Salters OCR B Chemistry ES

Dynamic Equilibrium

When the rate of forwards reactions equals the rate of backwards reactions. The concentration of each substance remains constant (not necessarily the same).

-only occurs in closed systems

How to calculate equilibrium constant

Kc = the concentration of products/ concentration of reactants

What happens to equilibrium when temperature increases?

-equilibrium shifts in endothermic direction to oppose increase in temperature

-Kc will decrease

What happens to equilibrium when temperature decreases?

-equilibrium shifts in exothermic direction to oppose decrease in temperature

-Kc will increase

Kc>>1

-equilibrium lies well over to the right

-many more products than reactants

Kc<1

-equilibrium lies slightly to the left

-more reactants than products

Kc=1

equilibrium is in the middle

What happens to Kc when there is a change in concentration?

Nothing. Kc is unaffected.

What happens to the equilibrium when pressure is increased?

The equilibrium shifts to the side with the fewest number of gas particles to reduce the pressure.

What happens to the equilibrium when pressure is decreased?

The equilibrium shifts to the side with the most gas particles to increase the pressure

Electrolysis

The breaking down of a substance using electricity.

What must the electrolyte be in electrolysis?

an ionic compound that is molten or in an aqueous solution

Anode

+ positive electrode

+ negative ions (anions) are attracted to the anode and give up electrons

Cathode

-negative electrode

-positive ions (cations) are attracted to the cathode and receive electrons

What are electrodes made of?

Inert and conductive materials e.g. platinum or graphite

What is the product at the cathode if the electrolyte is a group 1 or 2 metal or aluminium?

Hydrogen

2H2O(l) + 2e- -> 2OH-(aq) + H2 (g)

What is the product at the anode if the electrolyte contains a halide?

Halogen

2Cl- (aq) -> Cl2 (g) + 2e-

What is the product at the cathode in the electrolysis of metal that's not a group 1 or 2 metal or aluminium?

Metal

Cu2+(aq) + 2e- -> Cu(s)

What is the product at the anode if the electrolyte contains a sulfate or nitrate?

Oxygen

2H2O(l) -> O2(g) + 4H+(aq) + 4e-

What is the product at the cathode if the electrolyte is an acid?

Hydrogen

2H+(aq) + 2e- -> H2(g)

What is the product at the anode if the electrolyte contains a hydroxide?

4OH-(aq) -> O2(g) + 2H2O(l) + 4e-

Electrolysis in the purification of metals (eg. Copper) at the ANODE

-made from impure copper

-copper loses 2 electrons

- Cu(s) -> Cu2+(aq) + 2e-

-the anode wears away and becomes lighter

Electrolysis in the purification of metals (eg. Copper) at the CATHODE

-made from pure copper

-copper ions added to the solution from the anode gain electrons at the cathode to become copper again, this 'plates' the cathode

- Cu2+(aq) + 2e- -> Cu(s)

-the cathode will become heavier

Brine

-high concentration solution of salts

-found in sea water (made when mineral rocks dissolve in water)

-some halogens like chlorine can be extracted through the electrolysis of brine

Chlorine extracted from the electrolysis of brine, anode

Cl- ions from the salt make Cl2 gas

2Cl-(aq) -> Cl2(g) + 2e-

Chlorine extracted from the electrolysis of brine, cathode

H+ ions from the water make H2 gas

2H+(aq) + 2e- -> H2(g)

Chlorine extracted from the electrolysis of brine, solution

Sodium ions (Na+) are more reactive than hydrogen so remain in the solution. They react with hydroxide ions (OH- from the water) to make sodium hydroxide (NaOH)

Why must the sodium chloride solutions in the electrolysis of brine be concentrated?

If the solution is dilute the chloride ions don't release their electrons, instead OH- ions lose their electrons and water and oxygen are produced instead

4OH-(aq) + 2H2O(l) + O2 + 4e-

Extraction of bromine

-uses a more reactive halogen to extract it from its brine

-chlorine is more reactive than bromine

-bromine is produced which is condensed and purified into a liquid

2Br-(aq) + Cl2(g) -> Br2(g) + 2Cl-(aq)

Extraction of iodine

-uses a more reactive halogen to extract it from its brine

-chlorine is more reactive than iodine

-iodine is produced which is condensed and purified into a grey solid

2I-(aq) + Cl2(g) -> I2(aq) + 2Cl-(aq)

OIL RIG

Oxidation is Loss of Electrons, Reduction is a Gain of Electrons

Redox reaction

Involves reduction and oxidation

Reducing agents

Lose electrons and are oxidise themselves

Oxidising agents

Gain electrons and are reduced themselves

Oxidation number of every uncombined element

Always 0

e.g. Cl2, Fe, O2

Oxidation number of an ion

Always the charge on the ion

e.g. Cl- = -1

Oxidation number of groups 1, 2 and 3

Group 1 always = 1

Group 2 always = 2

Group 3 always = 3

e.g. in KCl, K = +1

Oxidation state of hydrogen

Always +1 except in hydroxides where it's -1

Oxidation number of oxygen

Always -2 (except it's -1 in peroxides and +2 in OF2)

What happens to the oxidation number in reduction?

It decreases

What happens to the oxidation number in oxidation?

It increases

Ammonium

NH4+

Hydrogen Carbonate

HCO3-

Manganate (VII)

MnO4 -

Nitrate (V)

NO3-

Sulfide

S²⁻

What must be balanced in redox reactions?

