paper 7 notes - user guide - chemistry final

7/29/2019 Paper 7 Notes - User Guide - Chemistry Final

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Paper 7 NotesChemistry

Health and Safety

Eye protection Laboratory coats Plastic gloves Substances spilled on skin Rinsed off immediately Chemicals never tasted Gases and vapours Never be smelt unless question instructs

Reducing risk (Likelihood of harm)

Smaller scale Lower concentration Careful planning + risk assessment Fume cupboard + eye goggles Temp kept low to reduce rate of reaction Alternative methods used which are less hazardous (Hazard Situation which is likely

to cause harm)

Accuracy

Titres average To two dps Two titres should be within 0.20cm3 Burette readings To the nearest 0.05cm3 Repeating the same experiment with the same apparatus wont increase accuracy

Same systematic errors could occur

To increase accuracy of titration:o Use white tile beneath conical flasko Ensure there are no air bubbleso Read burette at eye levelo Swirl flasko Ensure burette is vertical

Errors

Error in the reading = (Uncertainty in the equipment / The reading) * 100% Improper use of burette or pipette Inaccurate result in titration

Graphs

Dependent (Quantity being measured) Y-Axis Independent (Pre-determined quantity) X-Axis


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Laboratory skills

Burettes, pipette and measuring cylinders Should be rinsed with the solution they areto contain

Conical flask (Used in titration) + Volumetric flask (Used to prepare solution)Should be rinsed with distilled water

Burette should be read at eye level Thermometers + stirring rods removed from solution Drops could be lost

Flame tests

Dip a nichrome wire in concentrated hydrochloric acid, dip this in the solid, and holdwire in the hottest part of the flame, observe colour

Sodium Yellow Potassium Lilac Calcium Red-yellow Lithium + strontium Red Barium Green Magnesium No colour

Heating compounds

Carbonates of metals (except group 1) CO2 Group 1 nitrates (Except Lithium) O2 ONLY Nitrates (Other than Na or K) O2 and NO2 Hydrated saltsWater

Common gases

Oxygen Colourless gas, relights glowing splint Carbon dioxide Colourless gas, gives a white precipitate (turns cloudy) with

limewater (Calcium hydroxide solution)

Ammonia Colourless gas, turns moist red litmus paper blue, forms white smoke withHydrogen Chloride

HydrogenColourless gas, ignites with a pop Nitrogen dioxide Brown gas (BROWNITROGENDIOXIDE) Hydrogen chloride Steamy fumes on exposure to moist air, acidic, forms white

smoke with ammonia

Chlorine Pale green gas, bleaches moist litmus paper Bromine Brown gas Iodine Purple vapour Water vapour Turns blue cobalt chloride paper pink (Blue Pink, BP!!) Sulfur dioxide Turns filter paper soaked in acidified aqueous potassium dichromate

from orange to green Redox reaction

o Sulfur = yellow solid


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Dilute acids

Dilute Sulfuric acid (H2SO4) or Hydrochloric acid (HCl) added to a substance:

Carbonate CO2 evolved Metal H

2evolved

Chromate (VI) to dichromate (VI) Yellow solution turns orange Thiosulfate SO2 evolved and pale yellow precipitate formed

Reducing agents

Usually:

Decolourise aqueous acidified potassium manganite (VIII) May turns aqueous, acidified potassium dichromate (VI) from Orange to Green

Oxidising agents

Liberate iodine as a BROWN solution or BLACK solid from aqueuous potassiumiodide Iodine solution gives a blue-black coloration with starch

E.g: Aqueous chlorine and aqueous bromineDilute sodium hydroxide (NaOH)

Magnesium, calcium, strontium, bariumWhite ppte, insoluble in excess NaOH Sodium, potassium No ppte Ammonium No ppte, ammonia evolved on warming

Dilute ammonia solution (NH3)

