Chemistry - C3.4 - Analysis and Synthesis
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- Chemistry
- Analysing substancesEquilibriumCalculations, moles and yieldHaber and industrial processes
- GCSE
- AQA
- Created by: Katatatitee
- Created on: 05-05-14 15:52
C3.4.1 - Test for Positive Ions
- Positive ions identified by flame test or using sodium hydroxide solution
- Flame test: Li+ = crimson/red, Na+ = yellow, K+ = lilac, Ca2+ = red, Ba2+ = green
- Hydroxides of most metals that have ions with 2+ or 3+ charges are insoluable in water - when sodium hydroxide is added to solutions of these ions, precipitate of the metal hydroxide forms
- Al, Ca and Mg ions form white precipitates - when excess sodium hydroxide solution is added the aluminium hydroxide precipitate dissolves - Cu(II) hydroxide is blue, Fe(II) hydroxide is green, Fe(III) hydroxide is brown
- Show reactions of positive ions with sodium hydroxide solution by balanced ionic equations - Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)
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C3.4.2 - Test for Negative Ions
- Caronate ions: add dilute hydrochloric acid to substance and test gas given off in limewater - limewater goes milky it contains carbonate ions
- 2HCl(aq) + CaCO3(s) -> CaCl2(aq) + H2O(l) + CO2(g)
- Halide ions: add dilute nitric acid then silver nitrate solution - chloride ions give white precipitate, bromide ions give cream precipitate, iodide ions give yellow precipitate
- AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
- Sulfate ions: add dilute hydrochloric acid then barium chloride solution - white precipitate forms if sulfate ions are present
- BaCl2(aq) + MgSO4(aq) -> BaSO4(s) + MgCl2(aq)
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C3.4.3 - Titrations
- Acid + alkali -> salt + water - neutralisation - exact volumes used found through titration
- Pipette used to accurately measure volume of alkali put into conical flask with indicator - burette filled with acid and added gradually to flask
- When indicator changes colour - end point has been reached - colume of acid found from initial and final burette reading
- Titration should be done several times to improve repeatability of results
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C3.4.4 - Titration Calculations
- Concentration of solution meaured in g/dm³ or mol/dm³
- No of moles(mol) = C (mol/dm³) X V (cm³) / 1000
- No of moles = mass / molar mass
- Titrations used to find volume of solutions that react exactly - if concentration of one solution and volume of both is known, concentration of other solution can be calculated - concentrations calculated using balanced symbol equations and moles:
- Write equation, find ratio, find no. of moles in known solution, find no of moles in unknown solution (using ratio), find concentration/volume
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C3.4.5 - Chemical Analysis
- Scientists use a various methods to analyse substances fr purposes including environmental, medical and forensic investigations
- Can use 'wet chemistry' methods or GC-MS
- Qualitative - find out what substance sample is, quantative - how much is in a substance
- Large databses with analysis results have been built - used to identify substances in samples, identify individuals or monitor changes in amounts of substances over time
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C3.4.6 - Chemical Equalibrium
- Some reactions are reversible: products react together to make reactants
- In closed system no reactants/products can escape - for reversible reactions equilibrium is reached when rate of forward reaction = rate of reverse reaction - both reactions happen but amount of reactant and product are equal
- Amount of reactant/product from reversible reaction can be changed by changing conditions - important in industry in controlling reactions - eg. increasing concentration of reactant increases amount of product formed or if product is removed, more products form to try and reach equilibrium
- ICl + Cl2 ICl3
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C3.4.7 - Altering Conditions
- Change conditions of system at equilibrium: position of equilibrium shifts to try and cancel out change
- Changing pressure affects reversible reactions with different no of gas molecules on one side - eg. increase pressure: position of equilibrium shifts to reduce pressure, favoring reaction producting less gas) - forward reaction produces more gas: increased pressure = decreased amount of product: decreased pressure = increased amount of product
- Reversible reactions: exothermic endothermic - increasing temperature favors endothermic reaction: equilibrium shifts to lower temperature by taking in energy - forward reaction is exothermic: increased temperature = decreased amount of product: decreased temperature = increased amount of product
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C3.4.8 - Making Ammonia - the Haber Process
- Haber process used to manufacture ammonia: used to make chemicals eg. fertilisers
- Nitrogen (from air) and hydrogen (from natural gas) purified and mixed
- Gases passed over iron catalysts at ~450°C and at pressure of ~200 atmospheres - give fast rate of reaction and reasonable yield of ammonia
- N2(g) + 3H2(g) 2NH3(g)
- Some ammonia produced breaks down into N and H; yield ~15%
- Gases coming out of reactor cooled so ammonia condenses and separated from unreacted gases that are recycled so aren't wasted
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C3.4.9 - The Economics of the Haber Process
- Haber process: H and N react to make NH3(ammonia)
- products have fewer gas molecules than reactants: higher pressure = greater yield - more energy needed to compress gas and stronger reaction vessels: increased cost - pressure ~200 atmospheres: compromise between cost and yield
- Forward reaction is exothermic: lower temoerature = greater yield - but reaction rate decreaces with temperature and iron catalyst becomes ineffective: take long time for reaction to happen - temperature ~450°C: gives reasonable yield in short time
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