practicals

Question 1

method used to determine the enthalpy of combustion

Answer 1

measure 100cm3 of water in beaker

weigh a spirit burner contiaining liquid to be burnt

measure initial temp of water using thermometer

use spirit burner to heat water

stop heating after a reasonable temp rise, stir and measure final temp of water

reweigh spirit burner

calculate temp change and heat energy change using q = mcΔt

calculate the mass of fuel burnt by subtraction and find number of moles of fuel using mass/Mr

calculate enthalpy change per mole of fuel used

Question 2

why is value for enthalpy of combustion less exothermic than theoretical values?

Answer 2

heat loss to surroundings from spirit burner and calorimeter

loss of fuel from burner or water by evaporation

some heat used to raise temp of calorimeter

reaction unlikely to occur under standard conditions

Question 3

ways to improve calorimetry experiment

Answer 3

using draught shield to reduce heat loss

add lid on calorimeter

minimise distance between flame and calorimeter

insulating calorimeter/spirit burner

burn in supply of pure oxgen to minimise incomplete combustion

Question 4

method to determine enthalpy change of neutralisation

Answer 4

place polysstyrene cup in glass beaker

rinse measuring cylinder with 1 moldm-3 acid then measure 25cm3 and transfer to polystyrene cup

stir acid with themometer and record temp

rinse second measuring cylinder with 1 moldm-3 base and measure 25cm3

add base to acid and record highest temo reached

calculate temp change and heat energy change using q=mcΔt

calculate the amount of acid used, the moles of water formed and the enthalpy of neutralisation

Question 5

test for Mg2+

Answer 5

dissolve compound in water to form aqueous solution

add a few drop of NaOH (aq) at first then excess NaOH

positive result = white ppt forms

Question 6

test for hydroxide ions

Answer 6

warm with solid ammonium salt and test gas formed with red litmus paper or glass rod dipped in conc HCl(aq)

pungent ammonia gas released which changes red litmus paper to blue (as ammonia gas is alkaline) or produce white fumes with conc HCl

reaction : OH^- + NH₄⁺ → NH₃ + H₂O

Question 7

test for sulfate ions

Answer 7

reagent = acidified barium chloride solution

white ppt forms

Question 8

test for carbonate ions

Answer 8

add dilute HNO₃

test gas with limewater

effervesence and turns colourless limewater cloudy

Question 9

why is reflux often necessary involvign reactions with organic compound

Answer 9

organic compounds contain strong covalent bonds so organic reactions are slow

reflux increases the rate of reaction

Question 10

why are reactants added slowly to the reaction flask with cooling before reflux ?

Answer 10

the reaction is often exothermic so this prevents temp of reaction mixture from rising too rapidly and dangerously splashing and forming unwanted side-products

Question 11

purpose of condensor jacket in reflux apparatus

Answer 11

prevents vapour from escaping so the reactants can be boiled for long time without any loss of vapour

Question 12

why are anti-bumping granules used in simple distillation

Answer 12

ensure smooth boiling because liquid can splash over into the condesnor causing impure product or can cause an explosion

Question 13

what makes tertiary alcohols resistant to oxidation with acidified potassium dichromate

Answer 13

lack of H atom bonded to the C with the OH group attached

Question 14

test for esters

Answer 14

warm with ethanol and add a few drops of conc sulphuric acid and smell mixture

sweet smell if ester present

Question 15

test for carboxylic acids

Answer 15

sodium carbonate or Mg to test form phenol as not acidic enough to react with carbonates

effervescence and turns colourless limewater cloudy

effervescence - pop is heard when lighted splin is applied to gas produced (Hydrogen gas formed with Mg )

Question 16

purification of organism crude product

Answer 16

contamination occurs due to byproducts or unreacted reactant

solvent extraction used to purify product:

stopper and shake organic liquid with an aqueous solution (usually sodium hydrogen carbonate)

impurities more soluble in aqueous solution than in organic liquid so move into the aq sol

organic liquid and aq sol are immiscible so form separate layers

remove stopper and open tap to run off bottom layer into beaker

close tap partially when bottom layer nearly drained to slow down flow

open tap again and run off second layer into second beaker

discard aqueous solution

repeat process using a new sample of aqueous sol