AQA AS Physical Chemistry

Who described atoms as solid spheres that make up different elements and when?

John Dalton in the 19th century

Who suggested that atoms of a particular element had the same mass and atoms of different elements had different masses and when?

John Dalton in 1803

Who discovered the electron and when?

JJ Thomson, 1897 discovered electrons proving atoms weren't indivisible or solid.

New model named plum pudding model

Describe the plum pudding model

A ball of positive charge with negative electrons in embedded in it

Who disproved the plum pudding model and how?

Rutherford and Geiger + Marsden

By conducting the gold foil experiment. Positively charged alpha particles were fired at a very thin sheet of gold foil.

If the plum pudding model was correct then the charged particles would have mostly deflected off the gold foil, but most of the particles passed straight through.

Who discovered radioactivity and when? What did this show?

Henri Bequerel in 1896.

Particles could come from inside the atom.

State the three subatomic particles and name were they are found.

Protons - in the nucleus

Neutrons - in the nucleus

Electrons - in orbitals around the nucleus

What is the (relative) charge and the (relative) mass of protons?

Charge: +1.062 x 10^-19

Relative charge: +1

Mass: 1.673 x 10^-27 kg

Relative mass: 1

What is the (relative) charge and the (relative) mass of neutrons?

Charge: 0

Relative charge: 0

Mass: 1.675 x 10^-27 kg

Relative mass: 1

What is the (relative) charge and the (relative) mass of electrons?

Charge: -1.062 x 10^-19

Relative charge: -1

Mass: 0.911 x 10^-30 kg

Relative mass: 1/1840

What is an orbital?

A region of space where there is a 95% chance of finding an electron

How are the protons and neutrons held together?

By strong nuclear forces.

Why does the repulsion between protons not make the nucleus fall apart?

Nuclear forces are much stronger than the electrostatic forces that hold the electrons and protons together. This means that they can overcome the repulsion between the protons in the nucleus.

Why do elements in the same group have similar chemical properties?

They have the same number of electrons in their outer shell

Define the term 'isotope'

Atoms of the same element which have the same number of protons but different numbers of neutrons

Why do different isotopes of the same element react chemically in exactly the same way?

They have the same electronic configuration

What is carbon dating?

The process of using the radioactive isotopes in carbon-14. All living things contain carbon-14.

Carbon-14 decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food. Once the organism dies, however, it cannot absorb carbon-14. This means that the amount of the radiocarbon in its tissues steadily decreases.

Because carbon-14 decays at a constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.

What can carbon dating be used for?

To determine the age of carbon-based meterial up to 60,000 years old. However, it is more accurate for up to 2000 years old.

Describe the nuclear model.

Positively charged nucleus in a cloud of negative electrons.

Most of the atom is empty space.

Why did Neil's Bohr disagree with the nuclear model?

Believed electrons existed in shells rather than a random cloud .

When electrons moved between shells, a electromagnetic radiation was emitted or absorbed.

What is Bohr's atomic model?

Not all electrons in the same shell have equal energy.

Refined nuclear model includes sub shells.

Why is the refined Bohr model used instead of more accurate models that exist today?

As it is simple and explains many experimental observations

Who discovered the neutron and when?

James Chadwick in 1932

What did Gilbert Lewis do?

He put forward the ideas that:

1) Noble gases were inert due to their full outer shell.

2) Ions were formed when atoms gained or lost electrons

3) Atoms could bond by sharing electrons to form full outer shells

What is meant by the term 'electron cloud probability'

You can never really say where an electron is at any given moment. You can only state the probability that an electron will be found in a certain area of space.

What did Schrodinger discover?

Electrons had some of the properties of waves as well of those called particles.

What can Dalton's model of the atom be used for?

Working out the geometries of crystals

What can Bohr's model of the atom be used for?

Simple models of covalent and ionic bonding

Name the 4 subshells. Put them in order of energy from lowest to highest.

s, p, d, and f

How many electrons can each orbital hold?

two

How many orbitals are there in each sub-shell?

s = one

p = three

d = five

f = seven

How many electrons can each sub-shell hold?

s = two

p = six

d = ten

f = fourteen

How many electrons can an s sub shell hold?

Two electrons. These electrons both go in one orbital.

