AQA Chemistry Unit 3 (C3)

Just some brief notes required based on the Chemsitry unit 3 AQA specification 

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  • Created by: Triciaaa
  • Created on: 12-04-13 12:25

The Early Periodic Table (C3.1.1)

First editions of the periodic table

Newlands Law of Octaves

  • First published in 1864
  • Every 8th element had similar properties
  • However, the trend broke down on the 3rd row (transition metals were not the same)
  • His work was criticised because some groups didnt actually share similar properties e.g. carbon and titanium
  • He also merged metals and non metals together that was a problem
  • He didnt leave any gaps for future discoveries of elements

Dmitri Mendeleev 

  • The second edition was published in 1869 (just 5 years ager Newlands)
  • He placed the elements in order of their atomic mass
  • He also left gaps to keep similar properties in the same columns (vertical)
  • Willing to leave big gaps for the transition metals
  • His periodic table helped predict undiscovered elements by following the pattern
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The Modern Periodic Table (C3.1.2)

Electronic Structure

  • Early 20th century, protons, neutrons and electrons were discovered
  • Before, the table was arranged in order of atomic proton numbers but now it is categorised by the electronic structure e.g.[2,8,2]
  • Elements in the same group (vertical) have the same number of electrons on its outer shell (except the transition metals) 

Sheilding 

  • The protons that have a positive charge in the nucleus attract the electrons which holds it in place
  • An increased distance means the electron is getting further away from the nucleus
  • An increased shielding means there are inner electrons getting in the way 
  • Both and increase in distance and shielding means...
    = there's less attraction between the nucleus and electron
    = a higher energy level electron is easily lost
    = less likely to gain an electron (hence why Halogens are less reactive going down the group)  
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Trends Within the Periodic Table (C3.1.3)

Alkali Metals (Group 1)

  • All elements are alkali metals with similar properties
    - low density
    - react with non-metals = ionic compounds (With a +1 charge
    - react with water = hydrogen
    - dissolved in water = hydroxides (alkaline solutions) 
  • Trends (the further down the group...)
    - more reactive 
    - lower melting & boiling point 

Halogens (Group 7) 

  • All elements in group 7 are halogens with similar properties
    - react with metals = ionic compounds (With a -1 charge, halide ions
  • Trends (the further down the group...) 
    - less reactive, harder to gain electrons due to increased shielding
    - higher melting & boiling points 
  • More reactive halogens displace less reactive halogens from aqueous solutions
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Trends Within the Periodic Table (C3.1.3) Part 2

Transition metals 

  • Most transition metals are...
    -good conductors
    -dense
    -strong
    -shiny
    -hard
    -less reactive than group 1
    -useful catalysts e.g iron catalyst for the Haber process
  • Different transition metals have ions with different charges therefore they can form different coloured compounds
  • e.g. Potassium chromate = yellow
           Potassium magnate = purple  
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Hard and Soft Water (C3.2.1)

  • reacts with soap to form scum
  • more soap is needed to form a lather
  • soapless detergents don't form scum
  • caused by calcium and magnesium ions dissolved in water e.g. CaCO3
  • when heated, scale is formed reducing the effiency of heating systems and kettles
  • scale is a thermal insulator (therefore more energy is required to heat something up)
  • there are two main types of water:
    - Temporary hardness caused by hydrocarbonate ion
    - Permanent hardness caused by dissolved calcium sulfate
  • Temporary hardness is removed from boiling, it decomposes to form CaC03 which is insoluble (this is the limescale in kettles)  
  • Both types of water is softened by 2 types of methods:
    1. Sodium carbonate is added to react with Ca/Mg = insoluble precipitate
    2. Ion exchange columns (the Na/H ions exchange them for the Ca/Mg ions)  
  • Benefits
    1. healthy bones and teeth from Ca2+ ions
    2  less risk of developing heart disease because of the minerals
  • Soft Water readily reacts with soap to form a lathe
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Purifying Water (C3.2.2)

