OCR A Chemistry A Level Paper 1

Amount of substance equations

mass = mr x moles

mol = vol x conc

vol(gas) = mol x 24 (dm)

PV=nRT

(pressure, Pa, volume, m3, moles, -, 8.314, temp, k)

Empirical formula

The simplest whole number ratio of atoms of each element in a compound

Molecular formula

The number and type of atoms of each element in a molecule

Anhydrous

Contains no water molecules

Hydrated

Crystalline compound containing water molecules

Celsius to Kelvin

K=C+273

% yield and atom economy

% yield = (actual/ theoretical) x 100

atom economy = (desired products/all reactants) x 100

Cu2+ in aq solution

[Cu(H2O)6]2+

Pale blue

Fe2+ in aq solution

[Fe(H2O)6]2+

Pale green

Fe3+ in aq solution

[Fe(H2O)6]3+

Yellow

Mn2+ in aq solution

[Mn(H2O)6]2+

Pale pink

Cr3+ in aq solution

[Cr(H2O)6]3+

Green

Cu2+ and NaOH (aq)

Initially:

[Cu(OH)2(H2O)4] (s)

Blue precipitate

Excess:

No change

Fe2+ and NaOH (aq)

Initially:

[Fe(OH)2(H2O)4] (s)

Green precipitate

Excess:

No change

Fe3+ and NaOH (aq)

Initially:

[Fe(OH)3(H2O)3] (s)

Orange precipitate

Excess:

No change

Mn2+ and NaOH (aq)

Initially:

[Mn(OH)2(H2O)4] (s)

Pink precipitate

Excess:

No change

Cr3+ and NaOH (aq)

Initially:

[Cr(OH)3(H2O)3] (s)

Grey-green precipitate

Excess:

[Cr(OH)6]3- (aq)

Dark Green

Cu2+ and NH3 (aq)

Initially:

[Cu(OH)2(H2O)4] (s)

Blue precipitate

Excess:

[Cu(NH3)4(H2O)2]2+ (aq)

Dark blue

Fe2+ and NH3 (aq)

Initially:

[Fe(OH)2(H2O)4] (s)

Green precipitate

Excess:

No change

Fe3+ and NH3 (aq)

Initially:

[Fe(OH)3(H2O)3] (s)

Orange precipitate

Excess:

No change

Mn2+ and NH3 (aq)

Initially:

[Mn(OH)2(H2O)4] (s)

Pink precipitate

Excess:

No change

Cr3+ and NH3 (aq)

Initially:

[Cr(OH)3(H2O)3] (s)

Grey-green precipitate

Excess:

[Cr(NH3)6]3+ (aq)

Purple

Cu2+ and Cl-

[CuCl-(4)]2-

yellow

Cu2+ and iodide

off-white copper (I) iodide precipitate

Transition element

A d-block element that can form at least one stable ion with a partially filled d-orbital

have similar physical properties (high densities + mp)

have variable oxidation states cause 4s and 3d orbitals are very close in energy levels

Cu and Cr are exceptions to rule that 4s subshell is filled before 3d subshell

Complex ion

a metal ion which is surrounded by coordinately bonded ligands

Ligand

an atom/ion/molecule that donates a pair of electrons to a central metal ion

Monodentate ligands

Form one coordinate bond to a metal ion

e.g. H2O, NH3, Cl-, CN-

Multidentate ligands

Form more than one coordinate bond each

Bidentate= two

Octahedral

6 ligands

e.g. [Mn(H2O)6]2+

Tetrahedral

4 ligands

e.g. [CuCL4]2-

Cis-platin

[Pt(NH3)2Cl2]

used in cancer treatment (chemo)

- loses the 2 coordinate cl- ions and coordinating with 2 Nitrogen atoms in a DNA strand

-prevents dna from replicating and cell is unable to divide

Bronsted-Lowry acid

proton donor

e.g. HCl, H2SO4, H3PO4

Bronsted-Lowry base

proton acceptor

e.g. NaOH, NH3, KOH

Amphoteric

a substance that can act as both an acid and a base

acid + metal

acid + metal -> salt + hydrogen

e.g. 2H+ + Mg --> Mg2+ + H2

acid + carbonate

acid + carbonate -> salt + water + carbon dioxide

e.g. 2H+ + CuCO3 --> Cu2+ + H2O + CO2

acid + base

acid + base -> salt + water

base is solid metal oxide or hydroxide

e.g. 2H+ + MgO --> Mg2+ + H2O

Sorensen's pH scale

pH = -log[H+]

[H+] = 10^(-pH)

- low value of [H+] = high pH value

- high value of [H+] = low pH value

Strong acid

completely dissociates in a solution

HA (aq) --> H+ + A-

∴ [H+] = [HA]