In the example "1s2" what does the "1", "s" and "2" stand for?

1 = Electrons are in shell one

S = Electrons sit in the s sub-shell

2 = There are two electrons

What is electron spin?

Electrons have two states, 'spin up' and 'spin down'. In an orbital, each electron will be in a different spin state.

What is the total number of electrons in 4 energy levels?

1st — 1s = 2electrons

2nd — 2s 2p = 8 electrons

3rd — 3s 3p 3d = 16 electrons

4th — 4s 4p 4d 4f = 32 electrons

In what order do electrons fill shells?

Lowest energy first.

What happens to the electrons when they fill the p-oribital? Why?

They must be filled singly and then doubly. This is because there is less electron-pair repulsion.

(Think about people on the bus. You don't want to sit next to someone unless you absolutely have to)

How are ions different from atoms?

Ions have different number of protons and electrons

How are ions formed?

When atoms lose or gain electrons

-ive ions formed when electrons are gained

+ive ions formed when electrons are lost

Define the term 'relative atomic mass

The average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12.

Define the term 'relative isotopic mass'

The mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12.

Define the term 'relative molecular mass'

The average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12.

How can you measure relative masses?

Use a mass spectrometer

What are the 6 stages of time of flight (TOF) mass spectrometer?

1) Vacuum

2) Vaporisation and ionisation

3) Acceleration

4) Ion Drift

5) Detection

6) Data Analysis

Why is a vacuum important in mass spectrometry?

To prevent the ions that are produced from colliding with the air

What happens during the ionisation stage of mass spectrometry?

The sample is dissolved in a volatile solvent and forced through high voltage. This produces positive ions,

Name 2 methods of ionisation

Electrospray ionisation

Electron impact/ionisation

What happens during the acceleration stage of mass spectrometry?

1) Positive ions are attracted towards an negatively charged plate.

2) They accelerate more and move faster if the ions are lighter (have a lower m/z).

What happens during the ion drift stage of mass spectrometry?

1) Ions leave electric field at a constant kinetic energy.

2) Enter a region with no electric field so they drift

3) Lower m/z = higher drift speeds

What happens during the detection stage of mass spectrometry?

1) +ive ion hits -ively charged electric detector plate

2) +ive ion gains electron from the plate and become neutral

3) Movement of electrons generates a current

4) Size of the current gives a measure of the number of ions hitting the plate.

If kinetic energy is the same, what factor effects velocity?

Mass

What happens during the data analysis stage of mass spectrometry?

The signal from the detector passes to a computer which generates a mass spectrum.

What would a mass spectrum would look like?

X axis — m/z

Y axis — % abundance of isotopes

Height of each peak = relative isotopic abundance

How do you calculate relative atomic mass from a mass spectrum?

relative atomic mass (Ar) = combined mass of all isotopes ÷ combined abundance of all isotopes

Explain why mass spectrometry can be used to identify different isotopes of the same element.

Isotopes of the same element have different masses (remember: isotopes of the same element have different numbers of neutrons)

Define the term 'ionisation energy'

The energy required to remove a mole of electrons from one mole of gaseous atoms or ions.

Why is the first ionisation energy always endothermic (the values are always positive)?

Energy is always required to remove an electron, it is endothermic.

Electrons are attracted to the nucleus therefore energy is needed to remove them.

Explain what happens to the ionisation energies as you go down a group.

It decreases. This is because the outer electron is further away from the nucleus so it is easier to remove an electron.

Explain the general trend in ionisation energies across a period

Increases.

This is because:

1. Increase nuclear charge

2. Force of attraction between nucleus and outer electron increases.

3. Shielding stays the same

4. Harder to remove an electron

Why does the first ionisation energy drop between elements in groups 2 and 3 (Be-B and Mg-Al)?

Because:

1. Outer electron is in a p-orbital not an s-orbital

2. Outer electron further from nucleus

3. Less of an attraction between the outer electron and the nucleus

4. Easier to remove an electron

Why the drop in ionisation energies between elements in groups 5 and 6 (N-O and P-S)?

Because:

1. Same amount of shielding

2. Electron is being removed from the px orbital

3. The electrons in this orbital are already repelling each other

4. Easier to remove one of the electrons

Explain what happens to the ionisation energies across periods 2 and 3.

