Edexcel A Level Chemistry (Paper 3)

what can heat help a solid do?

dissolve

why should the volumetric flask be shaken when making a standard solution?

to ensure a uniform concentration

why is a volumetric pipette more accurate than a measuring cylinder?

smaller uncertainty

if the burette isn't rinsed with the substance that will be put in it, residual water might have what effect?

dilute the concentration leading to a larger titre

what is the problem with leaving the funnel in the burette?

small drops of liquid may fall from the funnel leading to a lower titre volume

why is it okay to rinse with distilled water in a titration?

it doesn't add any extra mols of reagents

what can you do to decrease apparatus uncertainties?

decrease sensitivity uncertainty by using a greater resolution or increase the size of the measurement

how would you reduce % uncertainty in a burette?

make the titre a larger volume by increasing conc and vol of the substance in the conical flask or decreasing the conc of the substance in the burette

how would you reduce % uncertainty in measuring mass?

use a balance that measures to more decimal places

is water a poor or good nucleophile?

poor

how does water react with halogenoalkanes

slowly in a substitution reaction

why does water go in the liebig condenser from the bottom?

to go against gravity allowing more efficient cooling and preventing the backflow of water

why should electric heaters be used to heat organic substances?

highly flammable

why should a reflux condenser never be sealed?

build up of gas pressure could cause explosion

what are anti-bumping granules?

prevent vigorous uneven boiling by making small bubbles form instead of larger bubbles

general method for purifying an organic liquid

put distillate of impure product into a separating funnel, was by adding saturated NaCl, allow layers to separate then run off the aq layer, run the organic layer into a dry conical flask and add drying agent (eg anhydrous calcium chloride), carefully decant the liquid into the distillation flask, distill to collect pure product

detailed method for preparing and purifying an ester

mix primary alcohol and carb acid, add conc sulphuric acid drop by drop (to prevent uncontrolled boiling) and cool in a water bath, when all acid has been added fit a reflux condenser to the flask and gently heat the mixture over an electric heating mantle, cool mixture and re-arrange apparatus for distillation, shake distillate in separating funnel with NaCO3 soln releasing the pressure at intervals, the lower aq layer is discarded, the crude ester is shaken in a separating funnel with CaCl soln removing any unreacted alcohol then discard the lower layer, the ester is run into a clean dry flask with anhydrous cacl, the ester is filtered into a clean dry flask with some anti-bumping granules and distilled

describe the steps in purifying an organic solid

dissolve the impure compound in a minimum vol of hot solvent, hot filter solution to remove any insoluble impurities, cool the filtered solution by inserting the beaker in ice, use buchner filtration to separate out crystals, wash the crystals with distilled water to remove soluble impurities, dry crystals between filter paper

what in an advantage of steam distillation?

the product distills at a lower temperature which can prevent decomposition

why should wire not be used in the salt bridge?

it will set up its own electrode system with the ions

components of the SHE

0 volts E, Hydrogen gas at 100kpa, H+ ions at 1moldm3, 298k, pt black catalyst (porous an can absorbs hydrogen gas)

Ecell

Ecell = Erhs-Elhs

why a re high resistance voltmeters used?

to stop the current from flowing

disadvantages of fuel cells

expensive, transporting pressurised liquid, limited lifetime, use of toxic chemicals in production

how can hydrogen be stored in fuel cells?

liquid under pressure, adsorbed onto a solid surface, absorbed within a solid material

transition metal colour arises from...

electronic transitions from ground to excited states between d-orbitals a quanta of visible light is adsorbed to promote electrons to higher energy levels, the light is absorbed and the opposite colour is transmitted

vo+

+5, yellow

vo2+

+4, blue

v3+

+3, green

v2+

+2, violet

test for carbon dioxide/ CO3^2-/HCO3-

dilute acid gives effervescence, limewater colourless to cloudy

sulphates test

BaCl2 gives a white ppt

test for halide ions

AgNO3 and nitric acid

AgCl + dilute NH3

complex ion

Silver Iodide + Ammonia Solution

no rection

AgBr + conc NH3

complex ion

ammonium test

react with warm NaOH to get NH3

uncertainties

burette is 0.05, pipette is 0.1, balance is 0.001