paper 3

Question 1

why is an electric heater used over a bunsen sometimes

Answer 1

volatile reactants that are flammable are not open to a naked flame

Question 2

describe the process of recrystalisation

Answer 2

• dissolve sample in a minimum volume of hot solvent
• this will remove impurities
• allow to cool to recrystalise
• then remove the solvent by filtering under reduced pressure in a buchner flask.
  wash with cold water and dry

Question 3

when heating in a crucible what is the bad thing about using large amounts of calcium sulphate

Answer 3

decomposition will be incomplete

Question 4

why should super small amounts of solid not be used when heating in a crucible

Answer 4

because the percentage uncertainties will be huge

Question 5

what si the point of the lid when heating in a crucible

Answer 5

it prevents loss of solid product - but notably should be loose fitting to allow gas to escape

Question 6

say the crucible when heating a solid product is wet what is the problem with this

Answer 6

water would be lost in the heating process as well as mass - the change in mass would be too great

Question 7

what are potential errors in using a gas syringe

Answer 7

*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 8

how do you make a solution

Answer 8

• Weigh the sample bottle containing the required mass of
  solid on a 2 dp balance
• Transfer to beaker and reweigh sample bottle
• Record the difference in mass
• Add 100cm3 of distilled water to the beaker. Use a glass rod to help stir
  -* Pour solution into a 250cm3 graduated flask via a funnel. * Rinse beaker and funnel and add washings from the
  beaker and glass rod to the volumetric flask. * make up to the mark with distilled water using a dropping
  pipette for last few drops. * Invert flask several times to ensure uniform solution.

Question 9

describe the method for carrying out a titration

Answer 9

*rinse equipment (burette with acid, pipette with alkali, conical flask
with distilled water) *pipette 25 cm3 of alkali into conical flask
*touch surface of alkali with pipette ( to ensure correct amount is
added) *adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point *phenolphthalein [pink (alkali) to colourless (acid): end point pink
colour just disappears] [use if NaOH is used] *methyl orange [yellow (alkali) to red (acid): end point orange]
[use if HCl is used] *use a white tile underneath the flask to help observe the colour
change
*add acid to alkali whilst swirling the mixture and add acid dropwise at
end point *note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two
readings within 0.1 of each other

Question 10

what is a good compound to neutralise acid

Answer 10

calcium carbonate

Question 11

in calorimetry why is it bad if the reaction is slow

Answer 11

If the reaction is slow then the exact temperature rise can be
difficult to obtain as cooling occurs simultaneously with the
reaction

Question 12

what errors can occur in calorimetry

Answer 12

• heat 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 heat
  absorbed by the apparatus. * reaction or dissolving may be incomplete or slow. * Density of solution is taken to be the same as water

Question 13

what can be done to improve the readings taken in calorimetry

Answer 13

take readings at regular intervals

Question 14

what can be done to increase the accuracy of temperature taken in calorimtry

Answer 14

• heat 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 heat
  absorbed by the apparatus. * reaction or dissolving may be incomplete or slow. * Density of solution is taken to be the sa

Question 15

why in rate equation reactions can large excess be used

Answer 15

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 16

what are the steps in an iodine clock reaction

Answer 16

• sodium thiosulfate, potassium iodide, starch and water are measured by burette and added to the beaker. sulfuric acid is in excess therefore can be measured by measuring cylinder.
• meaure 1cm-3 of 0.25moldm-3 hydrogen peroxide is added. this will start the reaction and therefore start the timer immediately when it is added

stop the timer when the solution turns blue black

Question 17

what are the ionic equations in the iodine clock reaction

Answer 17

H2O2 (aq) + 2I- (aq) + 2H+(aq) → I2 (aq) + 2H2O (l)

2S2O32- (aq) + I2 (aq) → 2I- (aq) + S4O62- (aq)

Question 18

in magnesium ribbon rate of reaction why is a gas syringe used

Answer 18

hydrogen has a very low density do measuring mass change in the reaction flask is difficult and inaccurate.

Question 19

what are some things to note to do in mg ribbon reaction

Answer 19

Make sure the plunger is fully inserted before you start the experiment otherwise you will have a volume error

If the magnesium does not look new and shiny, you may need to clean the surface with a bit of sandpaper

Make sure the plunger is secure and does not fall out of the barrel if the volume exceeds 100 cm3

Question 20

what apparatus do you need for measuring the emf of a cell

Answer 20

Two small beakers, around 75 cm3 capacity

Strips of suitable metals such as copper, zinc, iron and silver

1.0 mol dm-3 solutions of the metal ions (nitrates, chlorides or sulfates depending on their solubility)

A high resistance voltmeter (usually a digital multimeter has this)

Two sets of wires with crocodile clips

A salt bridge consisting of a strip of filter paper soaked in saturated potassium nitrate

Question 21

how do you measure emf

Answer 21

The strips of metals need to be freshly cleaned to remove any oxide coatings

This can be done with a piece of sandpaper

To support the metals, it is easiest to have long strips that can be folded over the side of the beaker and held in place with the crocodile clips

Fill up the beakers to about two thirds of the way with the metal ion solutions

Using tongs, dip a strip of filter paper into a beaker of saturated potassium nitrate solution and then place it between the two beakers making sure the ends of the strip are well immersed in the solutions

Connect the crocodile clips to the voltmeter, wait for a steady reading and record the measurement

Question 22

why must the salt bridge be changed between each reading

Answer 22

to prevent cross contamination of ions between half cells