Practical common questions

Question 1

Increasing distillation yield (2 points) with products that will oxidise further

Answer 1

Keep the temperature of the reaction mixture below the boiling point of propan-1-ol/below 97 °C
Cool the distillate / collecting vessel

Question 2

Irritant

Answer 2

dilute acid and alkalis- wear googles

Question 3

Corrosive

Answer 3

stronger acids and alkalis wear goggles

Question 4

Flammable

Answer 4

keep away from naked flames

Question 5

Toxic

Answer 5

wear gloves- avoid skin contact- wash hands after use

Question 6

Oxidising

Answer 6

Keep away from flammable / easily oxidised materials

Question 7

when heating in a crucible why shouldnt large amounts of solid be used

Answer 7

Large amounts of hydrated calcium sulfate, such as 50g,
should not be used in this experiment as the decomposition is
likely to be incomplete.

Question 8

why does the crucible need to be dry when heating

Answer 8

The crucible needs to be dry otherwise a wet crucible would
give an inaccurate result. It would cause mass loss to be too
large as the water would be lost when heating.

Question 9

when heating in a crucible why shouldnt small amounts of solid be used

Answer 9

Small amounts of the solid , such as 0.100 g, should not be used in this experiment as the percentage uncertainties in weighing will be too high.

Question 10

why use a lid when heating in a crucible

Answer 10

The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape.

Question 11

errors in gas syringe experiments

Answer 11

*gas escapes before bung inserted
*syringe sticks
* some gases like carbon dioxide or sulphur dioxide are
soluble in water so the true amount of gas is not
measured.

Question 12

errors when drawing gas syringe

Answer 12

ALWAYS add markings to show measurement made
make sure everything is connected so gas cant escape

Question 13

reason to invert in volumetric solution making

Answer 13

to ensure a uniform concentration.

Question 14

why not heat volumetric flask

Answer 14

Do not heat or put hot
solutions in the volumetric flask because the heat would cause
the flask to expand and the volume would then be incorrect.

Question 15

why volumetric pipette more accurate than cylinder

Answer 15

Using a volumetric pipette is more accurate than a measuring cylinder because it has a smaller uncertainty.

Question 16

in standard phrase titrate a with b which in which container

Answer 16

However, the standard phrase: titrate solution A
with solution B means that A should be in the
conical flask and B should be in the burette.

Question 17

why use conical flask

Answer 17

A conical flask is used in preference to a beaker because
it is easier to swirl the mixture in a conical flask without
spilling the contents.

Question 18

common error when using burettte

Answer 18

make sure the jet space in the burette is filled with the
solution and air bubbles are removed.
If the jet space in the burette is not filled properly prior to commencing the
titration it will lead to errors if it then fills during the titration, leading to a
larger than expected titre reading.

Question 19

degree of precision on burette

Answer 19

Even though a burette has marking reading to 0.1cm3
, the burette readings
should always be given to 2dp either ending in 0.00 or 0.05. 0.05cm3
is the
volume of 1 drop of solution delivered from a burette and so this is the
smallest difference in readings that can be measured. If the bottom of the
meniscus sits on a line it should end with a 0.00 as in the above example
9.00cm3. If the meniscus sits between two lines it should end 0.05. e.g. if the
bottom of the meniscus sits between the lines marked 9.1 and 9.2, you should
record 9.15

Question 20

phenolphthalein colour change and when used

Answer 20

If acid is added from the burette the colour change would
be pink (alkali) to colourless (acid): end point pink colour
just disappears [use with titrations using strong alkalis e.g.
NaOH ]

Question 21

Methyl orange colour change and when used

Answer 21

Methyl orange is a suitable indicator for neutralisation reactions where strong acids are used.
It is red in acid and yellow in alkali. It is orange at the end point.

Question 22

concordent titration results

Answer 22

within 0.1cm3 of each other

Question 23

titration between permanganate and iron (ii)

Answer 23

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq)—-> Mn2(aq) + 4H2O(l) + 5Fe3+(aq)

Question 24

acid used for permanganate titrations

Answer 24

dilute sulfuric acid needed to supply 8h+ ions - wont set up alternate redox reactions

Question 25

if not enough h+ ions added in permanganate titration what happens

Answer 25

mno2 formed which is brown masking the correct colour change and accurate endpoint wont be reached

Question 26

why not hcl/ hno3 in permanganate titrations

Answer 26

Cl- ions would be oxidised to Cl2 by MnO4- as the Eo MnO4-/Mn2+ > EoCl2/Cl- This would lead to a greater volume of manganate being used and poisonous Cl2 being produced. It cannot be nitric acid as it is an oxidising agent. It oxidises Fe2+ to Fe3+ as Eo NO3-/HNO2 > Eo Fe3+/Fe2+ This would lead to a smaller volume of manganate being used.

Question 27

permanganate with hydrogen peroxide equation titration

Answer 27

Overall 2MnO4 - (aq) + 6H+ (aq) + 5H2O2  5O2 + 2Mn2+ (aq) + 8H2O

Question 28

permanganate with ethanedioate equation titration

Answer 28

Overall 2MnO4 - (aq) + 16H+ (aq) + 5C2O4 2-(aq) 10CO2 (g) + 2Mn2+(aq) + 8H2O(l)

Question 29

why slow rxn between mno4- and c2o42-

Answer 29

The reaction between MnO4-and C2O42- is slow to begin with (as the reaction is between two negative ions). To do as a titration the conical flask can be heated to 60oC to speed up the initial reaction.

