Analysis and Synthesis

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  • Created by: Chesca
  • Created on: 17-05-13 11:08

Flame Tests

Some metal ions produce flames with colours

To test...:

  • j Put small amount of compound in nichrome wire loop
  • (wire loop dipped in concentrated HCI and heated to clean first)
  • Hold loop over bunsen burner

Lithium = crimson

Sodium = yellow

Potassium = lilac

Calcium = red

Barium = green

 

 

 

 

 

 

 

 

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Reactions with sodium hydroxide

Aliminium ions, calcium ions and magnesium ions

  • ALL form a white precipitate with sodium hydroxide

If more added to Aliminium- precipitate dissolves

  •  Doesn't dissolve in calcium and magnesium

Copper ions = blue precipitate

Iron 11 = green precipitate

Iron 111 = brown precipitate

 

 

 

 

 

 

 

 

 

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Negative ions - Carbonates

If add dilute  acid  to carbonate, it fizzes and produces CO2 gas

  • acid + carbonate - gas + water

In limewater - CO2 + calcium hydroxide = white precipitate

  • makes limewater cloudy

 

 

 

 

 

 

 

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Halides (chloride, bromide, iodide) + Sulfates

Add dilute nitric acid, then silver nitrate solution

  • if precipitate formed = halide present

Chloride ions = white precipitate

Bromide ions = cream precipitate

Iodide ions = pale yellow precipitate

Sulfates:

  • Test by adding dilute HCI and barium chloride solution
  • HCI removes carbonate ions
  • White precipitate formed shows sulfate ions (insoulable salt)

 

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Titrations

Acid + alkali = salt + water

must have same amount of acid = as alkali to neutralise

Titration: measures exact volumes of acid and alkali needed to react

End point: acid and alkali have reacted completely

Pipette: measures fixed volume of solution

Burette: measures volume of solution added

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Chemical equilibrium

Reversible reaction = products formed can form original again

so as concentration increases. rate at which they re-form to form reactants increases

Rate of forward reaction decreases, concentration of reactants descreases from the original max value

Eventually both forards and reverse reactions go at same rate reactant make products same rate as products make reactants

No change in the amount of products and reactant

Reaction reached equlibrium

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Pressure and equlibrium

Changing pressure ONLY affects if different number of molecules on each side of the equation

Changing concentration affects equlibrium.

  • if increases conc of reactant, position of equlibrium moves away from the stress (to the right)
  • If forward reaction produces MORE molecules of gas...
  • Increase in pressure, increases amount of products formed
  • Decrease in pressure, decreases amount og products formed
  • If forward reaction produces fewer molecules of gas..
  • increase in pressure, increases amount of product formed
  • decrease in pressure decreases amount of product formed
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Energy and equlibrium

In closed system, amounts of reactants and products in reaction at equlibrium depend on temperature

  • plan to get more products and less reactants by changing the temperature

If forward reaction is exothermic:

  • increase in temp decreases the amount of products formed
  • decrease in temp increases amount of products formed

If forward reaction is endothermic:

  • increase in temp increases amount of products formed
  • decrease in temp decreases amount of products formed

If increase temp, equlibrium shifts to try and reduce temp, so away from the stress.

  • reaction that is endothermic (taking energy) will cool it down
  • 
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The Haber Process

Plants need nitrogen to grow

  • plants absorb soluable nitrates from soil through their roots
  • when crop harvested - nitrates lost
  • farmers need to put nitrogen in soil

German Chemist = Fritz Haber

The Haber Process - allows us to turn nitrogen in the air into ammonia

  • ammonia makes fertilisers
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Titrations 2

Titration process:

  • Measure known vol of alkali/acid needed to react with the other with pipette
  • Add indicator solution to flask
  • Pour acid to use into burette, Record initial reading on burette (wash with distilled water  before)
  • Slowly add small amounts of acid
  • Until indicator changes colour
  • Record reading
  • Repeat the process 3 times
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The Haber Process 2

Raw materials for ammonia = nitrogen in air/hydrogen from natural gas

Nitrogen + hydrogen are purified

Passed over an iron catalyst at high temp (450) and pressure (200 atmospheres)

  • product of reversible reaction = ammonia.
  • Reaction in Haber Process = reversible
  • ammonia breaks back down to nitrogen and hydrogen
  • remove ammonia by cooling the gases, so liquifies

To achieve best possible yield....a compromise must be made :)

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Effect of Pressure

N2 + 3H2 = 2NH3

4 molecules on LHS  2 molecules on RHS

  • Increase pressure, shift to right - produce more ammonia

To get MAXIMUM yield of ammonia = need high pressure

  • however max pressure = expensive (pipes have to be v.strong!)

To get max....compromises...uses 200 atmospheres of pressure

  • gives a LOW yield but not as expensive

 

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Effect of Temperature

Forward reaction = exothermic

  • temp low, increases amount of ammonia in reaction at equilibrium
  • Low temp = slow rate of reaction :(
  • particles collide less often, as have less energy

Compromise = reasonably high temperature, even though reduces yield.

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Effect of Catalyst

Iron Catalyst - used to speed up reaction

  • speeds up both forward and reverse direction
  • Does NOT affect yield :)
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