Chemistry Milestone 1

IGCSE Edexcel Chemistry EOY revision

Covalent bond

The sharing of a pair of electrons between two atoms.

Attraction in covalent bonds

Shared electrons are strongly attracted to the nuclei of both atoms involved.

Strength of covalent bonds

Covalent bonds are strong.

Dot and cross diagrams

Diagrams showing covalent bonding using overlapping circles and dot-cross pairs.

Simple molecular structures

Usually gases, liquids, or solids with low melting and boiling points.

Low melting/boiling points of simple molecules

Due to weak intermolecular forces requiring little energy to overcome.

Effect of molecular size on melting/boiling points

Increases due to stronger intermolecular forces.

High melting points of giant covalent structures

Due to many strong covalent bonds needing a lot of energy to break.

Diamond, graphite, C60 fullerene

Their structures affect electrical conductivity and hardness.

Relative formula mass (Mr)

Calculated by adding relative atomic masses of atoms in a compound.

Mole

A term used to mean the amount of substance.

Mole calculation formula

Moles = mass(g) ÷ RFM.

Reacting masses

Calculations using masses of reactants/products and balanced equations.

Percentage yield

Calculated by comparing actual yield to theoretical yield.

Combustion in O2

Includes burning of magnesium, hydrogen, and sulfur; requires writing chemical equations.

Thermal decomposition of metal carbonates

Produces CO2, e.g., CuCO3 → CuO + CO2.

CO2 as a greenhouse gas

Contributes to climate change.

Test for oxygen

Relights a glowing splint.

Test for carbon dioxide

Turns limewater cloudy and extinguishes a burning splint.

Test for water

Turns anhydrous copper(II) sulfate from white to blue.

Heat vs temperature

Heat is energy; temperature is the average kinetic energy of particles.

Exothermic reaction

Gives out heat energy.

Endothermic reaction

Takes in heat energy.

Delta H (ΔH)

Symbol representing molar enthalpy change.

ΔH signs

Minus means exothermic; plus means endothermic.

Energy level diagrams

Used to show exothermic and endothermic changes.

Calorimetry experiments

Measure heat energy changes in chemical reactions.

Heat change formula

Q = mCΔT.

Specific heat capacity of water

4.2 J/g°C.

Molar enthalpy change (ΔH)

Calculated by dividing heat change by moles of substance.

Bond breaking

Endothermic.

Bond making

Exothermic.

Mexo Bend

Mnemonic for bond energy: Making = Exothermic, Breaking = Endothermic.

Enthalpy change from bond energies

Calculated using bond energy values.

Crude oil

A mixture of hydrocarbons.

Hydrocarbons

Made only of hydrogen and carbon.

Fractional distillation

Industrial process separating crude oil into fractions.

Fractions of crude oil

Refinery gases, gasoline, kerosene, diesel, fuel oil, bitumen.

Trends in crude oil fractions

Increasing boiling point, darker color, higher viscosity.

Complete combustion of hydrocarbons

Produces CO2 and H2O.

Incomplete combustion

May produce carbon monoxide, a poisonous gas.

Effect of carbon monoxide

Reduces the blood's oxygen-carrying capacity.

Nitrogen oxides

Formed in car engines from N2 and O2 due to high temperatures.

Sulfur dioxide

Formed from burning impurities in hydrocarbon fuels.

Pollutant gases

Nitrogen oxides and sulfur dioxide cause acid rain.

Problems of acid rain

Environmental damage from acidic precipitation.

Cracking

Converts long-chain hydrocarbons into shorter, more useful ones.

Catalytic cracking

Uses silica/alumina and 600-700°C to crack long alkanes.

General formula of alkanes

CnH2n+2.

Structural and displayed formulae of alkanes

Draw and name straight-chain isomers with up to 6 carbon atoms.

Methane substitution with bromine

Forms bromomethane under UV light.

Functional group in alkenes

C=C (carbon-carbon double bond).

General formula of alkenes

CnH2n.

Test for alkenes

Bromine water decolorises.

Addition reaction of alkenes

Alkenes + bromine → dibromoalkanes.

Alcohols to know

Methanol, ethanol, propanol (propan-1-ol), butanol (butan-1-ol).

Manufacture of ethanol (hydration)

Ethene + steam over phosphoric acid at ~300°C and 60-70 atm.

Manufacture of ethanol (fermentation)

Glucose fermented at ~30°C.

Choice of ethanol production method

Depends on availability of sugar cane and crude oil.

Oxidation of ethanol

By burning, microbial oxidation, or with potassium dichromate(VI) in acid.

Addition polymer

Formed by joining many monomers.

Repeat units of polymers

Draw for poly(ethene), poly(propene), poly(chloroethene), poly(tetrafluoroethene).

Monomer from polymer

Deduce monomer structure from polymer's repeating unit.

Disposal of addition polymers

Difficult due to inertness; burning releases toxic gases.