P3 Markschemes

Question 1

Why does carrying out a flame test on a mixture of potassium and strontium chloride not clearly show that two different metal ions are present

Answer 1

The red colour of the strontium ion will mask the lighter lilac colour of the potassium ion

Question 2

Why is the wire used in flame test made of in chrome and not iron

Answer 2

-nichrome produces no colour whereas iron can produce a colour
-nichrome is inert but iron reacts with oxygen and HCl

Question 3

Why is the nichrome wire in a flame test dipped in acid and heated before u add acid again and dip into the metal salt

Answer 3

To remove any residue of the previous sample being tested which cleans the wire

Question 4

Why is fresh HCl used when re-applying HCl to the newly cleaned nichrome wire

Answer 4

Because the previous acid can become contaminated with residue from previous tests

Question 5

Explain the formation of colour in a flame test made

Answer 5

Electrons absorb heat energy which promotes them to higher energy levels
Electrons then fall from this excited energy state to ground state, releasing energy as visible light as they do so

Question 6

Why can’t u just remove the excess water added to a volumetric flask above the fill line with a pipette

Answer 6

Removal of excess solution will remove some of the dissolved compound so the exact concentration of it will be unknown

Question 7

How to answer a question like “assess the suitability of this indicator for this titration” when given the titration curve

Answer 7

-state the pH range that the indicator changes colour in
-state the pH range of the vertical region of the titration curve
-state whether the pH range of the indicator lies completely within the vertical range of the titration curve (and thus if it’s suitable)
-if not suitable, state whether the colour change will be complete before or after the equivalence point is reached

Question 8

Why is the enthalpy change for the reaction of one mole of copper sulphate with five moles of water to form hydrated copper sulphate not able to be measured directly

Answer 8

You cannot react exactly five moles of water with one mole of copper sulphate (because the hydration doesn’t happen in a controlled manner) and you cannot measure the temperature change of a solid

Question 9

Why does the addition of HCl to an aqueous solution of sodium benzoate cause benzoic acid to precipitate out of solution

Answer 9

The benzoate ion gets protonated by the HCl to form benzoic acid which is less soluable in water than the sodium benzoate salt so it crystallises out of solution

Question 10

How to draw apparatus for filtration under reduced pressure

Answer 10

You draw a conical looking flask with a flat funnel that has filter paper in it, then draw a bung to connect the flask and funnel and draw a side arm labelled “pump” out of the side of the flask

Question 11

What is the expected observation when aromatic compounds undergo incomplete combustion

Answer 11

Black smoke

Question 12

Why must the front window of a fume cupboard be below the safety line even if the exhaust fan is switched on

Answer 12

-window above the safety line means the exhaust system is not strong enough to draw in fumes so the toxic fumes will escape into the lab

Question 13

Compare and contrast the preparation of Phenylamine from nitrobenzene with the preparation of butylamine from butanenitrile

Answer 13

-both are reduction reactions
-reagents for the preparation of Phenylamine from nitrobenzene are tin and concentrated HCl at 50c
-reagents for the preparation of butylamine from butanenitrile are hydrogen gas and a nickel catalyst at 150c

Question 14

Compare and contrast the basicity of Phenylamine and butylamine

Answer 14

-both are basic because of the lone pair on the nitrogen atom which can accept a proton
-Phenylamine is a weaker base because the lone pair of electrons on the nitrogen atom gets incorporated into the delocalised ring of electrons and so the electron density of the N atom decreases so it is less able to accept a proton

Question 15

How do u name an N-substituted amide

Answer 15

The prefix “N-“ is used to name the alkyl group attached to the nitrogen (e.g N-methylethanamude)

Question 16

Identify three differences in the esterification reaction when ethanoyl chloride vs ethanoic acid react with ethanol

Answer 16

-with ethanoyl chloride the reaction is irreversible but with ethanoic acid it is reversible
-with ethanoyl chloride the by product is HCl vs H2O with ethanoic acid
-the reaction with ethanoyl chloride is very fast and occurs at room temp but an acid catalyst is needed with ethanoic acid

Question 17

Outline a procedure from determining rate of reaction by collecting gas

Answer 17

-draw a diagram on a conical flask with a bung and delivery tube feeding into a gas syringe
-place calcium carbonate into the conical flask
-add hydrochloric acid and immediately place the bung back onto the flask
-record the volume of gas produced a regular time intervals and plot a graph of volume over time (gradient=rate)

Question 18

Why is dilute HCl needed in the test for sulphate ions (with barium chloride)

Answer 18

To react with any carbonate ions

Question 19

Why does 4-nitrophenol have a higher boiling point than 2-nitrophenol

Question 20

Describe how you could determine the maximum temperature change of a reaction between HCl and Na2CO3 by processing data

Answer 20

-place acid into a polystyrene cup and measure the temperature of the acid every minute for 5 minutes
-then add the sodium carbonate into
-stir and measure the temperature every 30 seconds for 10 minutes
-plot a graph of temperature over time
-draw 2 lines of best fit and extrapolate the lines to the exact time the sodium carbonate was added to determine the maximum temperature rise

Question 21

Define relative atomic mass

Answer 21

The weighted mean of the mass of an atom of an element relative to the mass of 1/12th mass of a carbon-12 atom

