Chemistry C5

Ocr, Gateway Chemistry C5

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The Mole

The mole= a name give to a certain number

One mole of atoms or molecules of any substance will have a mass in grams equal to there relative atomic mass (on the periodic table) or if its a molecule relative formula mass

Molar mass is another way of saying 'the mass of one mole' it is also measured in grams

Number of moles= mass in g / relative formula mass

The relative atomic mass of an element is the average mass on an element compared to the mass of an atom of carbon-12

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Reacting Masses

Mass is conserved during a chemical reaction. this means that the mass of reactants will always equal the mass of the products

calculating mass of reactants:

  • write out the balanced equation
  • work out the relative formula mass of the parts you want
  • apply the rule: divide to get one, then multiply to get all
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Empirical formula

Empirical formula= the smallest whole number ration of an atom in a compound

to work it out:

1. list the elements in the compound

2. underneath them write the mass (given in question)

3. divide each mass by the atomic mass for that element

4. turn the numbers you get into a simple ratio

5. get the ratio in its simplest form

remember to write the empirical formula at the end

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Electrolysis is the breaking down of a substance using electricity

an electric current is passed through a molten or dissolved ionic compound causing it to decompose

for the electrical circuit to be complete there must be a flow of electrons

  • Electrons are taken away from ions at the positive anode
  • passed by an external circuit they then go to the negative cathode
  •  they're given to other ions in the electrolyte

as ions gain or lose electrons they become atoms or molecules and are discharged from the solution

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Electrolysis in school lab

  • 2 graphite or platinum electrodes connected to DC power supply and placed in the electrolyte
  • positive ions go to the cathode
  • negative ions go to the anode
  • if molecules discharged are gases they are collected in test tubes
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Discharging ions from water

In aqueous solutions as well as the ions from the solute there are hydrogen ions (H+) and hydroxide ions (OH-) from the water

  • If the metal is very reactive its ions will stay in solution and H+ will be discharged at the cathode
  • if the non-metal is a halide ion (group 7) it will form the halogen if not oxygen will be discharged

e.g. A solution of Aqueous potassium sulfate (K2SO4)

contains 4 ions: K+ , SO4 2- , H+ and OH-

  • At cathode hydrogen is discharged: 2H+ + 2e- ----> H2
  • hydroxide ions lose electrons easier than sulfate ions, so oxygen is discharged at the anode: 4OH- - 4e- ----> O2 + 2H2O
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Electrolysis of molten salts

  • A salt will conduct an electric current when molten
  • the salt is always broken up into elements
  • positive metal cations are reduced (gain electrons) to the cathode
    • e.g. Pb 2+ + 2e- ---> Pb
  • Negative anions are oxidised (lose electrons) to atoms at the anode
    • e.g. 2Br- ---> Br2 + 2e-
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  • when a certain number of electrons flow away from the positive electrode the same number of electrons must flow onto the negative electrode
  • using electrolysis of copper sulfate solution:
    • negative electrode half equation: 1 mole of copper ions pick up 2 moles of electrons- makes 1 mole of copper atoms which sticks to the electrode causing mass to increase
    • positive electrode half equation- 1 mole of copper atoms loses 2 moles of electrons and is released as copper ions into the solution, copper is lost from the electrode- decrease in mass
    • 1 mole of copper is added to the negative electrode as 1 mole of copper is lost from the positive electrode- gain in mass is equal to the loss in mass
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Electrolysis- calculating masses

The amount of product made depends on the number of electrons transferred

if you increase the number of electrons you increase the amount of substance produced

  • can do this by increasing time or current

coulombs and faradays are amount of electricity

coulombs= amps time seconds (Q=It)

96000 coulombs= 1 faraday

one faraday (f) contains one mole of electrons

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Electrolysis- calculating masses

1. write out the balanced half equation

2. calculate the number of faradays

  • first do coulombs=amps *seconds
  • number of faradays= coulombs/ 96000

3. calculate the number of moles of products

  • divide the number of faradays by the number of electrons in the half equation

4. write in the relative atomic mass value to work out mass of solid products

  • mass= relative atomic mass * number of moles
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Concentration is a measure of how crowded things are

  • can be measure in moles per dm3
  • the mole solute you dissolve in a give volume the more crowded the solute particles are and the more concentrated the solution
  • 1 litre= 1000cm3 = 1dm3

concentration= number of moles / volume

remember to convert

  • grams per dm3 to moles per dm3
  • cm3 to dm3 divide by 1000
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It is important to get the concentration right:

  • food preparation
    • ingredients have to be at the right consistency so they have the right taste
  • medicines
    • if diluted too much may not work
    • not enough may have side effects
  • baby milk
    • too dilute baby won't get enough nutrient
    • not dilute enough could become overweight
  • weedkillers
    • need diluting so don't kill the plants
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Recommended Daily Allowances

