Chemistry Practical Theories Flashcards
Polyprotic titration: Suggest why the third end-point for complete neutralisation of polyprotic acid cannot be obtained by titration.
The pH change at the third end point is not sharp/ too gradual hence no acid-base indicators can be used as such indicators require the presence of rapid pH change within its working pH range to work.
Acid Base reaction: Suggest an explanation for the observations made when aqueous Al3+ and Cr3+ ions react with Na2CO3.
Cations have high charge density, thus O-H bonds in H2O ligands are polarised. Thus, the complex ions undergo partial hydrolysis to give H3O+ which subsequently reacts with the CO3 2- ions in an acid base reaction.
Acid Base reaction / Le Chatelier’s Principle: Explain how Na2CO3 (aq) forms ppt when added to Cu2+, and the ppt dissolves in CH3CO2H.
Blue CuCO3 was formed. CH3CO2H was added to remove the CO3 2- anion via acid-base reaction.
Concentration of CO3 2- thus decreases, position of equilibrium shifts and CuCO3 dissolves to give a blue solution.
Titration: Explain why it is necessary to use the dry apparatus in preparation of a saturated solution of Calcium Iodate (V).
To prevent the dilution of the saturated solution which results in a lesser amount of I03-/ Iodate reacted during titration and hence smaller titre volume.
Solubility equilibria: Suggest a reason why experimental value of Ksp differs from theoretical value, and suggest an improvement to attain a closer experimental value.
The experiment is not carried out at stated temperature of the theoretical value. Thus, position of equilibrium is shifted, resulting in the difference in Ksp value.
Use a thermostatically controlled water bath maintained at stated temperature.
Kinetics: Explain how the shape of your graph shows an autocatalysis reaction.
At the start - gradient of graph is gentle, the rate of reaction is slow as the catalyst has not yet been formed. In the middle - the gradient of graph is steeper, the rate of reaction is faster, showing that the product formed is also catalyst of the reaction and it speeds up the reaction. The last part of the graph - gentle gradient, the rate of reaction is slower, showing that the [reactants] is now low, reducing rate of reaction even though concentration of catalyst is high now.
Continuous experiment procedure: Explain why the time that each aliquot sample drawn does not have to be recorded.
The reaction continues in the removed sample at the same rate until the sample is quenched with large excess of cold water/ quenching reagent.
Continuous experiment procedure: Explain why a student does not need to be worried if he is unable to complete each titration within the interval.
The reaction stops upon quenching, and thus the amount of product formed does not change over time. Therefore the student does not have to complete each titration before removal of the next sample.
Titration: Explain why titration involving Hydrogen Peroxide gives rise to different titre value when the H2O2 is now left out, ceteris paribus.
Hydrogen peroxide decomposes over time. To reduce rate of decomposition of the H2O2 solution, the solution should be kept away from sunlight in a dark and sealed container, at low temperature.
or there could be a change in temperature of the H2O2 solution, thus the solution should be maintained at constant temperature with a thermostatically controlled water bath.
Solubility equilibria: Explain why NaOH forms ppt with Mg2+ but not with Ba2+
Ksp of Mg(OH)2 is smaller than Ksp of Ba(OH)2, hence Mg(OH)2 will precipitate as its ionic product will be more than its Ksp but Ba(OH)2 will not.
Kinetics: In a kinetics experiment, one major factor must be controlled. What is that factor and how it is to be controlled.
Temperature must be controlled. It should be controlled using a thermostatically controlled water bath.
Kinetics: In an Iodine Clock experiment, a measuring cylinder cannot be used to measure the volume of thiosulfate ions added. Why?
Thiosulfate is the limiting reagent and used to control the extent of all experiments, thus the number of moles must be measured accurately, using a burette. This also ensures that the amount of iodine produced to react with Thiosulfate across all experiments are relatively constant.
Chemical Energetics: In a temperature correction experiment, why the difference between theoretical and experimental enthalpy values cannot be due to the omission of the specific heat capacities of the apparatus in the heat change calculation.
Heat required to raise the temperature of the apparatus has not been included. This would result in a lower value for the heat change and so a lower enthalpy value than the published value (not a higher value).
Chemical Energetics: Why is enthalpy change of neutralisation between a weak base-strong acid reaction lower than that of a strong base-strong acid reaction.
Some energy have been absorbed to fully dissociate the weak base, and therefore the change in temperature of the solution is lower, resulting in a lower heat evolved.
Chemical Energetics: For the temperature correction method, identify and explain one significant error in determining the enthalpy change. Suggest one modification to minimise the error.
1) Heat capacity of the cup is not taken into consideration/ heat loss is not accounted for. Heat change will be smaller than actual, and enthalpy change calculated is less exothermic. To modify, consider the heat capacity of the cup and the heat change of the cup before calculating the molar enthalpy change.
2) Measuring cylinder does not measure the volume of solution accurately due to larger uncertainty in measurement. Heat change and thus enthalpy change calculated will be inaccurate. To modify, you a burette to measure the volume of solution added, which has a smaller uncertainty in measurement of volume than a measuring cylinder.
