core practical's Flashcards
give examples of irritants in a experiment and how you can reduce their risk
weak acids ands alkali
wear goggles
give examples of irritants in a experiment and how you can reduce their risk
strong acids and alkalis
wear goggles
how can you reduce risk around flammable substances
keep away from naked flame/use water bath or heating mantle
how can you reduce risk around toxic substances
wear gloves, avoid skin contact, wash hands after use
how can you reduce risk when using highly oxidizing substances
keep away from flammable/easily oxidized materials
what are potential errors when using a gas syringe
gas escaping before bung is inserted
gas may be soluble so a true reading will not be given
dependent on temp and pressure so these must be kept constant
when is heating in a crucible used
measuring mass lost in thermal composition
measuring mass gain when reacting magnesium in oxygen
why should the lid on the crucible be left on
improves accuracy as it prevents loss of solid from the crucible but should be loose to allow gas to escape
describe the process of making a solution
weigh the mass of solid in the test tube then transfer to a beaker
weigh the empty test tube
calculate the mass of solid
add 100cm^3 of distilled water and use glass rod to help dissolve
the solution may need to be heated gently to help it dissolve
pour solution into 250cm^3 graduated flask via a funnel
rinse the beaker and funnel and add washings to the flask
make up to the mark with distilled water
invert flask several times to ensure uniform solution
where should the graduated flask be filled to
the bottom of the meniscus should be on the line
why can graduated flasks not be heated
the heat would cause the flak to expand and then the volume would be inaccurate
what is the chemical equation for the reaction between ethanoic acid and calcium carbonate
2CH3COOH + CaCO3 -> (CH3COOH)2Ca + H2O + CO2
why is it more accurate to find the mass of a solid by weighing the weight the mass of solid in the test tube then transfer to a beaker then weighing the empty test tube after
as some of the solid may be lost when it is transferred to the test tube therefor it is important to re-weigh to ensure accurate results are obtained
how could you reduce gas lost from a reaction involving a gas syringe
having one person add the calcium carbonate and the other ready to put the bung in the test tube reduces the time for the gas to escape
how can you ensure an accurate mass is obtained on a balance
balance should be 2-3dp
weigh in a test tube
transfer to reaction vessel
re-weigh empty test tube
subtract the mass of the empty test tube from the first mass reading
why is a volumetric pipette better than a measuring cylinder
it has a smaller uncertainty
why should a pipette be rinsed with sodium hydroxide solution after washing with water (core prac 2)
to ensure there are no impurities left in the pipette that may disrupt the reaction
core prac 2 titration of sulfamic acid (in burette) and sulfuric acid (in flask) why is there no need for the conical flask to be dried in between titrations
the sulfamic acid will neutralize the sodium hydroxide therefor any left over residue will be neutral and have no effect on the reaction
what is the colour change of phenylalanine
colourless in acid <-> pink in alkali
what is the colour change of methyl orange
red in acid <-> yellow in alkali
what is the general method for carrying out a titration
- rinse equipment
- pipette 25cm^3 into conical flask
- fill burette with acid and ensure jet space is filled
- add a few drops of indicator to conical flask
- place a white tile under the conical flask to ensure colour change can be seen
- add acid slowly whilst swirling the misture going drop by drop near the end point
- note the burette reading at the start and end of the titration
- repeat titration until you get 2 concordant results
what does titrated solution A with solution B
A should be in the conical flask
B should be in the burette
why is a conical flask used over a beaker for titrations
easier to swirl the mixture without spilling the solution
describe how a pipette should be used
rinse with substance to go in it
fill till the bottom of the meniscus is on the graduation line
touch the surface of the solution with the pipette to ensure correct amount is added
why is it important to rinse equipment with the substance being added before carrying out a titration
any residual water may react with previous substances and and lower their concentration so a larger titer will be recorded
why should the funnel not be left in the top of the burette
small drops of liquid may fall from the funnel during the titration giving a lower titer volume
why must the jet space be filled before commencing a titration
remove air bubbles to ensure accurate titer calculating as if it is not filled the titer volume would be larger than expected
how should titer results be recorded
always to 2dp ending in 0.00 or 0.05
why should indicators be added in very small volumes
in high volumes they can effect the titration results as they are often weak acids/alkalis
