Chemistry C3

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The Early Periodic Table

  • Newland: - Put elements in order of weight

                   - Proposed theory of octaves (similar properties are repeated every eighth element)

                   - Did not leave space for undiscovered elements

                   - After calcium his law didn't work, so his table was rejected

  • Mendeleev: - Left gaps for undiscovered elements, in the place he thought they would go

                      - Put the elements in groups of similar properties

                      - His predictions of elements were proved correct, so people accepted his table

                      - Now the basis for our Periodic Table

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The Modern Periodic Table

  • Atomic number determinds the elements position in the table
  • Number of electrons in outer shell determinds its chemical properties
  • Larger the atom, the more reactive - weak intermolecular forces
  • As non-metals get larger, less reactive - they gain electrons rather than loose them
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Group 1 - Alkali Metals

  • Found in Group 1 of the periodic table
  • All have one electron in outer shell, which is lost in recations, so they form ionic compounds that have a single positive charge
  • Reactivity increases going down group, as the outer shell gets less strongly attracted to the nucleus
  • All metals that react readily with water and oxygen
  • Soft solids at room temperature, low melting and boiling points that decrease going down the group, low density so can float on water
  • React with water to produce hydrogen and the metal hydroxide 

    eg: sodium+hydrogen--->sodium hydroxide+hydrogen

  • React with halogens (group 7) to form salts
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Transition Metals

  • High melting and boiling points
  • Malleable, ductile, good conductors, strong, dense, good building materials or alloys
  • Often brightly coloured
  • Form positive ions of various charges
  • Dont react with water or oxygen at room temperature
  • Used as catalysts
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Group 7 - Halogens

  • Non-metalic elements, made up in pairs
  • Low melting and boiling points that increase going down
  • At room temperature: - Fluorine=Yellow Gas

                                     - Chlorine=Green Gas

                                     - Bromine=Red-Brown Liquid

                                     - Iodine=Gray Solid (but easily vapourises as a Violet Gas)

  • Negatively charged as 1-
  • Bond covalently with non-metals, forming molecules
  • Reactivity decreases going down
  • A more reactive halogen will displace a less reactive one
  • Reactivity decreases going down, as the attraction of the outer electrons to the nucleus decreases as the number of occupided energy levels increases
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Hard Water

  • Contains dissolved comounds such as calcium and magnesium
  • React with soap to make scum
  • Temporary hard water produces scale when it is boiled, reducing the efficiency of appliances
  • Hard water is healthier, calcium helps bones and teeth and can prevent heart disease
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Removing Hardness

  • Hard water can be softened in 2 ways:  

     - Heating it, temporary hard water contains hydrogen carbonate ions. They decompose when heated to form carbonate ions, water and carbon dioxide.The carbonate ions react with calium and magnesium to make calcium carbonate and magnesium carbonate which deposite as scale

     - Ion-exchange column, this is packed with resin beads of sodium and hydrogen. These displace the calcium and magnesium in the water as it flows through the resin beads.

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Water Treatment

  • Filtration or sedimentation, followed by disinfection cleans water
  • Chlorine is added to kill microbes
  • We can make pure water through distilation, but requires large amounts of energy and this makes it expensive
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Water Issues

  • Advantages and disadvantages to treatment of water: 

    - Chlorine is added to kill microbes but it is piosonous

    - Hard water causes problems in appliances, but soft water isnt was good for our health

    - Fluorine is added to stop tooth decay, but people want to choose whether to take extra fluorine or not

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Comparing The Energy Released By Fuels

  • The amount of energy released is usually measured in joules/J, but can be given in calories:    1 cal=4.2J
  • Simple calporimeters use the temporature change to calculate the amount of energy released
  • Use the equation: Q= mc/\T                                                                                               Q= amount of energy transferred (J)                                                                                     m= mass (g)                                                                                                                       c=specific heat capacity (J)                                                                                                 /\T=temperature change (oC)                                                                                              
  • If the reaction takes place in a solution we assume it behaves the same as water, 1cm3=1g
  • If a solid is added we assume the volume does not change
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Energy Level Diagrams

  • Exothermic reactions make bonds
  • Endothermic reactions break bonds
  • Exothermic reactions have the reactants at the top and products at the bottom, because when bonds are made energy is given out
  • Endothermic reactions have the products at the top and the reactants at the bottom, because when bonds are broken energy is taken in
  •  The activation energy is the energy need to start the reaction, using a catalyst decreases the activation energy
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Calculations Using Bond Energies

  • To calculate whether a reation is exothermic or endothermic we need to:

     - Calculate the total amount of energy needed to break all the bonds

     - Calculate the total amount of energy released in maing all of the bonds

     - Calculate the difference between the two totals

  • If the answer is negative then it is an exothermic reaction
  • If the answer is positive it is an endothermic reaction
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Fuel Issues

  • Hydrogen is an alternative to fossil fuels
  • Advantages: - Burns easily

                        - Releases large amounts of energy

                        - Produces no carbon dioxide when burnt, only water

                        - Can be burnt in combustion engines or used in fuel cells to power vehicles 

  • Disadvantages: - Has to be specially stored, these come with safety problems

                            - Vehicles that use fuel cells have to match the performance, conveinience,                                   cost of petrol/diesel to normal cars