Question 30

Readings def

Answer 30

the values found from a single judgement when using a piece of equipment

Question 31

measurement def

Answer 31

the values taken as the difference between the judgements of two values (e.g. using a burette in a titration)

Question 32

% uncertainty

Answer 32

% uncertainty = +_ uncertainty/ Measurement made on apparatus x 100

Question 33

Reducing uncertainties in a titration

Answer 33

Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty To reduce the % uncertainty in a burette reading it is necessary to make the titre a larger volume. This could be done by: increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette.

Question 34

Reducing uncertainties in measuring mass

Answer 34

Using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid. Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added.

Question 35

slow reaction means what in calorimetry

Answer 35

If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.

Question 36

errors in calorimetry experiment (5 of them)

Answer 36

* energy transfer from surroundings (usually loss) * approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water. * neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus. * reaction or dissolving may be incomplete or slow. * Density of solution is taken to be the same as water.

Question 37

calorimetry questions remember

Answer 37

Remember in these questions: sign, unit exo or endo, increase or decrease in temp of surroundings?

Question 38

reason enthalpy of hydration cant be done experimentally

Answer 38

This cannot be done experimentally because it is impossible to add the exact amount of water without the solid dissolving and it is not easy to measure the temperature change of a solid.

Question 39

errors in flame calorimetry

Answer 39

energy losses from calorimeter * Incomplete combustion of fuel * Incomplete transfer of energy * Evaporation of fuel after weighing * Heat capacity of calorimeter not included * Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Question 40

in dissapreaing cross what is rate why works

Answer 40

1/t t= time taken for cross to dissappear (fixed mass of sulfur produced) assume same mass of sulfur each time

Question 41

If titrating to find concentration of something as a reaction is happening what do you need to do?

Answer 41

Stop the reaction / quench By dilution / cooling / adding a reagent to react with H2O2/I-

Question 42

What needs to be drawn in filtering diagram

Answer 42

filter paper

Question 43

solid potassium dichromate danger what to do why

Answer 43

You must wear gloves when handling solid potassium dichromate(Vl) since it is highly toxic and a category 2 carcinogen; it is also an irritant. Avoid inhaling any dust.

Question 44

Where is the thermometer placed in distilation

Answer 44

Bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point

Question 45

Where does water flow in condenser

Answer 45

Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water.

Question 46

IMPORTANT IN REFLUX DIAGRAM and why

Answer 46

Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up

Question 47

anti bumping granules why

Answer 47

Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles

Question 48

Watch out when drawing reflux diagram

Answer 48

It’s important to be able to draw and label this apparatus accurately. * Don’t draw lines between flask and condenser. * Don’t have top of condenser sealed * Condenser must have outer tube for water that is sealed at top and bottom * Condenser must have two openings for water in and out that are open

Question 49

Hydrolysis def

Answer 49

the splitting of a molecule ( in this case a haloalkane) by a reaction with water

Question 50

rate of reaction with hydrolysis of haloalkanes

Answer 50

The precipitate only forms when the halide ion has left the haloalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different haloalkanes. The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane The weaker the bond, the easier it is to break and the faster the reaction.

Question 51

initial rate from conc time graph

Answer 51

The initial rate can be calculated from taking the gradient of a continuous monitoring conc vs time graph at time = zero

Question 52

large excess of reactants rate of reaction

Answer 52

In reactions where there are several reactants, if the concentration of one of the reactant is kept in a large excess then that reactant will appear not to affect rate and will be pseudo-zero order . This is because its concentration stays virtually constant and does not affect rate.

Question 53

ROR Technique measurement of the change in volume of a gas

Answer 53

change in the number of moles of gas in the reaction gas syringe

Question 54

ROR Technique Measurement of change of mass

Answer 54

This works if there is a gas produced which is allowed to escape. Works better with heavy gases such as CO2

Question 55

ROR Technique Titrating samples of reaction mixture with acid, alkali, sodium thiosulphate etc

Answer 55

Small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent

Question 56

ROR Technique Colorimetry.

Answer 56

If one of the reactants or products is coloured then colorimetry can be used to measure the change in colour of the reacting mixtures

Question 57

ROR Technique Measuring change in electrical conductivity

Answer 57

Can be used if there is a change in the number of ions in the reaction mixture

Question 58

ROR Technique Measurement of optical activity.

Answer 58

If there is a change in the optical activity through the reaction this could be followed in a polarimeter

Question 59

salt bridge facts quick

Answer 59

The salt bridge is used to connect up the circuit. The free moving ions conduct the charge. A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually Potassium Nitrate. It can also be a glass U tube containing a salt solution plugged with cotton wool The salt should be unreactive with the electrodes and electrode solutions.. E.g. potassium chloride would not be suitable for copper systems because chloride ions can form complexes with copper ions. A wire is not used because the metal wire would set up its own electrode system with the solutions

Answer 60

Note: in the electrode system containing two solutions it is necessary to use a platinum electrode and both ion solutions must be of a 1M concentration so [Fe2+] = 1M and [Fe3+ ] = 1M .

Question 61

calibration with pH curve

Answer 61

Calibrate meter first by measuring known pH of a buffer solution. This is necessary because pH meters can lose accuracy on storage. Most pH probes are calibrated by putting probe in a set buffer (often pH 4) and pressing a calibration button/setting for that pH. Sometimes this is repeated with a second buffer at a different pH

Question 62

improving accuracy of titration curves

Answer 62

Can also improve accuracy by maintaining constant temperature

Question 63

shape of pH curve at start and reason

Answer 63

At the start the pH rises quickly and then levels off. The flattened part is called the buffer region and is formed because a buffer solution is made