WJEC AS Chemistry Unit 2

Energy In

Energy In

Bonds in the reactant must be broken and this requires energy to be given to the reactant.

Energy Out

When the bond in the products are formed energy is released.

An endothermic reaction

More energy is taken in than released.

An exothermic reaction

More energy released than taken in.

Exothermic reaction

Release energy to the surroundings, there is a temperature rise, there is a temperature rise and ΔH is negative

Endothermic reactions

Takes in energy from the surroundings, there is a temperature drop and ΔH is positive.

Enthalpy H

The heat content of a system at constant pressure

Enthalpy change ΔH

The heat added to a system at constant pressure

Getting full mark enthalphy calculation

Enthalpy sign (+ or -) 3 sig fig Units

Enthalpy Changes conditon

1. 100kPa

2. 298K

3. All substances in stp

Two main energy types within water

Kinetic Energy - Movement of water Potential energy - Position of the atoms within the water

Calorimetry

To compare fuels need to measure how much energy they release

q=MCΔT equation

q = Energy Released ΔT = change in temperature M = Mass of water C = Specific heat capacity

Enthalpy changes Equation

• means exothermic

• means endothermic

Hess's Law

States that the total enthalpy change for a reaction is independent of the route taken from the reactant to the product.

Rate of Reaction

Change in the concentration of a reactant or product per unit time

Collision Theory

For a reaction between two molecules to occur an effective collision must take place

Relationship between concentration and rate of reaction

Increase in concentration causes an increase in rate of reaction because there are more molecules so an increased chance of collision

Relationship between temperature and rate of reaction

Increase in temperature causes an increase in rate of reaction because the molecules have enough kinetic energy to react when they collide

Relationship between particle size and rate of reaction

Increase in particle size causes a decrease in rate of reaction because it causes a decrease in surface area

Relationship between use of a catalyst and rate of reaction

A catalyst increases the rate of a chemical reaction without itself undergoing a permanent change

Relationship between light and rate of reaction

Some reactions have a faster rate of reaction in bright light e.g. photochlorination of methane

Activation Energy

The minimum energy required to start a reaction by breaking bonds

Boltzmann Distribution Curve

Graph showing the number of particles having each particular energy

Catalyst

Substance that increases the rate of a chemical reaction without being used up in the process. It increases the rate of reaction by providing an alternative route of lower activation energy

Homogenous Catalyst

Catalyst in the same phase as the reactants

Heterogenous Catalyst

Catalyst in a different phase as the reactants

Phase

Physically distinctive form of matter i.e. aq, g, s

Enzymes

Biological catalysts

Measuring rates of reaction

Change in gas volume, change in gas pressure, change in mass or calorimetry

Organic Chemistry

Study of carbon containing compounds. The simplest form of a hydrocarbon is a family called an Alkane.

Hydrocarbon

A compound which contain hydrogen and carbon

Saturated Compounds

Single covalent bond only (alkanes)

Unsaturated compound

Contains a C=C double bonds

Homologous series

It is a series of compounds with different functional groups i.e. alkenes - CnH₂n

Functional groups

Refers to the atom/group of atoms that gives the compound the characteristics properties

Empirical Formula

Simplest way of writing of writing a compound

Methane

CH₄

Ethane

C₂H₆

Propane

C₃H₈

Butane

C₄H₁₀

Pentane

C₅H₁₂

Hexane

C₆H₁₄

Molecular Formula

Tells us which element and how many of each are present

Displayed Formula

Shows every bond within the compound

Structural Formula

Shows the connection of different groups but not the bond e.g CH₃CH₂CH₃

Skeletal Formula

Only shows the carbon atoms and any other significant elements

Structural Isomers

They are compounds with the same molecular formula but with a different structural formula

Chaim isomerism

This occurs when the carbon chains of the molecule is arranged differently . Usually one is a straight chain and the other branched e.g CH₃CH₂CH₂CH₃

Positional isomerism

This occurs when the functional group is a different place in the molecule. e.g CH₂ClCH₂CH₃ 1-Chloropropane and CH₃CHClCH₃ 2-Chloropropane

Functional group isomerism

This occurs when the functional group compound is different e.g CH₃CH₂OCH₃ and ether and CH₃CH₂CH₂OH propan-2-ol

E-Z isomerism

An isomer which occurs in alkenes due to a restriction of rotation around the double bond

Z Isomer

When both of the high priority groups are opposite each other

E isomer

When the two high priority groups are on either end of the carbon

Van der Waals forces

They are dipole-dipole or temporary dipole-temporary dipole interactions between molecules. Since hydrocarbons are electronegative they are non-polar. They are temporary dipoles

Melting and boiling points

A branched chain has a much smaller melting point than a straight chained, as the surface area is much smaller.