  • Water must be purified to remove poisonous salts and microbes that cause diseases
  • Water at correct quality is produced by...
    - choosing an appropriate source
    - passing it through filter beds to remove solids
    - sterilising it with chlorine 
  • The main method used to treat water: 
    1. pass through a mesh screen to remove twigs 
    2. add chemicals to stick the solids+microbes together 
    3. filtered through gravel beds 
    4. chlorine is added to kill harmful microbes left 
  • Chlorine and Fluoride has several advantages and disadvantages:
    Benefits
    reduce tooth decay from fluoride
    prevent diseases from chlorine
  • Costs:
    increase in certain cancer from chorine
    produces toxic-by-products from chlorine 
    bone problems from fluoride
  • Pure water is produced by distillation (condensing) which requires a lot of energy 
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Energy from Reactions (C3.3.1)

Calorimetry 

  • Different fuels produce different amounts of energy 
  • We work out energy released by the following equation:
  • Q=mc∆t Specific heat capacity is 4.2
  • There are several consequences of fuel energy: releases C02, non renewable source

Exothermic and Endothermic Reactions

  • Exothermic: energy released from forming new bonds is greater than energy needed to break existing bonds. The products are at a lower energy than the reactants
  • Endothermic: the energy needed to break existing bonds is greater that the energy released from forming new bonds. The products are at a higher energy than the reactants. 
  • During a chemical reaction: 
    supplied = break bonds, released = new bonds formed 
  • Catalysts lower the activation energy > easier and quicker reactions 
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Energy from Reactions (C3.3.1) Part 2

Calculations: How to work out bond energy changes
The method is fairly straight forward if you remember these steps: 
1. write out the balanced equation for the reaction
2. draw the stick diagrams that match the substances
3. work out how much energy is required (bonds broken, energy release)
4. work out the number and type of bonds made in the product (energy out)
5. overall change = energy in - energy out (-exo,+endo) 

Hydrogen Fuel (+Fuel Cells)
- hydrogen + oxygen -> water
- burning in combustion is an exothermic reaction
- benefits: clean, no pollutant gases involved, renewable source
- costs: H+ is hard to store, very explosive, requires specialised and expensive machines
- fuels cells are electrical cells that is supplied with fuel+oxygen, it uses the energy to      generate electricity

(There is more info required about relating this topic to the car industry, basically its beneficial to as the only by products are WATER + HEAT) 

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Analysing Substances (C3.4.1)

Flame Tests (are used to identify anions ie. metal ions)
- when the metal compounds burn, they give off a specific colour
- Lithium (crimson) 
- Sodium (yellow)
- Potassium (lilac)
- Calcium (red) 
- Barium (green) 

Precipitates (are also used to identify anions) 
- when they react with NaOH, they produce a coloured precipitate becuase most metal hydroxides are insoluble
- Copper (II) Blue 
- Iron (II) Green 
- Iron (III) Brown 
- Aluminium White
- Magnesium White 
- Calcium White 

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Analysing Substances (C3.4.1) Part 2

Carbonates (are used to test for cations, negative ions)
- You can react carbonates with dilute acids that should give off carbon dioxide
- If it produces a white precipitate with lime water that goes cloudy, C02 is present
- acid + carbonate --> salt + water + carbon dioxide

Halide ions (are used to test for chlorides, bromides or iodide ions)
- Simply add dilute nitric acid followed by silver nitrate solution 
- Chloride (white)
- Bromide (cream) 
- Iodide  (yellow)

Sulfate ions (are used to test for sulfate ions)
- add sulfuric acid followed by barium chloride solutions
- if a white precipitate of barium sulate is formed, the orginal compound must have been a sulfate.  
- Barium + Sulfuric acid ---> Barium Sulfate  

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Analysing Substances (C3.4.1) Part 3

  • Titrations
    -are used to accurately measure how much alkali or acid is needed to neutralise the acid or alkali
    - We used indicators to identify when the solution has neutralised
    - Concentration is the measure of how crowded particles are (mol/dm3 or g/dm3)
  • Method
    1. pour the alkali and indicator in the flask
    2. add the acid to the burette 
    3. slowly drop the acid into the flask and see when the colour changes
    4. record the rough amount of acid used to neutralise
    5. repeat it 3x to be more precise and avoid contamination
     