what does 2 concorant results mean for titrations
2 readings within 0.1 of each other
why is it important to do repeat titrations
allows anomalous results to be spotted and discounted
what is the equation for the titration of acetic acid with sodium hydroxide
CH3CO2H + NaOH -> CH3CO2
-Na+ + H2O
what is the equation for the titration of sulfuric acid with sodium hydroxide
H2SO4 + 2NaOH -> Na2SO4 +2H2O
what is the equation for the titration of hydrochloric acid and sodium hydroxide
HCl + NaOH -> NaCl +H2O
what is the equation for the titration of sodium bicarbonate with hydrochloric acid
NaHCO3 + HCl -> NaCl + CO2 + H2O
what is the equation for the titration of sodium carbonate with hydrochloric acid
Na2CO3 + 2HCl -> 2NaCl + CO2 + H2O
how can you reduce percentage uncertainty in titrations
replacing measuring cylinder with pipettes or burettes which have lower apparatus uncertainty
increase the volume of the substance in the conical flak or lower the concentration of the substance in the burette
how can you reduce uncertainty when weighing substances
use a more accurate balance
weigh a larger mass
weighing sample in test tube before and after then calculating the difference
why does the pink colour of the phenylalanine fade after the end point for the titration of hydrochloric acid with sodium hydroxide
excess sodium hydroxide reacts with carbon dioxide in the air forming sodium carbonate and water
whats the equation for the reaction of sodium hydroxide with carbon dioxide
NaOH + CO2 -> Na2CO3 + H2O
why is it better to have a titer around 25cm^3 rather than 10cm^3
the larger the volume the smaller the percentage error
what type of a reaction is hydrolysis of halogenoalkanes
nucleophilic substitution
how do you measure the rate of hydrolysis in halogenoalkanes (core prac 4)
in separate test tubes place the different halogenoalkanes and add ethanol
place the halogenoalkane-ethanol solutions and the silver nitrate solution in a water bath so that is it all the same temperature then add the silver nitrate to the halogenoalkane-ethanol solution and measure the time taken for the precipitate to form
what trend is shown for the reactivity of halogenoalkanes (hydrolysis reaction core prac 4)
as you go down group 7 halogenoalkanes the time taken for a precipitate to form increases so rate of reaction decreases
as branching increases in time taken for the precipitate to form decreases so rate of reaction increases
what precipitate forms inthe hydrolysis of 1-iodobutane with silver nitrate solution
silver iodide
why is ethanol used for the hydrolysis of halogenoalkanes
to ensure the halogenoalkanes dissolve so that they can react with the water molecules as they are insoluble in water
why is water able to act as a nucleophile
it has a lone pairs of electrons from the oxygen
why is water used as a nucleophile for the hydrolysis of halogenoalkanes with silver nitrate solution instead of hydroxide ions
because hydroxide ions would form silver hydroxide instantly
what must be added if hydrolysis of halogenoalkanes is done with NaOH and not water (before silver nitrate is added)
HNO3 must be added to neutralize NaOH
what is the formula form silver nitrate
AgNO3
what are the reagents and conditions for the partial oxidation of primary alcohols and what will be observed
potassium dichromate(VI) and dilute sulfuric acid
warm gently and distill out the aldehyde as it forms
orange -> green
what is distillation
a separation technique used to separate an organic product from its reacting mixture, need to collect the distillate of the approximate boiling point range of the desired liquid
what is the equation for the oxidation of ethanol to ethanal
CH3CH2OH + [O] -> CH3CHO + H2O
how do you heat under reflux
substance is boiled in round-bottomed flask fitted with a vertical condenser and as it evaporates it is cooled by the water in the liebig condenser and so condenses back to a liquid and drops back into the flask to be heated again
why is heating under reflux used
allows heating for a long period of time
prevents the flask from boiling dry
prevents volatile reactants/products escaping
ensures even heating
why are anti-bumping granules used when heating under reflux
to allow smooth boiling and prevent bubbles forming and splashing hot liquid up the sides of the flask
what is the equation for the oxidation of ethanol to ethanoic acid
CH3CH2OH + 2[O] -> CH3COOH + H2O
summarise the experiment for the chlorination of 2-methylpropan-2-ol
add conc HCl
put rubber bung on and swirl
open bung to release pressure
add anhydrous CaCl2 and shake
distill the two layers in a separating funnel
top layer - organic product
lower layer - aqueous layer
2-methylpropan-2-ol has a bp of 82 and is soluble in water whereas 2-chloro-2-methylpropane has a bp of 51 and is insoluble in water
explain these differences
2-methylpropan-2-ol can form hydrogen bonds therefor is soluble
it has stronger intermolecular hydrogen bonds which require more energy to overcome
2-chloro-2-methylpropane cannot form hydrogen bonds therefor is not soluble
only forms intermolecular london forces and dipole-dipole interactions which require less energy to overcome