RDAs= amounts of nutrients (vitamins, food types etc)  that an average adult should eat for a healthy diet

The mass of salt in a product is approximately 2.5 * mass of sodium it contains

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  • titration's allow you to find out exactly how much acid is needed to neutralise a quantity of alkali
  • Using a pipette and pipette filler add some alkali (about 25cm3) to a conical flask along with 2 or 3 drops of indicator
  • fill a burette with the acid- make sure this is done below eye level
  • using the burette add the acid to the alkali a bit at a time giving the conical flask a regular swirl, go slowly when you think you are reaching the colour change
  • the indicator changes colour when all the alkali has been neutralised
  • record the amount of acid used to neutralise
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Consistent readings and the indicator

To increase the accuracy of the titration and to spot any anomalous results you need several consistent readings

  • the first titration is a rough titration to get the approximate idea of where the solution changes colour
  • you then need to repeat the whole thing a few times making sure you get around the same answer

Indicator used for titration's:

  • universal indicator shows a range of colours but during acid-base titration's you want a sudden colour change
  • want to use an indicator that does this e.g. acid--> alkali
    • litmus: red---> blue
    • methyl orange: red ---> yellow
    • phenolphthalein: clear---> pink
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pH curves

a neutralisation reaction= when an acid and an alkali react to form a salt and water

(draw in pH curves and label)

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calculating concentration for titration's

moles= volume*concentration/ 1000

1. calculate moles of substance 1

2. use balanced equation- ratio of moles

3. calculate concentration of substance 2

  • concentration= number of moles*1000 / volume
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Gas Volumes

Collection method:

  • gas syringe: give volume to nearest cm3 but can't use if reaction is too vigorous
  • upturned measuring cylinder or burette-
    • can use a delivery tube to bubble the gas into an upside-down measuring cylinder or gas jar filled with water
    • this method is no good for collecting things like hydrogen, ammonia or chloride as they dissolve in water
    • can also use an upturned burette which is slightly more accurate

can also measure the mass of gas produced:

  • carry out the experiment on a mass balance
  • as gas is released mass disappearing is easily measured on the balance
  • most accurate because mass balance is very accurate

One mole of any gas always occupies 24dm3 at room temperature and pressure

volume= moles * 24

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Reactions stop when one reactant is used up

the reactant that is used up first in the reaction is called the limiting reactant

if you halve the amount of limiting reactant the volume of gas produced will also halve (the amount of gas produced is proportional to the amount of limiting reactant)

If you double the amount of limiting reactant it might not be the limiting reactant anymore

Faster rates or reactions are shown by steeper curves

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A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants: A+B <---> C + D

Reversible reactants will reach equilibrium:

  • as the reactants (A+B) react their concentrations will fall- so the forward reaction will slow down- but as more products (C+D) are made their concentrations rise so the backward reaction would speed up
  • after a while the forward reaction will be going at the same rate as the backward one- equilibrium
  • at equilibrium both the reactions are still happening but there is no overall effect- this means that the concentrations of reactants have reached a balance and won't change
  • equilibrium is only reached if the reversible reaction take place in a 'closed system' a closed system just means that none of the reactants or products can escape
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Position of equilbrium

  • sometimes equilibrium will lie to the right- means lots of products and not much of the reactants
  • sometimes equilibrium will lie to the left- means lots of reactants but not much products
  • the position of equilibrium depends on the conditions

Adding a catalyst does not change the position of equilibrium:

  • they speed up both the forward and the backward reaction
  • adding a catalyst means that the reaction reaches equilibrium quicker but will have the same amount of product at the end
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Things that affect position of equilibrium

Temperature-all reactions are either endothermic or exothermic:

  • if decrease the temperature, equilibrium will move to try and increase it, equilibrium moves in the exothermic direction- produce more heat
  • if you raise the temperature, equilibrium will try and decrease it- moves in endothermic direction

Pressure- only affects gases

  • increase pressure, equilibrium tries to reduce it- moves in direction where fewer moles of gas
  • decrease pressure, equilibrium tries to increase it- moves in direction where more moles of gas


  • increase concentration of forward reactants, equilibrium tries to decrease it by shifting to right
  • if you increase the concentration of products equilibrium tries to reduce it by shifting to left
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Equilibrium tables and graphs

The haber process is a good example: N2 + 3H2 <----> 2NH3

  • table: as pressure increase the proportion of ammonia increases
  • graph: as temperature increase the proportion of ammonia decreases, the conditions that will give you most ammonia is high pressure and low temperature
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The contact process

used to make sulfuric acid:

