Chemistry

AS Chemistry - Module 2 - Foundation Physical and Inorganic Chemistry

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Enthalpy Change

Enthalpy Change - The amount of heat energy change in a reaction at constant pressure.

Standard Conditions= 100 KPa, 298 K and 1 mol dm-3

Exothermic - enthalpy change is negative as energy is lost to the system.

Endothermic - enthalpy change is positive as energy is gained form the system.

Standard State - the physical state of something under standard conditions.

Standard enthalpy change of formation - The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions. Elements = 0. Can be positive or negative.

Standard enthalpy change of combustion- The enthalpy change when one mole of a substance undergoes complete combustion in excess oxygen, under standard conditions. Is usually negative.

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Calorimetry

The energy released by burning a substance heats up water. The temperature rise of the water is linked to the amount of energy the substance contains.

q = mc'Tq = heat change m = mass of water c = specific heat capacity of water 'T = temperature change

Bomb calorimeter- Less heat lost to the surroundings, sample is electrically ignited, the water surrounding the calorimeter is the same temperature as the water in the calorimeter - heater, the heat capacity of the calorimeter is determined by burning a substance whose enthalpy change of combustion is known, more accurate.

Enthalpy Change for reactions in solution - In displacement and neutralisation reactions, as soon as the chemicals are mixed together any heat produced starts to be lost to the surroundings. The true temperature rise is difficult to obtain.

Record temperature of solution every half minute for 3 mins, at 3.5 mins add the solid and continue recording the temperature until ten mins has passed. Draw a graph of the results, extrapolate the graph back to 3.5 mins to find the temperature change when the solid was added. Use equation q = mc'T.

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Reduction of metal oxides by the electrolysis of m

The manufacture of aluminium is carried out by electrolysis of purified bauxite, Al2O3, which is dissolved in molten cryolite. The melting point of aluminium oxide is over 2000 degrees , dissolving in molten cryolite lowers the temperature to about 970 degrees.

At the cathode Al3+ +3e- = Al

At the anode 2O2- =O2 + 4e-

Some of the oxygen evolved reacts with the anodes at the high temperature:

2C + O2 = 2CO

C + O2 = CO2

Consumes large amounts of electricity and is only economic where electricity is fairly inexpensive. The process if continuous but regular additions of aluminium oxide is needed and the carbon electrodes need replacing as they are consumed. there is a potential environmental problem through waste cryolite causing fluoride pollution.

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Hess's Law

Hess's Law - The enthalpy change in a reaction depends only on the initial and final states and is independent of the route taken.

Enthalpy of formation cycles

Elements on the bottom

Arrows up

reaction = products - reactants

Enthalpy of combustion cycles

Combustion products on the bottom

Arrows Down

reaction = reactants - products

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Reduction of metal halides by active metals

Titanium is extracted from its chloride by reduction with an active metal, as traces of carbon, oxygen or nitrogen have undesirable effects.

The ore rutile, impure titanium(IV) oxide, is converted into titanium (IV) chloride using chlorine and coke at around 900 degrees Celsius.

TiO2 + 2C + 2Cl2 = TiCl4 + 2CO

Titanium (IV) chloride is a colourless liquid which fumes in moist air because of hydrolysis. It is purified from other chlorides by fractional distillation under an inert atmosphere of argon or nitrogen. The chloride is then reduced by sodium.

TiCl4 + 4Na = Ti + 4NaCl

An inert atmosphere of argon is used to prevent any contamination with O2 or N2.

Batch - high costs - chlorine and sodium produced first, high temps involved in both stages, precautions have to be taken when handling TiCl4, argon atmosphere has to be maintained - expensive.

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Bond Enthalpies

Standard molar bond enthalpy -the change when one mole of a particular bond is broken when the substance is in the gaseous phase under standard conditions.

Breaking Bonds requires energy - positive

Making Bonds releases energy - negative

It is exothermic if- heat is given out to the surroundings, the temperature goes up, there is a net release of energy in the reaction, the enthalpy change is negative.

It is endothermic if- heat is taken in from the surroundings, the temperature goes down, there is not a net release of energy in the reaction, the enthalpy change is positive.

Mean Bond Enthalpy- the average enthalpy required to make or break, one mole of a bond, taken as an average form many molecules containing the same bond e.g. C-H.

Bond dissociation- refers to a particular bond in a specific compound.

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Halides - Properties - down the group

Atomic radius - increases - number of electrons per atom increases, more energy levels occupied so atom size increases.

Electronegativity - decreases - atomic radii increases, so nucleus is more shielded, less nuclear attraction, so ability to attract electron density from a covalent bond is reduced.

Boiling Point - increases- as the molecules increase in size, the strength of the van der waals forces increase, causing the boiling point to increase.

Oxidising ability - decreases- atomic radii increases, strength of nuclear attraction reduced due to shielding, so atom gains electrons less readily, they become less reactive.

The more reactive element displaces the less reactive element

Reducing ability - increases- atomic radii increases, outer electrons are further away from the positive nuclear charge, the larger the ion, the easier it is to lose an electron.

