paper 3 Flashcards
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.
. Both oxides react with water to form hydroxides.
. Sodium oxide is more soluble than magnesium oxide.
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.
V2O5 + SO2 → V2O4 + SO3
V2O4 + 1/2O2 → V2O5
Describe how a calibration graph is produced during colorimetry and used to find the concentration of the iron(III) complex.
. measure absorbance for (a range of) known concentrations
. plot graph absorbance v concentration
. read value of concentration for the measured absorbance from this graph
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.
- Plot concentration v time and calculate gradient
- Plot gradient against concentration
- Straight line through the origin = first order.
State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.
pressure not constant in bomb calorimeter
Describe the conditions for a standard hydrogen electrode.
- H2 gas
- HCl at 1moldm-3
- Pt electrode
- 298 K, 100KPa
Identify a hazard of concentrated phosphoric acid.
Corrosive.
Suggest two ways in which the melting point of crude aspirin would differ from melting point of pure aspirin.
- Value would be lower
2. Range of values.
Describe two precautions when heating ethanol
- Ethanol is flammable - use a water bath
2. Heat below boiling point so it does not evaporate.
Describe how to determine enthalpy of solution of anhydrous MgCl2 (using 0.8g of MgCl2)
. 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
Explain the role of anti-bumping granules
They prevent the formation of large bubbles.
Explain why reflux is used
Allows reactant vapours to return to reaction mixture.
Why is benzene insoluble?
Benzene ring is non-polar.
Why could calculated enthalpy is different to data book
- Experiment not done under standard condition
- Heating up copper not considered
- Evaporation of alcohol
. Explain why thermometer 0.5C uncertainty was adequate for this experiment (enthalpy change)
Temperature rise is significantly bigger than uncertainty.
Why would you keep solution below 10 degrees in nitration?
To stop further substitution of nitro groups in benzene ring..
Reaction is exothermic
Method for enthalpy change using flame calorimetry
- Measure mass of spirit burner before and after
- Measure temperature change of water
- Measure volume of water in cup
How to calculate minimum temperature that is not influenced by surroundings.
- 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
Why might percentage yield not be 100% during preparation of aspirinn?
- Product lost in apparatus
- Product lost in recrystallisation
- Sample still wet
How do drying agents work
They absorb water vapour
Explain why a solution containing [Al(H2O)6]3+ has a pH < 7
[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.
Explain why copper (I) iodide is white
Full 3d sub shell
Cannot absorb visible light
Describe how a catalyst provides an alternate route
. Reactant adsorbed onto platinum surface
. Reaction happens on surface
. Desorption of product
Describe electrospray ionisation
. Dissolved in solvent
. Injected through needle at high voltage
. Gains a proton
Describe how to test for ammonium ions
. 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
Suggest a change to the enthalpy change experiment that would decrease the percentage uncertainty in the use of the same thermometer.
Increase the magnitude of temperature change by increasing the concentration of acid/alkali.
Why can you separate cyclohexene from aqueous solution in separating funnel
Liquids are immiscible - form separate layers.
Suggest a suitable reagent for the hydrolysis of a proteinn
Conc. HCl
Describe the method a student should use to make a volumetric solution
. 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
Anhydrous magnesium chloride can absorb water to form a hydrated salt. Explain why enthalpy change for this reaction cannot be determined directly by calorimetry
. Not possible to prevent some dissolving
Explain how to reduce percentage uncertainty in the use of the same thermometer during core prac 7 when measuring enthalpy change
Increase magnitude of temperature change by increasing concentration of acid/alkali
Explain the general method for calorimetry
. 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.
Why do we take readings at regular time intervals + extrapolate to time of addition?
If reaction is slow, exact temperature rise is difficult to obtain as cooling occurs simultaneously with reaction
Errors in calorimetry
. 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
Why can finding the enthalpy change to form a hydrated salt from an anhydrous salt cannot be done experimentally?
. Impossible to add exact amount of water without solid dissolving
. Not easy to measure temperature change of a solid
Method for iodine clock reaction
. 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
What causes high percentage error in iodine clock?
High concentrations with quick times
Method for titrating samples of reaction mixture with acid, alkali, sodium thiosulfate
. 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
Why is a large excess of reactants used in continuous monitoring method?
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
Give two reasons why it is essential to avoid making MnO2 in a titration between potassium manganate(VII) and iron(II) ions.
Can’t see end point due to brown colour
Larger titre needed than expected
