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chemistry > paper 3 > Flashcards

paper 3 Flashcards

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1
Q

Sodium oxide forms a solution with a higher pH than magnesium oxide when equal amounts, in moles, of each oxide are added separately to equal volumes of water.
. State why both oxides form alkaline solutions.
. Suggest why sodium oxide forms a solution with a higher pH than the solution formed from magnesium oxide.

A

. Both oxides react with water to form hydroxides.

. Sodium oxide is more soluble than magnesium oxide.

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2
Q

In the Contact process, sulfur(IV) oxide is converted into sulfur(VI) oxide using vanadium(V) oxide as a catalyst.
Give two equations to show how the vanadium(V) oxide acts as a catalyst in this process.

A

V2O5 + SO2 → V2O4 + SO3

V2O4 + 1/2O2 → V2O5

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3
Q

Describe how a calibration graph is produced during colorimetry and used to find the concentration of the iron(III) complex.

A

. measure absorbance for (a range of) known concentrations
. plot graph absorbance v concentration
. read value of concentration for the measured absorbance from this graph

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4
Q

The concentration of cisplatin is measured at one-minute intervals. Explain how graphical methods can be used to process the measured results, to confirm that the reaction is first order.

A
  • Plot concentration v time and calculate gradient
  • Plot gradient against concentration
  • Straight line through the origin = first order.
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5
Q

State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.

A

pressure not constant in bomb calorimeter

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6
Q

Describe the conditions for a standard hydrogen electrode.

A
  • H2 gas
  • HCl at 1moldm-3
  • Pt electrode
  • 298 K, 100KPa
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7
Q

Identify a hazard of concentrated phosphoric acid.

A

Corrosive.

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8
Q

Suggest two ways in which the melting point of crude aspirin would differ from melting point of pure aspirin.

A
  1. Value would be lower

2. Range of values.

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9
Q

Describe two precautions when heating ethanol

A
  1. Ethanol is flammable - use a water bath

2. Heat below boiling point so it does not evaporate.

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10
Q

Describe how to determine enthalpy of solution of anhydrous MgCl2 (using 0.8g of MgCl2)

A

. Measure water with measuring cylinder (100cm3) and add to insulated container
. Add known mass of MgCl2 (0.8g)
. Weigh before and after
. Measure initial temperature with thermometer
. Record temperature every 30s for 10 minutes.
. Plot temperature vs time
. Extrapolate lines to when solid was added (find initial and final temp)
. Final temp - initial temp = change in temp
. q=mc(change in temp)
. mol = mass/mr

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11
Q

Explain the role of anti-bumping granules

A

They prevent the formation of large bubbles.

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12
Q

Explain why reflux is used

A

Allows reactant vapours to return to reaction mixture.

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13
Q

Why is benzene insoluble?

A

Benzene ring is non-polar.

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14
Q

Why could calculated enthalpy is different to data book

A
  • Experiment not done under standard condition
  • Heating up copper not considered
  • Evaporation of alcohol
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15
Q

. Explain why thermometer 0.5C uncertainty was adequate for this experiment (enthalpy change)

A

Temperature rise is significantly bigger than uncertainty.

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16
Q

Why would you keep solution below 10 degrees in nitration?

A

To stop further substitution of nitro groups in benzene ring..
Reaction is exothermic

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17
Q

Method for enthalpy change using flame calorimetry

A
  • Measure mass of spirit burner before and after
  • Measure temperature change of water
  • Measure volume of water in cup
18
Q

How to calculate minimum temperature that is not influenced by surroundings.

A
  • Start clock when reagents mix
  • Measure temperature every minute for 5 minutes
  • Plot graph of temperature vs time
  • Extrapolate back to time of mixing to get temperature
19
Q

Why might percentage yield not be 100% during preparation of aspirinn?

A
  • Product lost in apparatus
  • Product lost in recrystallisation
  • Sample still wet
20
Q

How do drying agents work

A

They absorb water vapour

21
Q

Explain why a solution containing [Al(H2O)6]3+ has a pH < 7

A

[Al(H2O)6]3 + H2O –> [Al(OH)(H2O)5]2+ + H+

Al3+ has a high charge density and small size so releases H+ in water and weakens O-H bonds.

