a large angle. most of the atom is empty space. the positive charge is concentrated in the centre - the nucleus.

• Bohr - electrons move round the nucleus in shells of fixed size and energy

• Rutherford - named positive charge a proton

• Schrodinger - regions of space called orbitals where we are likely to find an electron

• Chadwick - discovered uncharged particles called neutrons

What is the relative mass of a proton, neutron and electron?

• Proton : 1

• Neutron: 1

• Electron: very small or 1/1840

What is the relative charge of a proton, neutron and electron?

• Proton: +1

• Neutron: 0

• Electron: -1

Why do isotopes have the same chemical properties?

They have the same electron configuration

What is Mass spectrometry and how is it used?

Mass Spectrometry is a modern analytical technique that chemists use to determine info about elements and compounds. It can be used to calculate the relative atomic mass of an element or the relative molecular mass of a compound

What are the four stages of TOF mass spectrometry

1) Ionisation

2) Acceleration

3) Separation of ions

4) Detection

What are the two different methods of ionisation?

1) Electron impact ionisation

2) Electrospray ionisation

Describe electron impact ionisation and give the general equation

• Sample is injected into spectrometer and vaporised

• High energy electrons from an electron gun are fired at the sample

• They knock off an electron from each particle to form positive ions

M (g) → M+ (g) + e-

Describe electrospray ionisation and give the general equation

• Sample is dissolved in polar solvent

• Injected through a fine hypodermic needle to give a fine mist

• Tip of the needle is attached to the positive terminal of a high voltage supply

• Particles gain a proton (H+ ion) from the solvent to form positive ions

M + H+ → MH+

Why is a volatile solvent used in electrospray ionisation?

To ensure it evaporates to leave positive ions (MH+)

The mass of the ion depends on…

the mass of ion

Describe acceleration in TOF

• An electric field is applied to accelerate the positive ions

• This gives all the ions with the same charge the same kinetic energy

• KE = ½ mv²

Describe separation in TOF, and what is the equation for TOF

The positive ions travel through the mass spectrometer to the detector

• Lighter ions travel faster and reach the detector first

• Heavier ions travel slower and reach the detector last

TOF = distance travelled / velocity

Describe detection in TOF,, include equations

A detector records the different ions as they arrive

• At the detector each positive ion gains an electron

• This generates a current

• M+ + e- → M

• The size of the current is proportional to the abundance of the ion

• The greater the abundance, the higher the current

What are the two key principles of TOF mass spectrometryr?

• The sample must be ionised so that it can be accelerated and detected

• The whole spectrometer is kept under vacuum to prevent ions from colliding with molecules from air

What is the equation for calculating the mass of one ion in kg?

Relative isotopic mass x 10^-3

divided by

6.022 × 10²³

In TOF, all ions with the same charge…

are given the same kinetic energy

What is the equation for calculating the time of flight

m1/(t1)² = m2/(t2)²

The relative atomic mass of magnesium is 24.3. Why is there no peak in the spectrum at 24.3?

24.3 is the weighted average mass of all the isotopes. The mass spectrum shows one peak for each isotope as each isotope has a different mass.

Isotopic mass

The mass of an isotope relative to 1/12th the mass of an atom of carbon-12

Relative atomic mass

Weighted average mass of all the isotopes

divided by

1/12 mass of one atom of carbon-12

As bromine is a diatomic molecule, peaks will be seen for both the ion and the atom themselves. What is the relative intensity of the peaks?

Relative intensities:

158: one

160: two

162: one

What is the molecular ion peak, M+, and what is it the same as?

32, which is the same as the Mr

What is the isotopic peak and why?

33, because some molecules will contain carbon-13 or hydrogen-2

Why are there peaks below m/z?

• Electron impact ionisation

• Causes some molecular ions to break apart

• Forming fragments

What is electron impact ionisation used for?

Elements, smaller molecules

What is electrospray ionisation used for?

Larger molecules such as proteins

What is the max. number of electrons in each main shell 1-4?

