AQA A-Level - Physical Chemistry

proton

negatively charged particles

relative mass 1

relative charge +1

neutron

relative mass : 1

relative charge: 0

electron, e-

relative mass: 1/2000

relative charge: -1

nuclear symbols

mass number - total number of protons + neutrons

atomic number - total number of protons in nucleus. identifies element.

all atoms of the same element have the same number of protons.

ions have different numbers of protons and electrons

atoms form ions by gaining or losing electrons

negative ions have more electrons than protons

eg: Br- (has 1 more electron than there are protons)

positive ions have fewer electros than protons

Mg2+ --> 2+ charge means there are 2 fewer electrons than protons. Mg has 12 protons so Mg2+ ,must have 10 electrons.

what is an isotope

how are their properties affected?

atoms of same element with different numbers of neutrons

number and arragement of electrons that decides the chemical properties of an element

also do have slightly different physical properties (such as different densities, rate of diffusion, etc)

physical properties tend to depend on mass of the atom

atomic number

number of protons in nucleus - identifies element

all atoms of same element have same number of protons

What did John Dalton propose ? (Model of Atom - 1)

described atoms as solid spheres and different spheres made up different elements

what did J. J. Thomson propose? (Model of Atom - 2)

showed that atoms weren't solid and indivisible

new model was known as plum pudding model

what did Ernest Rutherford propose? (Model of Atom - 3)

rutherford developed nuclear model of atom

tiny + charged nucleus surrounded by cloud of negative electrons

most of atom is empty space

how did Rutherford conduct his Gold Foil Experiment

fired + charged alpha particles at thin sheet of gold

plum pudding suggested alpha particles would be slightly deflected by positive pudding

but most particles passed straight through gold with only a small number being deflected backwards

what did Niels Bohr propose? (Model of Atom - 4)

Bohr suggested electrons exist in shells or obits of fixed energy

when electrons move between shells, electromagnetic radiation is emitted or absorbed

model fitted experimental observations of radiation emitted and absorbed by atoms

what was the Refined Bohr Model set to include? ( Model of Atom - 5 )

scientists later discovered that not all electrons in shell have same energy

refined model to include subshells

Relative Atomic Mass

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

Relative isotopic mass

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

Relative Molecular Mass

average mass of a molecule compared to 1/12th of the mass of a carbon-12 atom

what can a mass spectrometer be used to identify?

what can it be used to determine?

identify elements

determine relative molecular masses

Mass Spec (1) Describe Electrospray Ionization

Electrospray ionization - sample dissolved and pushed through small nozzle at high pressure

high voltage applied to it causing each particle to gain an H+ ion

sample turned into gas made up of positive ions

Mass Spec (1) Describe Electron Impact Ionization

Electron Impact ionization - sample is vaporized and 'electron gun' used to fire high energy electrons at it

> knocks one electron off each particle so they become +1 ions

Mass Spec (2) Describe Acceleration

+ charged ions are accelerated by electric field so they have same kinetic energy

(lighter ions will end up moving faster than heavier ions)

Mass Spec (3) Describe Ion Drift

ions enter a region with no electric field so they drift through it

lighter ions will drift through faster than heavier ones

Mass Spec (4) Describe Detection

detectors detect charged particles and mass spectrum produced

an electrical current is produced in detector when a charged particle hits it

lighter ions travel at higher speeds in drift region they reach the detector in less time than heavier ions

Exam Technique: Mass Spectrum

A graph with % abundance plotted against mass/charge, gained as a result from the mass spectrometer.

if the sample is an element, each line will represent a different isotope of the element

height of each peak gives relative isotopic abundance

Mass Spectrum with Electrospray Ionization

a H+ would have been added to each particle to form +1 ions -

so mass/charge ratio of each peak would be one unit greater than relative mass of each isotope

Mass Spectrum with Electron Impact Ionization

one electron has been knocked off each particle to turn them into +1 ions

so m/z ratio of each peak is same as relative mass of that isotope

How to calculate Ar (Relative Atomic Mass) from a Mass Spectrum

for each peak read the % abundance for the y axis and relative isotopic mass from x axis

multiply them together to get total mass of each isotope

add up all totals

divide by 100

Exam technique: If Relative Abundance isn't given as a percentage

multiply abundance and m/z together to get total mass of each isotope

add them together

then divide them by the sum of relative abundances (addition)

instead of 100

How is Mass Spectrometry used to Identify Elements

elements with different isotopes produce more than one line in mass spectrum since isotopes have different masses

can be used as 'fingerprints' to identify certain elements

(eg % abundance of one isotope may be 79 so its 24Mg, other is 10% so its 25Mg)

how is Mass Spectrometry used to Identify Molecules

molecular ion, M+ formed in mass spectrometer when one elctron is removed from the molecules

gives a peak int he spectrum with a m/z equal to Mr of molecule

can help identify unknown compound

Describe how electrons move around the nucleus.