Question 22

Why do you rinse crystals of ammonium cobalt sulphate (with water of crystallisation) with ice cold water and place them in a warm oven

Answer 22

-rinsed to remove impurities
-ice cold water to prevent the crystals redissolving
-warm oven to ensure water of crystallisation isn’t removed

Question 23

Why would the yield in preparing transition metal crystals be less than 100%

Answer 23

-some solution may have been lost during heating if it “spits out”
-some crystals remain in solution
-some crystals remain on filter paper
-some water of crystallisation lost in drying
-transfer losses from beaker to evaporating basin/funnel

Question 24

How would forgetting to use a lid to heat a hydrated salt impact the calculated number of moles of water of crystallisation

Answer 24

Some of the salt could be lost from the crucible during heating so loss of mass greater than expected and moles of water of crystallisation greater than expected

Question 25

Could u test for a carbonate ion in barium carbonate by using sulphuric acid

Answer 25

Usually carbonates react with acids to produce carbon dioxide but barium sulfate is also produced which is insoluable so the carbonate may not appear to react

Question 26

Why is the filter paper and funnel warmed in an oven during recrystallisation

Answer 26

To make sure the solution doesnt cool down to prevent crystallisation taking place in the filter paper which would reduce yield of product

Question 27

How does allowing the filtrate to cool and recrystallise and then filtering it under reduced pressure remove impurities from the crystalline product

Answer 27

Product is less soluble in cold solvent so the product crystallises out and soluble impurities present remain dissolved in solution. Filtering under reduced pressure removes the soluable impurities faster

Question 28

How would forgetting to remove an air bubble from the tip of a burette before titration impact the value of the titre

Answer 28

Titre value would be greater than expected as the titre value includes both the volume of the air bubble and sodium hydroxide solution

Question 29

Is it true that hydrogen ions cant be acting as a catalyst if one hydrogen ion is used in the first step on a reaction but 2 are formed as products in the first step and the third step

Answer 29

Yes they are still catalysing because one hydrogen ion is regenerated and so is acting as a catalyst and the other is lost from another step in the reaction (so is a by-product of reaction)

Question 30

Explain the problem which results from sulfur impurities in fuels

Answer 30

-acid rain is formed -because sulfur dioxide is formed which dissolves in water

Question 31

Explain the trend in reactivity of elements down group 2

Answer 31

Reactivity increases down the group because ionisation energy decreases meaning it is easier to lose electrons, because valence electrons are further from the nucleus and so these valence electrons are more shielded

Question 32

Explain why the reaction between calcium and excess sulfuric acid stops before all of the metal has reacted

Answer 32

Calcium sulfate forms which is insoluable and so it forms a layer on the surface of the acid which prevents reaction

Question 33

Why may a boiling tube with both a solid catalyst and wool soaked in a reactant at the bottom be kept at an angle

Answer 33

If it were horizontal then the reactant and catalyst could mix

Question 34

What’s the problem with heating a reactant rather than a catalyst sometimes

Answer 34

The reactant would evaporate and pass over the catalyst without reaction

Question 35

What could be the reason for using a Bunsen valve (a tiny hole for gas to pass through into water)

Answer 35

Prevents water from entering the delivery tube so the gas can leave

Question 36

How is the enthalpy change of hydration of magnesium ions different from the enthalpy change of hydration of calcium ions

Answer 36

-more exothermic because the magnesium ion is smaller than the calcium ion but has the same charge so has a stronger force of attraction to water molecules

Question 37

What would you need to make a manganese (II)/manganese (III) half cell connected to a zin/zinc half cell

Answer 37

-platinum electrode -manganese nitrate solution (Mn(NO2)2) and potassium manganate (KMnO4) -and sulphuric acid

Question 38

State the direction of electron flow in a hydrogen-oxygen fuel cell and why

Answer 38

From the negative to the positive electrode because hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode

Question 39

Advantages of hydrogen-oxygen fuel cells over petroleum fuel

Answer 39

-only product is water but petroleum create pollutants -less reliant on fossil fuels -more efficient energy production -smaller and lighter

Question 40

How does chromatography separate components of a mixture

Answer 40

Each component in the mixture is attracted to the mobile and stationary phase but more strongly to one than the other

Question 41

Describe how to remove an air bubble from the tip of the pipette

Answer 41

Expel some of the solution to remove the air bubble and suck more up with the tip of the pipette in solution

Question 42

Is a 100cm^3 measuring cylinder suitable to measure 2cm^3 of a solution

Answer 42

No because 2 is too small because the graduations are too big so the smallest measurement you can make is 10

Question 43

What process must you do to an impure sample of water that contained calcium ions after it has been boiled

Answer 43

Filter off the precipitate that forms

Question 44

Give experimental techniques that could be used to monitor the progress of the reaction between iodide a-nd hydrogen perioxide (iodine clock)

Answer 44

-colorimetry -quenching and then titrating with thiosulphate

Question 45

Why is acidifation required after alkaline hydrolysis of an ester

Answer 45

The hydrogen ions are needed to protonate the benzoate ion

Question 46

Why is the entropy change of the system positive when Ni(CO)4 is thermally decomposed to form solid nickel and CO gas

Answer 46

The number of moles of gas increases and this increase is larger in magnitude than the decrease from forming solid nickel