  • Calculations (1000cm3 = 1dm3) 
    1. write out the balanced equation 
    2. beneath it, write the info given in the question e.g. volumes or concentrations
    3. work out moles of one reactant (mole= conc x vol / 1000) or... mass=mole x Mr
    4. check out the ratio of moles (usually 1:1)
    5. work out the concentration (conc= mol x 1000/vol)  
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Making Ammonia (C3.5.1)

Haber Process 

  • Raw materials - nitrogen, hydrogen
  • Conditions - 200 pressures, 450ºc, iron catalyst 
  • Used to make fertilisers 

Method

1. High pressures favour forward reactions (smaller no. of molecules on that side) 
2. 200 pressures gives the best percentage yield that comprimises with the price
3. Forward reaction is exothermic but increasing the temp will move the equilibrium the other way
4. Therefore, it favours lower temperatures (however, low temp=low rate of reaction)
5. 450ºc is a compromise between percentage yield and the speed of reaction 
6. Ammonia gas condenses, liquifies and is removed
7. Unused hydrogen and nitrogen is recycled 

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Making Ammonia (C3.5.1) Part 2

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants

  • Equilibrium is the amount of reactants and products reaching a balance and stays there
  • Closed systems is where no reactants can escape 
  • Reversible reactions occur in closed systems 
  • Equilibriums are reached when the reaction occur at exactly the same rate in each direction

Temperature:
= if raised, the endothermic reaction will increase to use up/take in heat (yield increases for endo, yield decreases for exo) 
= if reduced, the exothermic reaction will increase to give out heat (yield increases for exo, yield decreases for endo)

Pressure:
= if raised, the pressure will encourage the reaction that produces less volume 
= if lowered, the pressure will encourage the reaction that produces more volume  

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Alcohols (C3.6.1)

Properties of Alcohols

- Functional group -OH
- flammable (burn in air > C02 + H2O) 
- dissolved in water = neutral solutions
- reacts with Na = hydrogen and alkoxides 
- used as fuels and solvents

Uses 

Solvents: perfumes, aftershave lotions, methylated spirit (cleans paint brushes) 
Fuels: spirit burners, gasohol

Oxidation

- oxidation of ethanol produces ethanoic acid
- this is catalysed by bacteria using 02 in the air
- ethanoic acid is in vinegar (a week acid aqueous solution)
- weak acids e.g. ethanoic produce H+ less readily = partially dissociated
- strong acids e.g. hydrochloric produce H+ more readily = fully dissociated  

 

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Carboxylic Acids (C3.6.2)

Properties

- functional group -COOH
- methonoic, ethanoic, propanoic acids are the first 3 members of a homologous pair 
- react with carbonates = Co2
- react with alcohols (with an acid catalyst) to produce esters
- dissolves in water = acidic solutions
- do not ionise completely (forming weak acidic solutions)
- weak acids have high pH

Common Acids

  • ethanoic acid-> oxidising ethanol or oxidising agents
  • ethanoic acid-> dissolve in water to make vinegar for flavouring or preserving 
  • citric acid-> oranges and lemons 

Uses
-soaps, detergents
-esters  

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Esters (C3.6.3)

Properties

  • Functional group -COO-
  • acid catalysts e.g. sulfuric acid are used to produce esters
    Alcohol + carboxylic acid --> ester + water 
  • Ethanoic acid + ethanol --> ethyl ethanoate + water 
  • Esters have pleasant smells, volatile compounds and dont mix well with water

Uses

  • Perfumes (Aromas) 
  • Flavourings
  • Ointments
  • Solvents for paints, ink, nail varnish remover

Problems

  • irritates mucous membrane in the nose + mouth 
  • extremely flammable (heavier than air) 
  • toxic (due to synthetic food additives) 
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Comments

Rianna Gokani

fantastic, a huge help, thank you

Turnip44

these are soooooooo gooood.............

nafeesah291

REALLY DETAILED BUT UNDERSTANDABLE WHICH IS GREAT! THANK U

هزل, يتبادل المزاح

معمع10/10 

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