electrons have fixed energies

move around nucleus in shells

the further a shell is from nucleus, higher its energy

Electron Sub-shell + Orbital Table

two electrons in each orbital spin in opposite directions

Explain the structure of Transition Metals and Electronic Structure

Chromium + Copper donate one of their 4s subshells to 3d subshells

They're 'happier' with a more stable full or half-full d subshell

when they become ions they lose their 4s electrons before their 3d

(since it comes before 3d on periodic table)

Exam Technique: Electronic Structure Decides Chemical Properties of Element (S,P,D,0)

S block elements (group 1 + 2) have 1/2 outer shell electrons

easily lost to form positive ions with inert gas configuration

(eg Na = 1s2s2p3s --> Na+ = 1s2s2p6)

P block elements (group 5,6 +7) can gain 1, 2 or 3 electrons to form negative ions with inert gas configuration

(O: 1s2s2p4 --> O2-: 1s2s2p6)

D block elements (transition metals) tend to lose s and d electrons to form positive ions

Group 0 (inert gases) - completely filled s and p sub-shells so no need to gain or lose/share electrons - they are inert because of full subshells

what is First ionization energy

energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Exam Technique: Important points about ionization energies

1) must use gas state symbol (g) since ionization energies are measured for gaseous atoms

2) Always refer to 1 mole of atoms, rather than to a single atom

3) Lower the ionization energy, the easier to form an ion

High Ionization Energy (Factors affecting Ionization Energy)

high ionization energy means theres a high attraction between electron + nucleus so more energy is needed to remove electon

Shielding (Factors affecting Ionization Energy)

lessening of the pull of nucleus by inner shells of electrons.

as number of electrons between outer electrons and nucleus increases

outer electrons feel less attraction towards nuclear charge

Nuclear Charge (Factors affecting Ionization Energy)

more protons in nucleus, the more positively charged the nucleus is and the stronger the attraction is for electrons

Distance from Nucleus (Factors affecting Ionization Energy)

attraction falls off very rapidly with distance

electron close to nucleus will be much more strongly attracted than one further away

Successive Ionization energy

A measure of the energy required to remove each electron from an atom, leaving only the nucleus.

Second Ionization Energy

energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions

*Exam Tech: Writing Equations for nth Ionization Energy

X(^N-1)+ --> X^n+ + e^-

How do Successive Ionization Energies give evidence for sub-shell Structure

within each shell successive ionization energies increase

electrons are being removed from an increasingly positive ion so there's less repulsion amongst remaining electrons

so they are held more strongly by the nucleus

big jumps in ionization energy happen when a new shell is broken into - electron being removed from a shell closer to nucleus

Exam technique: How to identify Periodic Table Group Number from Graph of Successive Ionization Energies

count how many electrons removed before big jump to find group number

(eg in sodium, one electron is removed before first big jump)

Exam Technique: How to identify Electronic Structure from Graph of Successive Ionization Energies

can be used to predict electronic structure of elements

working from RIGHT TO LEFT

count how many electrons in each shell, starting from the first

What is the Trend in First Ionization Energies

the first ionization energies down a group decrease

the first ionization energies across a period generally increase

Why does Ionization Energy Decrease Down Group 2

each element down group 2 has an extra electron shell compared to the one above

so extra inner shells will shield the outer electrons

extra shells mean that outer electrons are further away from nucleus so the nucleus's attraction will be greatly reduced

Why does Ionization Energy Increase Across a Period

ionization energies increase across a period

since number of protons is increasing so a stronger nuclear attraction

extra electrons are roughly the same energy level even if outer electrons are in different orbital types

means there are generally little extra shielding effect/distance to lessen attraction from nucleus

Why do drops between groups 2 and 3 shows sub shell structure (Evidence for Theory of Subshells)

eg: Be and B

Outer electron of group 3 is in a 'p' orbital rather than a 's'.

p orbital has a slightly higher energy than the s orbital so the electron is further from the nucleus

p orbital has additional shielding provided by s2 electrons

both factors together are strong enough to override effect of increased nuclear charge, resulting in ionization energy dropping slightly

Why does the drop between groups 5 and 6 provide evidence for Electronic Structure Model?

eg: P and S

shielding is identical to both atoms and electron is being removed from identical orbital

in phosphorus - electron being removed from a slightly occupied orbital (3p3)

in sulfur electron is being removed from orbital containing 2 electrons (3p4)

repulsion between 2 electrons means electrons are easier to remove from shared orbitals

Number of particles equation

number of particles = number of moles x Avogadro's constant

Amount of a substance in moles Equation

Number of moles = mass of substance / Mr

Concentration of a Solution equation + units

concentration = mol dm-3

number of moles = conc (mol dm-3) x volume (dm3)

*Exam Technique: Different Units provided

You may be provided a solution that's in cm3 instead of dm3 and told to calculate the mass of a compound.