Formula: 2n²

Main shell one: 2 e-

Main shell two: 8 e-

Main shell three: 18 e-

Main shell four: 32 e-

How many orbitals are in each subshell?

s: 1

p: 3

d: 5

f: 7

What is the max. number of electrons in each subshell?

s: 2 e-

p: 6 e-

d: 10 e-

f: 14e-

What’s different about Cu and Cr in terms of electron configuration?

Cu and Cr are more stable with half full and full 3d sub shells respectively

Describe the factors which influence first ionisation energy?

1) Amount of shielding - inner shells of electrons shield the outer electron from the nuclear attraction

2) Number of protons - more protons leads to a greater attraction between nucleus and outer electron

3) Atomic radius - the distance of the electron being removed from the nucleus

The smaller the ionisation energy…

The easier it is for the outer electron to be removed

Down a group, first ionisation energy…

Decreases.

As you go down a group

• atomic radius increases (there are more shells)

• there is more shielding

So the attraction between the nucleus and outer e- decreases, so less energy is needed

Why is there a small dip in first ionisation energy between the group 2 and 3 elements (eg from Be → B and Mg → Al)

The outer electron in B is in the 2p subshell which is further away from the nucleus with a higher energy level and more shielded. So there is less attraction between the nucleus and the outer electron in B

Why there is a small dip in first IE between nitrogen and oxygen and again between phosphorous and sulfur?

In O there is a pair of electrons in the 2p orbital, these repel and so the electron is easier to remove.

Why is there a big drop in first IE between the end of one period and the start of the next? (eg. Ne → Na)

The outer electron being removed in Na is in the s subshell which is further from the nucleus with more shielding. Therefore the attraction between the nucleus and outer electron is less in Na, so less energy is needed to remove the electron.

A mole of any substance contains the same number…

of particles as a mole of any other substance.

Formula to find out mole?

n = m/mr

How many grams is in 1 mg?

1×10^-3 grams

How many grams is in one tonne?

1 tonne is 1 × 10^6 grams

What is the formula for the number of particles?

number of particles = mole x avogadro’s constant (6.02×10²³)

what is the formula to calculate the mole in volumes of solutions?

mole = volume x concentration

Methyl orange’s colour in acid, base, and end point colour?

Methyl orange

Acid: red

Base: yellow

End point: orange

Phenolphthalein colour in acid, and base?

Acid: colourless

Base: pink

How to make a standard solution?

• Record the mass of a weighing boat and solid

• Tip solid into a beaker and record the mass of the weighing boat

• Determine the mass of solid used by calculating the difference between the two masses (weigh by difference)

• Dissolve the solid in deionised water

• Using a funnel, transfer into a volumetric flask, include washings

• Make up to the graduated line by carefully adding deionised water, making sure the meniscus sits on the line eye-level

• Stopper the flask and invert to mix the contents thoroughly

Practical method to find the concentration of a solution of a base?

• Fill a burette with a standard solution of the acid

• Pipette a known volume of base into a conical flask

• Add a few drops of indicator to the base in the flask

• Add the solution of the acid from the burette until the indicator just changes colour. This is known as the end-point

• Record the volume of acid added, record your burette readings to the nearest 0.05 cm³

• Perform a rough tiration and then sufficient accurate ones until the titres are concordant (within 0.10 cm³ of each other)

• Calculate the mean titre from the concordant titres to two decimal places

What is the formula for percentage error?

uncertainty/value x 100

Hydrated salt

Salt which contains water

Water of crystalisation

Water molecules which form an essential part of the crystalline structure of a hydrated salt

Percentage yield formula

actual yield/theoretical yield x 100

Four reasons for a low percentage yield?

1) Incomplete reactions/reversible reactions

2) Side reactions

3) Loss of product during transfer or purification steps

4) Impurities in reactants

Atom economy formula?

Mr of desired product/Mr of all products x 100

Atomic number

The number of protons in the nucleus of an atom

Mass number

The number of protons and neutrons in the nucleus of an atom

Isotopes

Atoms of the same element with a different number of neutrons

Why do Isotopes have the same chemical properties?