Ka

Ka = [H+][A-]/[HA]

-changes with temp (values usually standardised at 298K)

Larger Ka = eqm is further right = greater acid strength

Weak acid

partially dissociates in solution

Ka for weak acids

Ka=[H+]^2/[HA]

Approximations:

1) [H+]eqm = [A-]eqm

dissociation of H+ in water will be small so neglected

approcimation breaks down for very weak/dilute acids (pH>6)

2)[HA]start > [H+]eqm

therefore [HA]eqm = [HA]start

dissociation of weak acids is small, conc. of acid is much greater than conc. of H+ ions at eqm

not justified for stronger weak/dilute acids (Ka> 10^-2))

Ionic product of water

Kw = [H+][OH−]

Strong base

e.g. NaOH --> Na+ + OH-

[OH-] = [NaOH]

Weak base

pOH = -log[OH-]

[OH-] = 10^(-pOH)

pH + pOH = 14

Buffer solution

A system that minimises pH changes when small amounts of an acid or a base are added

Contains weak acid and conjugate base

Methods for preparing weak acid buffer solutions

1) CH3COOH(aq) --> H+(aq) + CH3COO-(aq)

CH3COONa(s) + aq --> CH3COO-(aq) + Na+(aq)

2) partial neutralisation of a weak acid

Excess CH3COOH(aq) + NaOH(aq) --> CH3COONa(s) +H2O(l)

How buffers work: Ha --> H+ + A- (reversible)

Adding small amount of acid, H+ :

H+ conc increases

H+ reacts w/ conjugate base A-

eqm shifts left

reduces H+ conc, pH maintained

Adding small amount of alkali, A- :

OH- conc increases

OH- reacts with H+ to form water

HA dissociates to form more H+ ions

eqm shifts right

increases H+ conc, pH maintained

pH titration curve

Periodicity

repeating pattern of trends in physical and chemical properties (across period)

Mendeleev's arrangement

elements arranged according to:

-relative atomic mass

-properties

-left gaps for undiscovered elements

-predicted properties of elements that would fill gaps

Arrangment now

elements arranged according to:

-increasing proton number

-elements in period show repeating trends in physical + chemical properties

-elements in groups show repeating/similar chemical properties

Atomic number

number of protons in the nucleus of an atom

Comparing Beryllium and Boron

2p subshell in boron has higher energy that 2s subshell in beryllium

2p electron easier to remove

1st I.E lower for boron

Comparing Nitrogen and Oxygen

In oxygen, the paired electrons in one of the 2p orbitals repel each other (like-charges)

Easier to remove electron from oxygen

1st I.E lower for oxygen

1st Ionisation energy (I.E)

Amount of energy required to remove one mole of electrons from one mole of atoms in a gaseous state

1st: X(g) --> X+(g) + e-

2nd: X+(g) --> X2+(g) + 2e-

Ionisation energy down a group

atomic radius increases

shielding increases

nuclear charge decreases

nuclear attraction to outer electrons decreases

1st I.E decreases

Ionisation energy across a period

atomic radius decreases

similar shielding

nuclear charge increases

nuclear attraction to outer electrons increases

1st I.E increases

Ionisation graphs

Large jump = new shell

e.g. oxygen

large increase from 6-7th I.E

therefore must have 6 electrons in outer shell

therefore must be in group 6

Ionic bonding

Strong electrostatic attraction between oppositely charged ions (between metals and non-metals)

Melting/boiling point:

-high

-large amount of energy required to over come electrostatic attraction between oppositely charged ions

Electrical conductivity:

solid- no, fixed lattice

molten/aq- yes, mobile ions

Simple Covalent bonding

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms (between 2 non-metals)

Melting/boiling point:

-low

-only london forces

Electrical conductivity:

no, no mobile charge carriers

Giant covalent bonding

Many atoms joined covalently and arranged in a giant regular lattice

Melting/boiling point:

-high

-lots of strong covalent bonds

Electrical conductivity:

diamond-no

silicon-no

graphene- yes

Solubility:

-insoluble

-covalent bonds holding atoms in lattice are too strong to be broken by interactions with solvents

diamond + silicon are tetrahderal, 109°

Metallic bonding

Strong electrostatic attraction between cations and delocalised electrons

Melting/boiling point:

-high

-many electrostatic forces to be broken, lots of energy needed

-giant lattice structure

Electrical conductivity:

yes- delocalised electrons carry charge

Solubility:

-insolubly

-any interactions lead to a reaction, not dissolving

Enthalpy change (△H)

Amount of heat evolved or absorbed in a reaction carried out at constant pressure