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Tests For Positive Ions

  • Flame test: - Lithium=Crimson

                      - Sodium=Yellow

                      - Potassium=Lilac

                      - Calcium=Red

                      - Barium=Green

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Tests For Positive Ions

  • Sodium hydroxide test: - When sodium hydroxide is added to a solution of 2+ or 3+ ions a                                            precipitate of the metal hydroxide is formed
  • Aluminium, calcium and magnesium form white precipitates, if excess sodium hydroxide is added the precipitate of aluminium hydroxide dissolves
  • Copper (II) hydroxide=Blue                                                                                                   Iron(II) hydroxide=Green                                                                                                       Iron(III) hydroxide=Brown                                                  
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Tests For Negative Ions

  • Carbonate ions: - Add dilute hydrochloric acid to the substance to see if it fizzes, if it does                                 collect the gas in a delivery tube and test in limewater, if the limewater goes                           cloudy then there is a carbonates present.
  • Halide ions: - Add dilute nitric acid and the silver nitrate solution,                                                               Chloride ions=White precipitate                                                                                           Bromide ions=Cream precipitate                                                                                           Iodide ions=Yellow precipitate
  • Sulfate ions: - Add dilute hydrochloric acid and then barium chloride solution. If a white precipitate forms then sulfate ions are present
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Titrations

  • A titration measures accurately how much acid and alkali react together completely
  • We use an indicator the end point (the point at which enough acid has reacted with enough alkali to make the solution neutral) 
  • Titration calculations can be completed using:             M

                                                                                 C    V

  • C=concentration                                                                                                                 M=moles                                                                                                                           V=volume                                                                                                              

       

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Chemical Analysis

  • Environmental Monitoring Medicine and Forensic Science all need to analyse substances
  • The results of their analysis are often matched to existing things stored on a database 
  • Qualitative methods find out simply if a substance is in a sample eg: tests for positive and negative ions
  • Quantitive methods tell us how much of a substance is present in a sample eg: gas chromatography or titrations
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Chemical Equilibrium

  • Reversible reactions are ones where the products react together again to make the reactants
  • In a closed system, no reactants or products can escape.
  • Equilibrium is reached when the rate of the forward reaction is equal to the rate of the reversed reaction. Both reactions continue to happen but the amouns of reactants and products remain constant
  • The amount of products and reactants made can be changed by changing the conditions
  • Increasing the concentration of the reactants will make more products as the system tries to achieve equilibrium
  • If a product is removed, more reactants will react to try to achieve equilibrium, so more products are made
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Altering Conditions

  • Changing conditions can make the position of equilibrium favour one side of the reaction, therefore more products or reactants are formed
  • Pressure: - The forward reaction produces more molecules of gas:

                      increase in pressure=decrease in products formed (vice versa) 

                    - The forward reaction produces fewer molecules of gas:

                      increase in pressure=increase in products formed (vice versa)

  • Temperature: - The forward reaction is exothermic:

                           increase in temperature=decrease in products formed (vice versa)

                         - The forwadr reaction is endothermic:

                           increase in temperature=increase in products formed (vice versa)

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Haber Process

  • Nitrogen from air, mixes with hydrogen from water or natural gas to make ammonia
  • Gases are passed over an iron catalyst
  • It is a reversible reaction
  • Conditions are 450 oC and a pressure of 200 atmospheres
  • Yield is only 15% because some of the ammonia breaks down to make hydrogen and nitrogen again
  • The nitrogen and hydrogen formed are recycled, this can happen because it is done in a closed system, so no reactants can escape or be put in
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Economics of the Haber Process

  • A higher pressure would produce more ammonia, but can not be contained
  • A lower temperature wold produce more ammonia but it would take too long
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Structures of Alcohols

  • Alcohols are part of the Homologous series, they are alkanes and alkenes made up of hydrogen and carbon only
  • Alcohols have the functional group: -O-H, so one hydrogen atom from each alkane molecule is replaced with an -O-H
  • Methanol
  • Ethanol
  • Propanol
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Structures of Carboxylic Acids

  • Carboxylic acids have the functional group -COOH, therefore three hydrogen atoms get replaced with -COOH
  • Methanoic acid:
  • Ethanoic acid:
  • Propanoic acid:
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Structures of Esters

  • Esters have the functional group -COO-. If the H atom in a carboxylic acid is replaced by a hydrocarbon group the compound is an ester.
  • Ethyl ethanoate: 
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Properties and Uses of Alcohols

  • Alcohols are used as solvents and fuels, and ethanol is the main alcohol in alcoholic drinks
  • Alcohol is burnt in air to form carbon dioxide and water
  • Alcohols react with sodium to from a solution and give off hydrogen gas
  • Ethanol can be oxidised to ethanoic acid, by a chemical oxidising agent or by microbes. Ethanoic acid is vinegar
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Carboxylic Acids and Esters

  • Carboxylic acids have a pH less than 7, and react well with carbonates
  • Esters are made my reacting a carboxylic acid and an alcohol together with an acid catalyst
  • Esters are used in flavourings and perfumes
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Organic Issues

  • Alcohols, carboxylic acids and esters can all benefit society, however they can be abused: alcohols and solvents are addictive
  • In the future ethanol and esters could be used in biofuels, when crude oil runs out
  • However future use of boifuels might conflict with the need to feed the world 
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