They have the same electron configuration

Isotopic mass

The mass of an isotope relative to 1/12th the mass of an atom of carbon 12

Relative atomic mass

The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon 12

Explain isotopic peaks

Some molecules will contain 13C or 2H

Outlier Elements for electron configuration

Cr and Cu are more stable. Chromium has a half full 3d subshell and Copper has a full 3d sub shell.

Ionisation energy

The energy needed to remove one mole of electrons from one mol of atoms in the gaseous state

Relative atomic mass

The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon 12

Relative formula mass

The mass of one formula unit of an ionic compound relative to 1/12th the mass of an atom of carbon 12

The mole

The amount of substance that contains as many particles as there are in exactly 12 grams of carbon 12

Empirical formula

The simplest whole number ratio of atoms of each element present in a molecule

Molecular formula

The actual number of atoms of each element present in a molecule

Ionic Bond

The electrostatic attraction between oppositely charged ions

Metallic Bond

The attraction between positive ions and delocalised electrons

What is the charge on CN

negative one

what is the charge on HPO4

negative 2

what is the charge on SO3 and SO4

negative 2

In the periodic table the elements are arranged in order of

increasing atomic number

A period

horizontal row of elements in the periodic table

A group definition, and why do they have similar properties

a vertical column. elements in a group have similar chemical properties because they have the same number of outer shell electrons

First ionisation energy

Energy needed to remove one mole of electrons from one mole of atoms in the gaseous state

Explain the trend of first ionisation energy across a period. Are there any dips across period 2 and 3?

• First Ionisation energy increases

• Across a period…

• Electron being removed is in the same shell with similar shielding

• There are more protons in the nucleus

• Nuclear attraction to the outer electron increases

• More energy is needed to remove it

• There is a dip between group 2 + 3 and 5 + 6

Explain the trend of atomic radius across a period

Atomic radius decreases across the period, there are the same number of shells/similar shielding and more protons in the nucleus so nuclear attraction is greater and electrons are held more tightly

Are cations smaller/larger than their atoms and why?

• Cations are smaller than their atoms

• Same number of protons are attracting fewer electrons

• They are held more tightly

Are anions smaller/larger than their atoms and why?

Anions are larger than their atoms

Same number of protons attracting more electrons.

Held less tightly

Electronegativity

The power of an atom to attract the pair of electrons in a covalent bond

Explain the trend of electronegativity across a period

Electronegativity increases across the period, as there are more protons in the nucleus and a same number of shells with similar shielding. therefore the pair of electrons in the covalent bond are more strongly attracted

Why is there variation in melting and boiling points in period 3?

Due to changes in structure and bonding across the period

What is the structure of the elements Na to Al

Giant metallic lattice and metallic bonding

Explain the trend of the melting and boiling point from Na to Al?

Melting and boiling point increases.

Charge on the ion increases

More delocalised electrons

So metallic bonds are stronger

More energy is needed to overcome them

What is silicon’s structure, bonding, and boiling point?

Silicion is a giant covalent lattice with very strong covalent bonds between silicion atoms which must be broken under high energy

What is the structure, explain the melting and boiling points, and the order of P4, S8, Cl2?

• Simple molecular structure

• Weak van der waals forces between molecules.

• Order of melting/boiling point is S8 > P4 > Cl2

• The bigger the molecule, the more electrons

• So the stronger the Van der Waals forces between the molecules

What is the structure of Ar?

Simple atomic, very weak van der waals forces between atoms

Explain how the first ionisation energy varies across period 2 vs 3

First IE of period 2 are all above period 3

Less shells and shielding

Stronger nuclear attraction to outer electron

Why does Al have a higher melting point than Mg?

• Has greater charge density as Al3+ is more highly charged than Mg2+

• There are more delocalised electrons in Al than in Mg

• Stronger forces of attraction between Al3+ ions and delocalised electrons

• Al3+ is smaller than Mg2+ so more closely packed

• Metallic bonds are stronger

• So more energy is needed to overcome