PAGS - OCR Chemistry

PAG 1-Heating in a crucible - for oxidation and decomposition of crystals

Weigh empty crucible and lid, add mg ribbon , heat with bunsen burner, lift lid with tongs to allow oxygen to enter, cool , weigh crucible and contents again , reheat crucible and reweigh until you reach constant mass

why heat to constant mass

to ensure reaction is complete

why should small amounts of solid not be used

percentage uncertainties would be too high

why should too large amounts of solid not be used

decomposition is likely to be incomplete

PAG 1-Measuring gas volumes diagrams

ensure no gaps in diagram , gas syringe with measurement markings - could have an upturned measuring cylinder with trough of water instead

potential errors using a gas syringe

gas escaping before bung is inserted , co2 and so2 is insoluble in h20 so true gas amount is unknown

PAG 2- Making a standard solution

weigh sample bottle containing required mass of solid on a 2dp balance , transfer to beaker , reweigh empty sample bottle, record difference in mass , add 100cm3 of distilled H20 to beaker and stir with glass rod , pour solution into the 250cm3 graduated flask via a funnel , rinse beaker and funnel and add washings from beaker and glass rod to vol flask , make up mark with distilled H20 using a dropping pipette for the last few drops , invert flask several times to ensure uniform solution.

why should you not heat or add hot solutions to flask

the flask will expand therefore the volumes would not be accurate

PAG 2- Diluting a solution

pipette 25cm3 of original solution into 250cm3 volumetric flask , make up to mark with distilled water using a dropper pipette , invert flask several times to ensure uniform solution

why is using a volumetric pipette better than using a measuring cylinder

It has a smaller uncertainty

PAG 2 - acid-base titrations

rinse burette with alkali then fill burette with alkali , touch surface of solution with pipette to ensure correct amount of solution is added , add to acid solution in conical flask with indicator added , keep swirling conical flask until a colour change is reached , then stop the titration , repeat until concordant results are found

phenolphthalein

colorless in acid, pink in base

methyl orange

Red in acid, yellow in alkali

concordant results

within 0.10cm3 of each other

manganate redox titration

grind up tablets with little H2SO4 1M using a pestle and mortar , use a funnel to transfer paste to 100cm3 volumetric flask , add sufficient H2S04 to make up solution to 100cm3 , use stopper flask and shake to ensure contents are throughly mixed , titrate 10cm3 portions of solution with 0.005 potassium manganate , end point purple colour

manganate redox titration equation

MnO4- + 8H+ + 5Fe2+ ----> Mn2+ + 4H20 + 5Fe3+

MnO4-

purple

Mn2+

colourless

uncertainty- balance

+/- 0.001g

uncertainty-burette

+/- 0.1 cm3

uncertainty-volumetric flask

+/- 0.1cm3

uncertainty-25cm3 pipette

+/- 0.1 cm3

how to work out percentage uncertainty

uncertainty/measurement on apparatus x 100

how to work out percentage uncertainty of burette

0.10/average titre result x 100

how to reduce uncertainty in a burette

use a larger titre by increasing the volume and concentration of substance in conical flask or decreasing concentration of substance in burette

how to decrease apparatus uncertainty

using apparatus with finer scale divisions or increase size of measurements made

PAG 3-Measurement of an enthalpy change

used to measure enthalpy change of combustion

calorimetry

wash equipment, polystyrene cup , measure out desired volumes of solutions or powder , thermometer , measure initial temperature, add the substance and stir to even out the temperature , then record temperature change every 2-3 minutes

errors in calorimetry method

energy loss to surroundings , approximation of specific heat capacity , incomplete reaction , neglecting specific heat capacity of water

calorimetry graph

difference between both lines is enthalpy change

how to work out the enthalpy change

q=mc∆T then ∆H=q/n make sure q is in joules then answer in kj

what sign must go before enthalpy change of combustion

negative

flame calorimetry

measure mass of spirit burner before and after heating , volume of water and temperature change

errors in flame calorimetry

heat loss to surroundings , incomplete combustion , non standard conditions

PAG 4-Testing for cations group 2

flame tests , adding NaOH

flame tests

lithium

red

sodium

yellow orange

potassium

lilac

calcium

brick red

barium

apple green

copper

blue-green

Testing for NH4+

add 0.1 moldem-3 of ammonium chloride then add sodium hydroxide and shake the mixture , warm the mixture slightly with a water bath , test fumes using damp red litmus paper if present should turn blue

testing for anions

group 7

testing for sulfate ions

add BaCl2 with acidified HCL , if sulfate ions present a white precipitate of barium sulfate will form

testing for halide ions

add silver nitrate solution

testing for carbonates

Add dilute acid to a solid carbonate and look for any effervescence and bubble through limewater if it goes cloudy , positive result

insoluble salts

- silver and lead chloride , bromides and iodides
- strontium , barium, and lead sulfate
- all carbonates and hydroxides except sodium, potassium, and ammonium

filteration

A process that separates materials based on the size of their particles.

vacuum filteration

air outlet - water pump
Buchner flask - thicker glass walls to cope with vacuum
buchner funnel

When is vacuum filtration used?