Increase

Because:

1. Same amount of shielding

2. More protons in nucleus

3. Greater attraction between outer electron and nucleus.

4. Harder to remove an electron.

Define the term 'mole'

The amount of substance that contains 6.022 x 10^23 particles

Define the term 'concentration'

The amount of atoms or particles in a given volume

Define the term 'solvent'.

A liquid in which another substance is dissolved

Define the term 'solute'.

Substance that is being dissolved

Define the term 'solution'.

A liquid consisting of a uniform mixture of two or more substances

What two factors affect the volume of a gas?

Pressure and temperature

Define the term 'empirical formula'

The simplest whole number ratio of atoms within a compound

Define the term 'molecular formula'

The actual number of atoms within a compound

Define the term 'stoichiometry'

The ratio in which the reactants react and the products are produced in simple, whole numbers

What information can we work out from balanced equations?

1) The amount of substances that react together and are produced.

2) States of matter

3) Stoichiometry

4) Molar ratio

What are the rules for balancing equations?

1) Use the correct formula

2) Change the coefficient only

3) Coefficient tells you how many moles are reacting

4) When dealing with ionic equations, the total charges on each side must balance

What are spectator ions?

Ions that do not take part in the reaction

Describe a practical method for conducting a titration.

1) Fill a burette with the acid of known concentration

2) Use a volumetric pipette to measure the amount of alkali and add it to a conical flask

3) Add a few drops of indicator to the conical flask

4) Add the acid from the burette dropwise until the indicator changes colour

5) Repeat until results are concordant

Name two indicators that can be used for titrations

Methyl orange

Phenolpthalein

What factors affect atom economy?

1) Quantity of reactants (you can only make as much as your limiting reagent allows)

2) Inefficient reactions

3) Purity of reactants

What factors affect percentage yield?

1) Product lost during separation (e.g. filtration)

2) Side reactions

3) Incomplete reactions

4) Impure reactants

5) Inefficient reactions

6) Quantity of reactants (you can only make as much as your limiting reagent allows)

What are the rules of chemical sustainability

Reduce the use of hazardous chemicals

Reduce the amount of waste

Recycle the waste

Use fewer chemicals

Reduce the amount of energy the process requires

Use renewable feedstocks

Manufacture biodegradable products

Safety

Establish international co-operation

Define a 'compound'

A substance consisting of two or more elements that are chemically combined.

Name the three types of intramolecular bonding and indicate which is the strongest

1) Covalent (weakest)

2) Ionic

3) Metallic (strongest)

What is the difference between intramolecular and intermolecular forces of attraction?

Intramolecular - holds the atoms WITHIN a molecule together

Intermolecular - holds DIFFERENT molecules together

What type of atoms are involved in ionic bonding?

Non-metal and metal

What type of atoms are involved in covalent bonding?

Non-metals only

What type of atoms are involved in metallic bonding?

Metals only

Define an ionic bond

The electrostatic force of attraction between oppositely charged ions

Explain how an ionic bond is formed.

Atoms gain or lose electrons to become ions.

Ions of opposite charge are attracted, electrostatically.

What is the formula of a sulphate ion?

(SO4)2-

What is the formula of a hydroxide ion?

(OH)-

What is the formula of a nitrate ion?

(NO3)-

What is the formula of a carbonate ion?

(CO3)2-

What is the formula of an ammonium ion?

(NH4)+

How do you work out the formula of an ionic compound?

Use swap and drop with the charges

E.g- Mg 2+ and Cl-

MgCl2

Na+ (+1) NO3- (-1)

NaNO3

What is a lattice structure?

A regular, repeating pattern of atoms or ions

State four properties of ionic compounds

High melting and boiling points

Conduct electricity

Brittle

Soluble in water

Why do ionic compounds conduct electricity when molten or dissolved, but not when solid?

They have ions which are free to move and carry a current.

Why do ionic compounds have high melting and boiling points?

Giant ionic lattices held together by strong electrostatic forces.

These require a lot of energy to break.

Why do ionic compounds dissolve in water?

Water molecules are polar. The slight negative and slight positive pull charged part of ions away from the lattice causing it to dissolve

Why are ionic compounds brittle?

They exist in a lattice structure. When the ions are forced out of position, ions with the same charge line up and cause repulsion.