CHARGE as well as atoms

Iodine-sodium thiosulfate titration

-used to find concentration of oxidising agent

-add excess potassium iodide solution to KIO3

IO3-(aq) + 5I-(aq) + 6H+(aq) -> 3H20(l) + 3I2(aq)

-titrate sodium thiosulfate into KIO3 solution until it turns a pale yellow colour

-add starch, it turns dark blue, makes it easier to see colour change

I2(aq) + 2S2O3 2-(aq) -> S4O6 2-(aq) + 2I-(aq)

-use titration to find concentration of IO3-

How to get more accurate results in a titration

-rinse burette

-wash conical flask between repeats

-read from the bottom of the meniscus

-record results to 2d.p

-repeat until concordant results

-use freshly made solutions

Fluorine state at room temperature

yellow gas

Chlorine state at room temperature

pale green gas

Bromine state at room temperature

brown, volatile liquid

Iodine state at room temperature

silver solid

Uses of chlorine

-sterilise drinking water

-swimming pools

-bleach

-making paper

Use of fluorine

toothpaste

use of bromine

fire-retardant materials

use of iodine

disinfectant

What happens to electronegativity down group 7?

It DECREASES because the atoms get larger in size which means there is a larger distance between the positive nucleus and bonding electrons and there is more shielding

What happens to charge density down group 7?

it decreases

What happens to boiling point down group 7?

it INCREASES because instantaneous dipole-induced dipole bonds increase due to the increasing size and relative mass of the atoms

solubility of chlorine in hexane

organic layer- Pale green (more vivid)

polar layer - pale green

solubility of bromine in hexane

organic layer- red

polar layer - red/brown

solubility of iodine in hexane

organic- violet

polar - brown

How to test for chloride

Add dilute nitric acid, then silver nitrate

WHITE precipitate forms

Ag+(aq) + Cl-(aq) -> AgCl(s)

How to test for bromine

Add dilute nitric acid, then silver nitrate

CREAM precipitate forms

Ag+(aq) + Br-(aq) -> AgBr(s)

How to test for iodine

Add dilute nitric acid, then silver nitrate

YELLOW precipitate forms

Ag+(aq) + I-(aq) -> AgI(s)

How to test for fluorine

Add dilute nitric acid, then silver nitrate

no precipitate forms

silver chloride in ammonia

dissolves to give a colourless solution

silver bromide in ammonia

precipitate is almost unchanged using dilute ammonia solution, but dissolves in concentrated ammonia solution to give a colourless solution

silver iodide in ammonia

precipitate is insoluble in ammonia solution of any concentration

Preparing hydrogen chloride

NaCl(s) + H2SO4(aq) -> NaHSO4(aq) + HCl(g)

white fumes produced

Preparing hydrogen bromide

- NaBr(s) + H2O4(aq) -> NaHSO4(aq) + HBr(g)

- bromine ions produced are strong enough to reduce sulfuric acid to sulfur dioxide

- 2H+(aq) + 2Br-(aq) + H2SO4(aq) -> SO2(g) + Br2(l) + 2H2O(l)

-gas produced will be a mixture of hydrogen bromide, sulfur dioxide and bromine vapour (because exothermic)

-if phosphoric acid is used no reduction takes place and pure HBr is formed

Preparing hydrogen iodide

- NaI(s) + H2SO4(aq) -> NaHSO4(aq) + HI(g)

- iodine ions produced are even stronger reducing agents than in hydrogen bromide. They reduce sulfuric acid to hydrogen sulfate gas

- 8H+(aq) + 8I- + H2SO4(aq) -> H2S(g) + 4I2(s) + 4H2O(l)

- if phosphoric acid is used no reduction takes place and pure hydrogen iodide is formed

What happens to the thermal stability down group 7?

it DECREASES because bond strength between hydrogen and halogen decreases down the group

What happens to hydrogen fluoride when it's heated?

it's not broken down into its elements

What happens to hydrogen chloride when it's heated?

it's not broken down into its elements

What happens to hydrogen bromide when it's strongly heated?

it forms some brown bromine gas

2HBr(g) -> H2(g) + Br2(g)

What happens if a red hot needle is plunged into hydrogen iodide?

large amounts of purple gaseous iodine are formed

2HI(g) -> H2(g) + I2(g)

Acidity of hydrogen halides

HCl, HBr, and HI form stronger acids than HF, in solution, because they dissociate into their ions

e.g HCl(aq) -> H+(aq) + Cl-(aq)

or H20(l) + HCl(aq) -> H3O+(aq) + Cl-(aq)

What happens to strength of halide ions as reducing agents down the group?

-it INCREASES

-because as you go down the group the ionic radius increases

-so distance between nucleus and outer electrons gets larger, more shielding and weaker attractive forces

-the outer electron is lost more readily

Hydrogen fluoride and sulfuric acid

no reaction

Hydrogen chloride and sulfuric acid

no reaction

Hydrogen bromide and sulfuric acid

makes sulfur dioxide, SO2

Hydrogen iodide and sulfuric acid

makes hydrogen sulfide, H2S

Reactions of hydrogen halides and ammonia

make salts

e.g. NH3(g) + HCl(g) -> NH4Cl(s)

form white misty fumes

Chlorine risks

-toxic and corrosive

-oxidising agent so must be kept away from flames

Chlorine transportation

Transported and stored under pressure as a liquid in a small container

What is atom economy?

Mass of desired product/mass of all products x 100

How efficient a reaction is

100%= sustainable, less waste, less by-products so saves time and money