1. make sulfur dioxide-usually burning sulfur in air

  • sulfur+oxygen ----> sulfur dioxide
  •    S   +  O2    ------> SO2

2. sulfur dioxide is then oxidised with help of catalyst to make sulfur trioxide

  • sulfur dioxide + oxygen <----> sulfur trioxide
  • 2SO2    +   O2     <----> 2SO3

4. sulfur trioxide is used to make sulfuric acid

  • sulfur trioxide + water <----> Sulfuric Acid
  • SO3  + H2O -----> H2SO4
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Conditions used to make SO3


  • oxidising sulfur trioxide is exothermic (gives out heat) so to get more product you'd think the temperature should be reduced
  • but reducing temperature slows reaction down so 450'c used as an optimum temperature as a compromise


  • there are 2 moles of product compared to 3 moles of reactant, so to get more product you would think pressure should be increased but increasing pressure is expensive so atmospheric pressure is used


  • to increase rate of reaction a vanadium pentoxide catalyst is used, it doesn't change position of equilibrium
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Acids release protons in water, and when they ionise they release H+ ions

Strong acids (e.g. sulfuric,hydrochloric) ionise almost completely in water

weak acids (e.g ethanoic, citric and carbonic) ionise only very slightly

  • ionisation of weak acids is reversible because only a few H+ are released the equilibrium lies well to the left

The pH of an acid or alkali is a measure of concentration of H+ ions in solution- measured with universal indicator

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Strong and concentrated acids

Acid strength= what proportions of the acid molecules ionise in water

concentration= how many moles of acid molecules there are in a litre of water, basically how watered down the acid is

  • describes the total number of acid molecules
  • more moles of acid per dm3 the more concentrated the acid is

Strong acids are better electrical conductors than weak acids

  • hydrochloric acid has a much higher electrical conductivity than the same concentration of ethanoic acid- to do with the concentration
  • greater concentration of ions in the strong acid means more charge can be carried
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Reaction rate of strong and weak acids

Strong and Weak acids react with reactive metals and with carbonates in the same way

  • difference between the reactions is the rate, ethanoic acid will react more slowly than the same concentration of hydrochloric acid
  • all to do with the equilibrium of ethanoic acid
  • when you put a weak acid into water it releases few H+ ions which means the concentration of H+ ions decreases so the equilibrium shifts to compensate meaning more H+ ions are released- these ions react and the equilibrium shifts as more ions are removed more are supplied
  • completely different to when have a strong acid.
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Amount of gas produced

Amount of gas produced depends on the amount of acid

  • hydrochloric acid will react faster than ethanoic acid but the amount of product made will be the same
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Weak acids

are actually useful- can be more useful that dilute strong acids

  • strong acid reacts very fast- might not be useful if you want a more controllable reaction like with kettle descalers
  • strong acid would react quickly with the scale and may also react with the metal kettle.
  • a weak acid still removes the scale but the lower concentration of H+ ions means the reaction will be slower and easier to control
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Precipitation reactions

Precipitation reactions make an insoluble substance- normally involves 2 solutions reacting together

the insoluble substance= the precipitate

most precipitate reactions involve ions, to react with each other these ions need to collide so they are able to move

this means the ionic substances have to be molten or in solution

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Ionic equations of precipitate reactions

  • you can tell when its a precipitation reaction because start off with 2 solutions (aq) and end up with a solid (s)
  • The spectator ions= ions which were dissolved in the solution before the reaction and are still dissolved afterwards
  • an ionic equation concentrates only on the useful bits of the reaction
    • e.g. Ba 2+ (aq) + SO4 2- ----> BaSO4 (s)
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Test for sulfates and Halides

can use precipitation reactions to identify substances

test for sulfate ions:

  • add dilute HCl followed by barium nitrate
  • a white precipitate of barium sulfate means the original compound was a sulfate

test for chloride, bromide or iodide

  • add dilute nitric acid followed by lead nitrate
    • chloride gives white precipitate of lead chloride
    • bromide gives a cream precipitate of lead bromide
    • iodide gives a yellow precipitate of lead iodide
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Preparing insoluble salts

you must pick the right reactants

  • need some ions, need to be in solution so can move about, then:

1. check each reactant and product for hazardous information

  • add 1 spatula of lead nitrate to a test tube and fill it will distilled water, shake to make sure all lead nitrate is dissolved, then fo the same with 1 spatula of sodium iodide, tip the 2 solutions into a small beaker, stir and salt should precipitate out

2. put a folded piece of filter paper into filter funnel and stick in conical flask, pour contents into middle of filter paper, swill out beaker with distilled water and tip through filter paper- to get all product

3. rinse the contents of filter paper with distilled water to make sure all soluble salts washed away and scrape lead iodide onto fresh filter paper and leave it to dry

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These are great- Thank you : )



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