Analysis and Synthesis
- 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
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
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
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)
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
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
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
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
-
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
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
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 :)
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
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.
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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