Exothermic

system--> surrounding

△H is -ve

Endothermic

surrounding--> system

△H is +ve

Activation energy (Ea)

Minimum energy required for a reaction to take place/break bonds

Standard conditions

Standard pressure= 100 kPa

Standard temp= 298K (25°C)

Standard conc= 1 mol dm-3

Standard state

Standard enthalpy change of reaction (△rHθ)

Enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation, under standard conditions + standard states

(if molar quantity is divided by 2, △rHθ is divided by 2)

△H (KJ mol-1) = sum bond enthalpies in reactants- sum bond enthalpies in products

Standard enthalpy change of formation (△fHθ)

Enthalpy change when one mole of a compound is formed from its elements in their standard states + under standard standard conditions

△fHθ for any element in standard state in 0

Standard enthalpy change of combustion (△cHθ)

Enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions + standard states

Standard enthalpy change of neutralisation (△neutHθ)

Enthalpy change that accompanies the reaction between an acid and base to form one mole of water under standard conditions + standard states

(value is same for all neut reactions, -57)

Measuring enthalpy change

1) q= mc△T

m= mass heated (of water/aq solution)

c= 4.18

T= temp change

2) Calculate moles

3) Calculate △H

△H = q/ mol

Born-Haber cycle

Rates equation

Rate= conc/time

Rate ∝ [A]^a

Rate= k[A]^a [B]^b

Orders of reactions

Zero order:

Rate∝ [A]^0

conc has no effect on rate

First order:

Rate∝[A]

if conc is doubled, rate is doubled

Second order:

Rate∝[A]^2

Conc time graph zero order

Straight line with negative gradient

Conc time graph first order

Constant half life

Conc time graph second order

Half life increases as time increases

How to work out rate constant for first order reaction

method 1: calculate tangent of curve at half life

method 2: K= ln2/half life

Rate-conc graph zero order

Rate=K

therefore K= y-intercept

Rate-conc graph first order

Rate∝K

Rate= K[A]

therefore K= gradient

Rate-conc graph second order

Rate= k[A]^2

K determined by plotting second graph of rate against conc^2

Rate-determining step

the slowest step in a reaction mechanism, the one determining the overall rate

If something shows in rate equation but not overall equation, its a catalyst

Arrhenius equation

k = Ae^(-Ea/RT)

R= gas constant (8.314)

T=temp (k)

Ea= activation energy

Increase in temp= rate increases and rate constant increases

Catalyst= Ea lowers = rate increases

Logarithmic form of Arrhenius equation

lnk = -Ea/RT + lnA

A plot of LnK against 1/T gives downswards linear graph of type y=mx+c, where m= -Ea/R and c=lnA

Average bond enthalpy

The energy required to break one mole of a specified type of bond in a gaseous molecule

measure of average strength on covalent bond

always endothermic ( positive value)

BENDO MEXO

Hess' Law and enthalpy cycles

FUCD- formation up, combustion down

Entropy

dispersal of energy and disorder within the chemicals making up a chemical system (JK-1 mol-1)

Predicting entropy changes

1) At 0k there is no energy and all substrates have an entropy of 0

2) Entropy increases with changes in state to give more random particles

More random system (e.g. gas) = more positive entropy change

3) Changes in number of gas molecules

e.g. more gas molecules in reactants = entropy decreases = entropy is negative

4) Dissolving in crystalline solid (ionic)

solid is highly ordered, solution is disordered --> entropy increases

Standard entropy (Sθ)

The entropy of one mole of a substance under standard conditions

△Sθ = sum of Sθ products - sum of Sθ reactants

Gibbs free energy

overall change in energy during a chemical reaction

△G = △H -T△S

convert △S to kilo by dividing by 1000

Feasibility of a reaction △G

△G > 0 then reaction isnt feasible

△G <0 then reaction is feasible

note: may be feasible, the reaction may not actually happen in any sensible time

△G as a graph

△G= y axis

△H= y intercept

△S= gradient

T= x axis

Mean titre

sum of concordant titres / number of concordant titres

concordant titres are within 0.1 of each other

% uncertainty

Uncertainty x 2 /mean titire x 100

Test for ammonium (NH4 +)

Dilute NaOH + gently heat

may be able to smell ammonia

moist pH indicator --> blue

NH4 +(aq) + OH-(aq) --> NH3(g) + H2O(l)

Test for Carbonate (CO3 2-)

Dilute acid

Efforvesence

Gas (CO2) turns limewater milky

CO3 2-(aq) + 2H+(aq) --> CO2(g) + H2O(l)

Test for sulphate (SO4 2-)

Dilute HCl + Barium chloride

white precipitate (barium sulfate) formed

Ba2+(aq) + SO4 2-(aq) --> BaSO4(s)