So, you convert the units to dm3

Calculate the number of moles (m=c x v)

and then

mass = number of moles x Mr

Ideal Gas Equation

PV=nRT

where:

p = pressure (Pa)

V= volume (m3)

n = number of moles

R = the molar gas constant

T = temperature. (K = °C + 273)

ensure everything is in the CORRECT units when you conduct calculations

*Exam Technique: Balancing Equations

1) work out how many of each atom you have on each side

you don't need to have full amounts (as in you can do 1 1/2 mol)

*Exam Techniques: Writing Ionic Equations

in an ionic equation, only the reacting particles (and products they form) are included

ionic substances in the equation will dissolve, breaking up into ions in solution

so eg:

the full balanced inital equation

> HNO3 + NaOH --> HaNO3 + H2O

rewritten equation with all of the ions that are in the reaction mixture

> H+ + NO3- + Na+ + OH- --> Na+ NO3- + H2O

cross out any ions that appear on both sides of the equation

H+ + OH- --> H2O

make sure all the charges are balanced and ✔

Empirical Formula and Molecular Formula

empirical formula - gives the simplest whole number ratio of atoms of each element in a compound

molecular formula - gives the actual number of atoms of each element in a compound

> made up of a whole number of empirical units

*Exam Techniques: Empirical Formula Calculations

1) will be provided with empirical formula and ACTUAL mR and having to find the molecular formula.

calculate the empirical mass of molecule and then compare with Mr & multiply by common multiple

--

2) may be told to work out empirical formula of a compound from percentages of different elements it contains

"a compound is found to have % composition of ......"

in 100g, divide its mass by the mr

and then divide it by the smallest figure of numbers in elements

that gives the ratio, which gives you the empircal formula

Theoretical Yield

the maximum amount of product that can be produced from a given amount of reactant

assumes no chemicals are lost in the process.

Actual Yield

actual mass of product (the actual yield) will always be less than the theoretical yield.

many reasons for this, e.g not all the 'starting' chemicals reacting fully

Percentage Yield Equation

percentage yield = actual yield/theoretical yield x 100

What is the purpose of percentage yield

it tells you how wasteful a process is, based on how much of the product is lost during the process

however it doesn't measure how wasteful the reaction is.

100% yield could still be wasteful if the a lot of the atoms of the reactants are by-products

What is Atom Economy

measure of the proportion of reactant atoms that become part of the desired product (rather than by-product) in balanced chemical equation

Atom Economy Equation

Mr of desired product/Mr of all products x 100

Importance of Atom Economies

companies in chemical industry try to use processes with high atom economies

processes with higher atom economies are better for environment since they produce less waste.

> any waste made needs to be disposed of safely, so the less made the better

make more efficient use of raw materials so they are more sustainable (use up natural resources slowly)

less expensive, companies will have to spend less on separating desired product from waste products + treating waste

Ionic Bonding

electrostatic attraction holds positive and negative ions together - it's very strong.

simplest ions are single atoms that have lost/gained electrons so they have a full outer shell

Compound Ions

ions made up of groups of atoms with an overall charge

Examples:

Sulfate: SO4^2-

Hydroxide: OH-

Nitrate: NO3-

Carbonate: CO3^2-

Ammonium: NH4 +

*Exam Technique: How to figure out formula of an ionic compound

overall charge of any compound is 0

so all the negative charged in the compound must balance all the positive charges

Giant Ionic Lattice (Why is it called "giant" , what is a lattice + example)

lattice - regular structure

structure is called 'giant' since its made up of the same basic unit repeated over and over again

Example: Sodium Chloride

> Na+ and Cl- ions are packed together. Lattice is cube shape.

Ionic Compound Properties (Conductivity)

conduct electricity when molten or dissolved - but not when they're solid

> ions are free to move in a liquid and they carry a charge

> in a solid the ions are fixed in position by strong ionic bonds

Ionic Compound Properties (Melting Point)

have high melting points

> giant ionic lattices held together by strong electrostatic force

> takes loads of energy to overcome these force so melting forces are very high

Ionic Compound Properties (Polarity)

tend to dissolve in water

water molecules are polar - part of molecule has small negative charge and other bits have small positive charges

charged parts pull ions away from the lattice causing it to dissolve

Define Molecule + bonding in molecules

molecules form when two or more atoms bond together

held together by strong covalent bonds

(e.g chlorine gas, carbon monoxide + ethanol are all molecules)

Covalent Bonds

bonds creates by two (or more) atoms sharing electrons so they've both got full outer shells of electrons

happens between non metals

both of the positive nuclei are attracted electrostatically to the shared electrons.