22
Q

Explain why copper (I) iodide is white

A

Full 3d sub shell

Cannot absorb visible light

23
Q

Describe how a catalyst provides an alternate route

A

. Reactant adsorbed onto platinum surface
. Reaction happens on surface
. Desorption of product

24
Q

Describe electrospray ionisation

A

. Dissolved in solvent
. Injected through needle at high voltage
. Gains a proton

25
Q

Describe how to test for ammonium ions

A

. Place ammonium chloride in test tube
. Add NaOH
. Heat gently in water bath
. Put damp red litmus paper in mouth of test tube - it will turn blue

26
Q

Suggest a change to the enthalpy change experiment that would decrease the percentage uncertainty in the use of the same thermometer.

A

Increase the magnitude of temperature change by increasing the concentration of acid/alkali.

27
Q

Why can you separate cyclohexene from aqueous solution in separating funnel

A

Liquids are immiscible - form separate layers.

28
Q

Suggest a suitable reagent for the hydrolysis of a proteinn

A

Conc. HCl

29
Q

Describe the method a student should use to make a volumetric solution

A

. Weigh solid in plate and reweigh to find mass
. Dissolve in water in cylinder
. Transfer with washings into 250cm3 volumetric flask
. Make up to 250cm3 then shake

30
Q

Anhydrous magnesium chloride can absorb water to form a hydrated salt. Explain why enthalpy change for this reaction cannot be determined directly by calorimetry

A

. Not possible to prevent some dissolving

31
Q

Explain how to reduce percentage uncertainty in the use of the same thermometer during core prac 7 when measuring enthalpy change

A

Increase magnitude of temperature change by increasing concentration of acid/alkali

32
Q

Explain the general method for calorimetry

A

. Wash equipment with solution to be used
. Dry cup
. Use polystyrene in glass beaker for insulation and support
. Measure desired volumes with volumetric pipette, transfer to insulated cup
. Measure initial temperatures of solution for 2 minutes
. Then add second reagent, and record temperature after addition for 5 minutes.

33
Q

Why do we take readings at regular time intervals + extrapolate to time of addition?

A

If reaction is slow, exact temperature rise is difficult to obtain as cooling occurs simultaneously with reaction

34
Q

Errors in calorimetry

A

. Energy transferred to surroundings
. Neglecting specific heat capacity of calorimeter
. Reaction/ dissolving may be incomplete or slow
. Density of solution is assumed to be the same as water
. Incomplete reaction
. Incomplete transfer of energy
. Measurements not carried out under standard conditions

35
Q

Why can finding the enthalpy change to form a hydrated salt from an anhydrous salt cannot be done experimentally?

A

. Impossible to add exact amount of water without solid dissolving
. Not easy to measure temperature change of a solid

36
Q

Method for iodine clock reaction

A

. Measure required volumes of potassium iodide, sodium thiosulfate, starch and water into small conical flask
. Measure hydrogen peroxide into a test tube
. Pour hydrogen peroxide into conical flask and start timer
. Mix and stop timer at first hint of blue
. Repeat with different concentrations of potassium iodide

37
Q

What causes high percentage error in iodine clock?

A

High concentrations with quick times

38
Q

Method for titrating samples of reaction mixture with acid, alkali, sodium thiosulfate

A

. Small samples removed from reaction mixture
. Quench by dilution/cooling
. Titrate with suitable reagent:
- NaOH can be titrated with an acid
- H+ can be titrated with an alkali
- I2 could be titrated with sodium thiosulfate

39
Q

Why is a large excess of reactants used in continuous monitoring method?

A

If one reactant concentration is kept in a large excess then that reactant will appear not to affect rate and will be pseudo-zero order, as concentration stays constant and does not affect rate

40
Q

Give two reasons why it is essential to avoid making MnO2 in a titration between potassium manganate(VII) and iron(II) ions.

A

Can’t see end point due to brown colour

Larger titre needed than expected

chemistry (8 decks)

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