Fossil Fuels

A fuel which is collect from organisms which lived long ago

Advantages of fossil fuels

Always available all of time Available in a variety of forms (Coal/Natural gases)

Disadvantages of fossil fuels

Non renewable, reserves are used faster than they are made Combusted and produce CO₂, this is a greenhouse gas which absorbs infrared radiations. Increase temperature of earth Sulfur dioxide reacts with water to form acid ran which can cause serious damage to buildings (Carbonate)

Alkanes

Saturated hydrocarbons Physicals properties depend on the molecules mass, small alkanes are gases a room temperature and larger are liquids.

Reactions of alkanes

The hydrogen and carbon electronegativity is similar as they are non-polar meaning they are generally non reactive. They take place in two reactions Halogenation and combustion

Combustions

Alkanes reactive with oxygen and the reaction is exothermic Complete combustion: 2C₂H₆ + 7O₂ → 2CO₂ + 3H₂O Incomplete combustion: 2C₃H₆ + 7O₂ → 3CO + 4H₂O

Complete Combustion

Combustion that occurs with excess oxygen carbon dioxide and water are produced

Incomplete Combustion

Combustion that occurs with insufficient oxygen, carbon monoxide and water are produced

Halogenation

A reaction between a organic compound and halogen, reaction will only take place under the condition of UV light

Stage 1 - Initiation

Ultraviolet light breaks the bond (Homolytic Fission) to form two radicals. Cl₂ → 2Cl°

Stage 2 - Propagation

Radicals are very reactive . Radicals are used as the reactant and then forms a radical as a product. Cl° + CH₄ → CH₃° + HCl CH₃ +Cl₂ → CH3Cl + Cl°

Step 3 - Termination

All radials come together Cl° + CH₃° → CH₃Cl

Homolytic bond fission

A covalent bond breaks, splitting one electron to each atom

Radical

Very reactive species of unpaired electrons

Alkenes

Saturated hydrocarbon containing a C=C bond. Very reactive compared to alkanes CnH₂n

Bonds within a alkene

C=C is called an end-on-overlap, this type of bond is called a sigma bond (σ). These are strong covalent bonds. P orbitals have one electron in the orbital. These overlap sideways to produce a pi-bond. Pi-bonds are covalent bonds but weaker than sigma.

Reaction of alkene - Electrophilic Addition

Where a substance break heterolytically and lead to an overall addition to the substance

Heterolytic Fission

Where a bond beaks unevenly.

Testing for bromine

Bromine water is brown

Monomer

Small molecule which can make a polymer

Polymerations

The joining of very large number of monomer molecules to make a larger polymer

Formation of a polymer

When the double bond molecule cross over to make single bonds, a repeated unit.

Repeat unit

A sections of polymer which repeat to make the whole polymer.

Halogenoalkanes

An alkane in which one or more hydrogen have been replaced by a halogen.

Structure of halogenoalkanes

They contain a carbon to a halogen bond, since halogen are more electornegative than carbon, its polar.

Nucleophilic substitution

A nucleophile has a lone pair that can be donated to the electron deficient centre. Alkane → Alcohol

Nucleophilic substitution conditions

Aqueous solution to be dissolved in water and a reflux

Reflux

A process of continuous evaporation and condensation

Hydrolysis

Chemical breakdown of reaction due to water

Test for halogenoalkanes

Water is used to hydrolyse Halogenoalkanes but this is rather slow, so NaOH is used. (aq) Silver nitrate is added and the NaOH is neutralised by dilute nitric acid, NaOH could interfere.

Colour of Halogen with addition of Ag⁺

Chlorine - White Precipitate Bromine - Cream Precipitate Iodine - Yellow Precipitate

Elimination Reaction

CH₃CH₂CH₂Cl + NaOH → H₂O + CH₃CHCH₂ + NaCl Ethanolic sodium hydroxide used, to be dissolved in.

Elimination Reaction Process

A small group of atoms break away to form a larger molecule and never replaced. The reaction condition is anhydrous (No water)

Unsymmetrical elimination reaction

Two products are formed, which ever has the most stable carbocation is the major product

Uses of Halogenoalkanes

Can be used as solvent, anaesthetics and refrigerants

As solvent

Contains part polar section due to the carbon halogen bond and non-polar section due to the alkyl chain. Meaning they can mix with a variety polar and non polar substances

As anaesthetics

Many used as a general anaesthetics and trichloromethane was the first substance to be used.

As refrigerants

They are gases at room temp but the permanent dipole attraction means the boiling point is also close to room temp. Liquids that are easily evaporated or liquefied. CFCs since the heat need to change the liquid to a gas is removed from the fridge to cool content

Alcohol

A homologous series containing -O-H as the functional group, its connected to the carbon atom.

Naming alcohol

Add -ol to the root name

Classification of alcohol

Similar to the carbocation, the inductive effect, primary (least stable) and tertiary (most stable).

Fermentation

An enzyme catalysed reaction that converts sugar to ethanol. 30°-40° and yeast are the conditions.

Catalytic Hydration

Where an alkene is converted into an alcohol using steam. C₂H₄ + H₂O ⇌ CH₃CH₂OH

Industrial preparation of ethanol

300° 60-70 atmospheres Phosphoric acid catalyst