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Reactions of Halides

Chlorine + Br- = yellow/orange solution Cl2 + 2Br- = 2Cl- + Br2

Chlorine + I- = purple/brown solution Cl2 + 2I- = 2Cl- + I2

Bromine + I- = purple/brown solution Br2 + 2I- = 2Br- + I2

Silver Nitrate - Cl- =White ppt Br- = Cream ppt I- = Yellow ppt

AgNO3 + KCl = AgCl + KNO3

Dilute ammonia - Cl- = soluble Br- = Insoluble I- = insoluble

Conc ammonia - Cl- = soluble Br- = soluble I- = insoluble

NaCl + H2SO4 = NaHSO4 + HCl

NaBr +H2SO4 = HBr + NaHSO4

2HBr + H2SO4 = Br + SO2 + 2H2O

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Chlorine

Chlorine + water = hydrochloric acid + chloric(I) acid

Cl2 + H2O = HCl + HClO

The reaction between chlorine and water is a redox reaction as the chlorine has been oxidised CLO- and reduced to Cl-. This is a disproportionation reaction (a reaction in which one substance is both oxidiesd and reduced)

Chlorine is used in water treatment Chloric(I) acid and sodium chlorate(I) are used as germicides (disenfectant) to kill bacteria which live in water.

Cl2 + 2NaOH = NaCl + NaClO + H2O

Chlorine reacts with cold dilute sodium hydroxide to produce bleach. This reaction is a disproportion reaction.

Bleach is sodium chlorate(I)- NaClO.

ClO- ions act as a bleaching agent, which kills bacteria and oxidises dyes and stains to make them colourless.

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Oxide and Sulphide ores

Sulphur dioxide dissolves in water in the clouds to form sulphurous acid:

SO2(g) + H2O(l) = H2SO3(aq)

Some of the sulphur dioxide is oxidised to sulphur trioxide.

2SO2(g) + O2(g) = 2SO3(g)

This gas dissolves in water to form sulphuric acid.

SO3(g) + H2O(l) = H2SO4(aq)

These acids can then fall as acid rain, damaging plants and polluting lakes.

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Iron

Initially, coke reacts with the hot air blast in a strongly exothermic reaction.

C(s) + O2(g) = 2CO2(g)

The carbon dioxide forms reacts at high temperatures with unreacted coke to form carbon monoxide.

CO2(g) + C(s) = 2CO(g)

The carbon monoxide reduces most of the iron(III) oxide at around 1200 degrees C.

Fe2O3(s) + 3CO(g) = 2Fe(l) + 3CO2(g)

Coke reacts directly with the iron oxide in hotter parts.

Fe2O3(s) + 3C(s) = 2Fe(l) + 3CO(g)

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Slag, Steel, Carbide Formation

Limestone is added it decomposes in the heat to form calcium oxide and carbon dioxide

CaCO3(s) = CaO(s) + CO2(g)

Calcium oxide combines with acidic oxides in the furnace to form ****.

CaO(s) + SiO2(s) = CaSiO3(l)

Iron - Steel Iron desulphurised using magnesium Mg + S = MgS

BOS Oxygen is blown onto the molten iron, calcium carbonate is added, oxygen reacts with carbon which is removed as carbon monoxide. Also reacts with impurities to form oxides that react with the CaO to produce ****.

Titanium oxide is heated with carbon, carbides are potential catalysts TiO2 + 3C = TiC + 2CO

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The Effects of Various factors on the position of

Concentration-

Increase conc of reactants = right, decrease conc of reactants = left, increase conc of products = left, decrease conc of products = right.

Pressure-

Increased pressure = moves to side with fewest gas molecules, Decreased Pressure = moves to side with more gas molecules.

Temperature-

Increased temperature = moves in the direction of the endothermic reaction +ve, Decreased temperature = moves in the direction of the exothermic reaction -ve.

Catalyst-

No effect on equilibrium positions - obtained faster.

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Haber Process - production of ammonia

N2 + 3H2 = 2NH3 -92 KJ Mol-1

Raw materials cheapyl available from air (N2), Natural Methane gas (H2) and water (H2).

High Yield = equilibrium needs to move to the right = low temperature and high pressure.

However = Low temperature means a low rate of reaction so not cost effective. High pressure needs to be maintained - expensive and dangerous.

Compromise =

450 degrees celsius still low but at a realistic rate.

Iron catalyst - increase reaction rate so equilibrium reached quicker

High pressure - 250 Atm = able to be maintained safely

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The contact process - production of sulphuric acid

2SO2 + O2 = 2SO3 -197 KJ Mol-1

High yield = equilibrium needs to move to the right = low temperature and high pressure.

However = low temperature means rate is too low, high pressure means it is expensive and potentially dangerous.

Compromise= temperature of 450 degree Celsius = rapid rate, vanadium(V) oxide catalysts to speed up rate, 1 Atm of pressure = low but yield is still high.

SO3 must then be converted to sulphuric acid . not reacted directly with water as too dangerous as exothermic and acid formed would be vaporised.

It is reacted with existing sulphuric acid to produce oleum (H2S2O7) which can be converted safely, by reacting with water to form 2 moles of sulphuric acid.

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