if large amounts of solid are used

how is the vacuum produced

apparatus is connected to a water pump

PAG 5- Preparation of a pure organic liquid

distillation , using a separating funnel , boiling point determination e.g purifying a haloalkane from an alcohol or cyclohexene from hexanol

distillation

used to separate an organic product from its reaction mixture

why does the water go in the bottom of the condenser in distillation

goes against gravity causing efficient cooling and prevents backflow of water

using a separating funnel

put impure product into a separating funnel , wash product using sodium hydrocarbonate or sodium chloride solution , allow layers to seperate in the funnel , run the organic layer into a clean conical flask and add a drying agent to and dry the organic liquid carefully decant the liquid into the distillation flask , distill to collect the pure product

drying agents

anhydrous sodium sulfate , calcium chloride

organic layer

layer with the lower density - upper layer

drying agent should be....

insoluble in the organic liquid , not reactive with the organic liquid

decant

pour off the organic liquid gently and leave the drying agent in the conical flask

what does NaHCO3 do

neutralises any remaining reactant acid

what does sodium chloride do

separates the organic layer from the aqueous layer

preparing and purifying an ester

propanol and propanoic acid then add c.H2S04 and keep the contents of the flask cooled and shaken in an ice water bath , reflux , distillation , distillate in a separating funnel , anhydrous calcuim chloride , filter into dry flask with a few anti bumping granules

why is anti bumping granules used

to prevent vigorous uneven boiling / to allow liquid to boil more gently

reflux

heating using a vertical condenser allowing continuous boiling and condensing of a liquid

PAG 6- Recrystallisation

dissolve impure compound in minimum volume of hot solvent , filter the solution to remove any insoluble impurities and retain filterate. Allow the filterate to cool and crystallise. Wash the crystals with cold ethanol or cold water and then dry crystals in the open or using an oven

loss of yield during recrystallisation

loss of crystals during filteration and washing , some product stays in solution after recrystallisation and side reactions occurring

why should you never seal end of the condenser during reflux

build up gas pressure leading to apparatus exploding

Melting point determination

If there is impurities how would the melting point differ

the melting point would be lowered and sample would melt over a range of temperatures

thin layer chromatography

a separation technique that involves the separation of small molecules as they move through a silica gel

Process of TLC

pencil line 1cm above bottom of TLC plate , add solvent and ensure a lid is used , when solvent is about 1cm from the top of the plate mark solvent front with a pencil, allow plate to dry using a fume cupboard , use a locating agent

locating agent

ninhydrin - amino acids
UV or iodine crystals
used to make spots on chromatogram more visible

why is a lid used in TLC

to prevent evaporation of solvent

Rf

distance moved by spot/distance moved by solvent

PAG 7- tests for alkenes

bromine water - orange to colourless

Test for aldehydes

Tollens reagent: silver mirror , Fehlings test - brick red precipitate

Test for carboxylic acids

carbonate ions - CO2 formed/effervescence

tests for alcohols

acidified dichromate orange to green
primary alcohols - aldehyde distillation , carboxylic acid reflux
secondary alcohol - ketone

test for haloalkanes

warm with aqueous silver nitrate in ethanol - white precipitate

test for phenols

iron (111) chloride test
purple

phenols and carbonates

don't react carbonates ions fizz with sodium but no reaction with carbonates

PAG 9- measuring rate of reaction by a continuous monitoring method

measuring change in volumes of gas , measuring changes in concentration via a continuous rate method

measuring change in gas volumes

use gas syringe to see volume of gas collected

initial rate

instantaneous rate at t=0 of a concentration time graph , fastest rate

why is the measure of initial rate preferable

we know the concentrations at the start of the reaction

excess of a reactant doesn't...

affect rate , zero order

half life of a first order reaction

independent of the initial concentration

PAG 10- measuring the rate of reaction by an initial rate concentration

gradient of a continuous monitoring method or using a clock reaction

clock reactions

Measures the time from the start of reaction until there is a fixed concentration

how is the end point of clock reactions determined

when one limiting reactant runs out leading to a sudden colour change

Example of clock reaction

hydrogen perioxide , sodium thiosulfate iodine titration

how to determine order of reaction of clock reactions

repeating experiment several times and varying concentration of a reactant several times by keeping concentrations of other reactants the same

how can initial rate of reaction be represented

1/t

describe the clock reaction method

put reactants into separate test burettes , measure out required volumes of potassium iodide , starch , and sodium thiosulfate and water a conical flask , measure hydrogen peroxide into a test tube and pour into the conical flask and immediately start timer , end point is when first blue/black colour appears

working out rate order graphically

log rate=log k + nlog conc

measuring rate of reaction changes with temperature

sodium thiosulfate and hydrochloric acid

method for sodium thiosulfate and hydrochloric acid.

place test tubes each with 2 reactants into a water bath to ensure they are are at the right temperature ,first place thiosulfate into a conical flask on top of a cross then pour in the hcl , start stopwatch and swirl , stop clock when cross disappears , repeat for other temperatures

how to calculate Ea from a graph

lnk = -Ea/RT + lna

how to titrate samples of a reaction mixture with acid , alkali or sodium thiosulfate

take out small samples of the reaction mixture , quenching via dilution or cooling then titrate with a suitable reagent