WJEC AS Chemistry Unit 2

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138 Terms

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Energy In
Bonds in the reactant must be broken and this requires energy to be given to the reactant.
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Energy Out
When the bond in the products are formed energy is released.
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An endothermic reaction
More energy is taken in than released.
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An exothermic reaction
More energy released than taken in.
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Exothermic reaction
Release energy to the surroundings, there is a temperature rise, there is a temperature rise and ΔH is negative
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Endothermic reactions
Takes in energy from the surroundings, there is a temperature drop and ΔH is positive.
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Enthalpy H
The heat content of a system at constant pressure
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Enthalpy change ΔH
The heat added to a system at constant pressure
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Getting full mark enthalphy calculation
Enthalpy sign (+ or -)
3 sig fig
Units
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Enthalpy Changes conditon
1. 100kPa
2. 298K
3. All substances in stp
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Two main energy types within water
Kinetic Energy - Movement of water
Potential energy - Position of the atoms within the water
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Calorimetry
To compare fuels need to measure how much energy they release
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q\=MCΔT equation
q \= Energy Released
ΔT \= change in temperature
M \= Mass of water
C \= Specific heat capacity
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Enthalpy changes Equation
- means exothermic
+ means endothermic
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Hess's Law
States that the total enthalpy change for a reaction is independent of the route taken from the reactant to the product.
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Rate of Reaction
Change in the concentration of a reactant or product per unit time
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Collision Theory
For a reaction between two molecules to occur an effective collision must take place
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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
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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
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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
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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
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Relationship between light and rate of reaction
Some reactions have a faster rate of reaction in bright light e.g. photochlorination of methane
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Activation Energy
The minimum energy required to start a reaction by breaking bonds
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Boltzmann Distribution Curve
Graph showing the number of particles having each particular energy
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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
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Homogenous Catalyst
Catalyst in the same phase as the reactants
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Heterogenous Catalyst
Catalyst in a different phase as the reactants
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Phase
Physically distinctive form of matter i.e. aq, g, s
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Enzymes
Biological catalysts
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Measuring rates of reaction
Change in gas volume, change in gas pressure, change in mass or calorimetry
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Organic Chemistry
Study of carbon containing compounds. The simplest form of a hydrocarbon is a family called an Alkane.
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Hydrocarbon
A compound which contain hydrogen and carbon
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Saturated Compounds
Single covalent bond only (alkanes)
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Unsaturated compound
Contains a C\=C double bonds
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Homologous series
It is a series of compounds with different functional groups i.e. alkenes - CnH₂n
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Functional groups
Refers to the atom/group of atoms that gives the compound the characteristics properties
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Empirical Formula
Simplest way of writing of writing a compound
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Methane
CH₄
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Ethane
C₂H₆
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Propane
C₃H₈
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Butane
C₄H₁₀
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Pentane
C₅H₁₂
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Hexane
C₆H₁₄
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Molecular Formula
Tells us which element and how many of each are present
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Displayed Formula
Shows every bond within the compound
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Structural Formula
Shows the connection of different groups but not the bond
e.g CH₃CH₂CH₃
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Skeletal Formula
Only shows the carbon atoms and any other significant elements
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Structural Isomers
They are compounds with the same molecular formula but with a different structural formula
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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
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Positional isomerism
This occurs when the functional group is a different place in the molecule.
e.g CH₂ClCH₂CH₃ 1-Chloropropane
and CH₃CHClCH₃ 2-Chloropropane
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Functional group isomerism
This occurs when the functional group compound is different e.g CH₃CH₂OCH₃ and ether and CH₃CH₂CH₂OH propan-2-ol
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E-Z isomerism
An isomer which occurs in alkenes due to a restriction of rotation around the double bond
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Z Isomer
When both of the high priority groups are opposite each other
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E isomer
When the two high priority groups are on either end of the carbon
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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
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Melting and boiling points
A branched chain has a much smaller melting point than a straight chained, as the surface area is much smaller.
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Fossil Fuels
A fuel which is collect from organisms which lived long ago
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Advantages of fossil fuels
Always available all of time
Available in a variety of forms (Coal/Natural gases)
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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)
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Alkanes
Saturated hydrocarbons
Physicals properties depend on the molecules mass, small alkanes are gases a room temperature and larger are liquids.
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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
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Combustions
Alkanes reactive with oxygen and the reaction is exothermic
Complete combustion: 2C₂H₆ + 7O₂ → 2CO₂ + 3H₂O
Incomplete combustion: 2C₃H₆ + 7O₂ → 3CO + 4H₂O
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Complete Combustion
Combustion that occurs with excess oxygen carbon dioxide and water are produced
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Incomplete Combustion
Combustion that occurs with insufficient oxygen, carbon monoxide and water are produced
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Halogenation
A reaction between a organic compound and halogen, reaction will only take place under the condition of UV light
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Stage 1 - Initiation
Ultraviolet light breaks the bond (Homolytic Fission) to form two radicals. Cl₂ → 2Cl°
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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°
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Step 3 - Termination
All radials come together
Cl° + CH₃° → CH₃Cl
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Homolytic bond fission
A covalent bond breaks, splitting one electron to each atom
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Radical
Very reactive species of unpaired electrons
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Alkenes
Saturated hydrocarbon containing a C\=C bond. Very reactive compared to alkanes
CnH₂n
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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.
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Reaction of alkene - Electrophilic Addition
Where a substance break heterolytically and lead to an overall addition to the substance
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Heterolytic Fission
Where a bond beaks unevenly.
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Testing for bromine
Bromine water is brown
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Monomer
Small molecule which can make a polymer
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Polymerations
The joining of very large number of monomer molecules to make a larger polymer
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Formation of a polymer
When the double bond molecule cross over to make single bonds, a repeated unit.
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Repeat unit
A sections of polymer which repeat to make the whole polymer.
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Halogenoalkanes
An alkane in which one or more hydrogen have been replaced by a halogen.
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Structure of halogenoalkanes
They contain a carbon to a halogen bond, since halogen are more electornegative than carbon, its polar.
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Nucleophilic substitution
A nucleophile has a lone pair that can be donated to the electron deficient centre. Alkane → Alcohol
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Nucleophilic substitution conditions
Aqueous solution to be dissolved in water and a reflux
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Reflux
A process of continuous evaporation and condensation
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Hydrolysis
Chemical breakdown of reaction due to water
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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.
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Colour of Halogen with addition of Ag⁺
Chlorine - White Precipitate
Bromine - Cream Precipitate
Iodine - Yellow Precipitate
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Elimination Reaction
CH₃CH₂CH₂Cl + NaOH → H₂O + CH₃CHCH₂ + NaCl
Ethanolic sodium hydroxide used, to be dissolved in.
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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)
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Unsymmetrical elimination reaction
Two products are formed, which ever has the most stable carbocation is the major product
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Uses of Halogenoalkanes
Can be used as solvent, anaesthetics and refrigerants
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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
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As anaesthetics
Many used as a general anaesthetics and trichloromethane was the first substance to be used.
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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
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Alcohol
A homologous series containing -O-H as the functional group, its connected to the carbon atom.
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Naming alcohol
Add -ol to the root name
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Classification of alcohol
Similar to the carbocation, the inductive effect, primary (least stable) and tertiary (most stable).
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Fermentation
An enzyme catalysed reaction that converts sugar to ethanol. 30°-40° and yeast are the conditions.
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Catalytic Hydration
Where an alkene is converted into an alcohol using steam.
C₂H₄ + H₂O ⇌ CH₃CH₂OH
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Industrial preparation of ethanol
300°
60-70 atmospheres
Phosphoric acid catalyst