MagnesiumWhite ppte, insoluble in excessAqueous barium chloride

SulfateWhite ppte (BaSO4) Insoluble in dilute HCl SulfiteWhite ppte (BaSO3) Dissolves in dilute HCl CarbonateWhite ppte (BaCO3) Dissolves with effervescence

If HCl added before aqueous BaCl, only sulfate will form as a ppte

Test for halides

Carbonates removed and solution neutralized by adding dilute nitric acid before theaqueous silver nitrate

Add aqueous silver nitrate ChlorideWhite ppte which darkens in sunlight (AgCl) Dissolves in dilute

aqueous NH3 Bromide Cream ppte which darkens in sunlight (AgBr) Dissolves in concentrated

aqueous NH3 IodideYellow ppte which doesnt darken in sunlight (AgI)Doesnt dissolve


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Concentrated sulfuric acid Burns skin, corrosive, very exothermic

Added to solid halide Potentially hazardous reaction, use fume cupboard, small scale:

Halide Observation on addingconcentrated H2SO4

Observed reaction products

Chloride Steamy fumes, vigorousreaction

HCl

Bromide Steamy fumes, vigorousreaction, brown vapour

HBr, Br2, SO2

Iodide Steamy fumes, vigorousreaction, black solid, purplevapour

HI, I2, S, H2S

Chemical test

Test Observation InferenceWarm with acidifiedpotassium dichromate

(VI)

Orange to Green Primary, secondary alcohol,aldehyde

Shake with brominewater

Yellow solution is decolourised Alkene

PCl5 (Phosphorous(V) Chloride)

Steamy fumes of HCl that turndamn blue litmus paper red

OH group in alcohols and carboxylicacids

Cl replacesOH

Boil with Fehlingsor Benedicts

solution

Blue solution gives red ppte Aldehyde

Add sodium or

potassium carbonate

Effervescence Carboxylic acid

Add a small piece ofsodium

EffervescenceSodium dissolvedWhite solid formedTemperature gets hot

Alcohol, carboxylic acid

Chlorine Yellowy/greeny gas Bromine Red/brown liquid Iodine Black/gray solid


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Gas syringe

Distillation:

-> Collection vessel shouldnt be sealed

-> Water should go in against direction of gravity


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Reflux:

Separating funnel

Aqueous layer is always denser so it goes to the bottom.

To separate:

Run off lower layer to waste

Run off upper layer (the desired product) to flask


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Acids

Hydrochloric acid HCl Sulfuric acid H2SO4 Nitric acid HNO3

Alkane CnH2n+2

Alkene CnH2n

Alcohol CnH2n+2O

Halogenoalkane CnH2n-1X

Aldehyde CnH2nO

Ketone CnH2nO

Carboxylic Acid CnH2nO2

Random notes collected from mark schemes

Drying agent Calcium chloride

Filter paperMight absorb some of the product

Indicator of iodine and sodium thiosulfate Starch, which changes from blue-black to

colourless

Sample washed with carbonate Neutralises acid

Reflux primary alcohol Carboxylic acid

Distill primary alcohol Aldehyde

Purify sample by distillation

Group 1 and 2 oxides and hydroxides are white solids

Solution should be continuously stirred so that the heat is evenly distributed

Putting lid on cup gives more accurate results

Alkene + Bromine waterDouble bond broken, Br added to each C (Two Brs added)

Decant Pour off liquid, leaving solid behind

Preparing saturated solution:

1. Add excess of solid to water until no more dissolves2. Head it3. Leave to cool4. Decant

Standard enthalpy change in kJ mol-1

, we assume density of solution is 1g cm-1

Always recheck numbers youre using, and recheck calculations!

Limewater Calcium hydroxide Ca(OH)2 in water


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For quicker reaction Secondary halogenoalkane

No water added Lower yield, dehydration to alkene

Yield is not 100% because:

Reaction didnt go to completion Side reactions Solution lost

o In aqueous layerso In apparatus

paper 7 notes - user guide - chemistry final

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