Giant Covalent Structure (Macromolecular)

huge network of covalently bonded atoms

carbon atoms can form this type of structure since they can each form four strong covalent bonds

(e.g graphite + diamond)

Structure of Graphite

carbon atoms arranged in sheets of flat hexagons covalently bonded with 3 bonds each

fourth outer electron of each carbon is delocalised

weak bond between layers in graphite are easily broken so the sheets can slide over each other

delocalized electrons in graphite aren't attracted to any particular carbon atoms so they are free to move along the sheets carrying a charge

layers are quite far apart compared to the length of the covalent bonds so graphite has a low density

Properties of Graphite

> an electrical conductor

due to strong covalent bonds in hexagon sheets, graphite has a very high melting point (over 3900K)

insoluble in any solvent - covalent bonds in sheets are too strong to break

has a low density and used to make lightweight strong sports equipment

Diamond Structure

made up of carbon atoms

each carbon covalently bonded to four other carbon atoms.

atoms arranged into a tetrahedral shape

Diamond Properties

due to strong covalent bonds:

> diamond has very high melting point

> is extremely hard - used in diamond-tipped drills + saws

> vibrations travel easily through stiff lattice so its a good thermal conductor

> can't conduct electricity - all outer electrons held in localised bonds

> diamond won't dissolve in any solvnet

> cut diamond to form gemstones. structure makes it refract light which is why it sparkles

Dative Covalent Bonding

Where both electrons are provided by one atom for a covalent bond.

Charge Clouds

bonding pairs and lone pairs of electrons

area where you have a really big chance of finding an electron pair

electrons don't stay still - whizz around charge cloud

Bonding Pairs and Lone Pairs repel each other

electrons all negatively charged so charge clouds will repel each other as much as they can

+

pairs of electrons in outer shell of an atom will sit as far apart from each other as they can

Valence-Shell Electron Pair Repulsion Theory

shape of charge cloud affects how much it repels other charge clouds

greatest angles are between lone pairs of electrons

+

bond angles between bonding pairs often reduced since they are pushed together by lone pair repulsion

3 types of VSEPR theory angles

Lone-pair / lone-pair angles are the biggest

Lone-pair / bonding pair angles are the second biggest

Bonding pair/bonding pair angles are the smallest

Exam Technique: Using Number of Electron Pairs to predict the shape of a molecule

work out which central atom

use periodic table to work out number of electrons in outer shell of the central atom

add one to this number for every atom the central carbon is bonded to

divide by 2 to find the electron pairs on the central atom

compare number of electron pairs to the number of bonds to find the number of lone pairs and the number of bonding pairs on the central atom

Shape of molecule: 2 electron pairs

no lone pairs

linear

180°

Shape of molecule: 3 electron pairs

no lone pairs

trigonal planar

120°

Shape of molecule: 4 electron pairs, no lone pairs

no lone pairs

tetrahedral

109.5°

Shape of molecule: 4 electron pairs, 1 lone pair

1 lone pair

trigonal pyramidal

107°

Shape of molecule: 4 electron pairs 2 lone pairs

2 lone pairs

bent

104.5°

Shape of molecule: 5 electron pairs, no lone pairs

no lone pairs

trigonal bipyramidal

90°

Shape of molecule: 5 electron pairs, 1 lone pair

1 lone pair

seesaw

87°

Shape of molecule: 5 electron pairs, 2 lone pair

2 lone pairs

T-shaped

88°

Shape of molecule: 6 electron pairs, no lone pairs

no lone pairs

octahedral

90°

Shape of molecule: 6 electron pairs, 2 lone pairs

2 lone pairs

square planar

90°

Electronegativity

A measure of the ability of an atom in a chemical compound to attract electrons

What makes a Covalent Bond Polar

in a covalent bond between two atoms of different electronegativities

the bonding electrons will be pulled towards the more electronegative atom

> makes the bond polar

greater the difference in electronegativity, the more polar the bond

Which Covalent Bonds are Non-Polar

covalent bond between two atoms of the same element is polar since the atoms have equal electronegativities (eg H2)

some elements (like C and H) have similar electronegativities so bonds between them are non polar

Pauling Scale

the unit used to measure electronegativity