chem practical (energectics) Flashcards
Explain why it is NOT necessary to measure the volume of sulfuric acid more accurately than by using a measuring cylinder in ai)
Sulfuric acid used in ai) merely provides an acidic medium for the titration and it will be used in excess. Hence, its exact amount is not important and its measurement does not need to be very accurate
Explain how the volume of FA2(from the burette) obtained in ai) would change if HCl is used instead of H2SO4
Ci- in HCl will be oxidised by KMnO4 to give Cl2 gas. Hence, more KMnO4 will be required & resulting in a larger volume of FA2 obtained
The error (uncertainty) associated with each burette reading is +- 0.005cm3. Explain how an error of +-0.10cm3 may arise when using a burette to obtain a titre volume
There may be errors from both initial and final burette readings
[energetics] What is the precaution to take note when adding solid sodium hydrogencarbonate into aqueous hydrochloric acid in a polystyrene cup?
Solid sodium hydrogencarbonate reacts very vigorously with acid. If it is added too quickly to acid, the effervescence produced by the reaction will cause spillage of the reaction mixture.
Thus, adding of solid sodium hydrogencarbonate to the acid must be carefully done to prevent spillage of the reaction mixture.
Quickly replace the lid once all the solid sodium hydrogencarbonate has been added to the acid.
[energetics] Explain why there is a need to reweigh the weighing bottle after the addition of solid sodium hydrogencarbonate into hydrochloric acid in the polystyrene cup.
There are residual solid in the weighing bottle. Thus, it is important to obtain the mass of the residual solid so that the mass of solid sodium hydrogencarbonate transferred can be accurately determined.
[energetics] why both volumes and conc of reagents need to be known?
to find the limiting reagent to find the enthalpy change (=Qsystem/no. moles of limiting reagent)
[energetics] Suggest a reason why it is not advisable to use a much larger volume of FA 3 (reagent which is in excess)
The amount of heat released will still be the same since the volume of limiting reagent (NaOH(aq)) remains the same. If a much larger volume of FA 3 is used, the heat released will spread over a larger total volume, resulting in a smaller temperature change, which will cause a greater % error in temperature measurement.
Hence, the smaller increment T is, the greater the percentage error in increment straight T, since the uncertainty associated with the thermometer is fixed when the same thermometer is used.
[energetics] Suggest a reason why this neutralisation reaction is carried out via direct mixing of the reactants instead of adopting the cooling correction method.
Neutralisation reaction is instantaneous, hence there is likely minimal heat loss to the environment. Hence, it is not necessary to adopt the cooling correction method to account for heat lost to the environment.
[energetics] suggest why there is no sharp increase from the initial temperature of the reaction mixture to the maximum temperature of the reaction mixture at the time of mixing.
time is needed for the reaction to be completed.
Heat is also continuously lost to the surroundings (cooling) as the reaction takes place.
to correct heat lost to the surroundings, extrapolation can be made from the best-fit lines of both the temperature changes
why bulb of thermometer should not touch the bottom of the metal calorimeter?
Metal is a good conductor of heat. If the thermometer bulb touches the metal directly, it will result in reading the temperature of the quickly heated (and hotter) metal rather than the slowly heated (and cooler) solution. In that case, it becomes difficult to tell the actual temperature of the water.
Before the spirit lamp was lit, why might the temperature of the water in the calorimeter be changing?
- The water was stored in a cooler place and is now absorbing heat from its warmer surroundings, hence temperature increases.
- During setting up, heat could have been introduced to the set up from our body or due to friction. This heat energy takes time to dissipate into the surroundings.
- If the environment is particularly windy, there might be increased rate of evaporation, which decreases the temperature of water in the can. (cuz the more water vapour present, the harder it is to evaporate more water.)
(difference might be insignificant so)
Contrary to expectation, when the spirit lamp is put out, the temperature reading on the thermometer continues to climb for a short while before it stops (and then starts to fall). Suggest why the temperature reading can continue to rise even after the spirit lamp is put out.
Transfer of heat energy takes time, and so the excessive heat on the calorimeter or other parts of the water took time before becoming homogeneous across the entire calorimeter set-up.
Explain why it is necessary to add starch for the experiment to be carried out successfully
addition of starch allows distinct colour change from ___ to __ after a fixed amount of ___ produced is used up
[energetics] state and explain if the enthalpy change of neutralisation between KOH and HCl is the same as that between KOH and CH3COOH
No.
HCl is a strong acid.
CH3COOH is a weak acid so it is slightly dissociated in aq soln. Since ionisation, which involves bond breaking, is an endo process, some of the energy evolved from the neutralisation process is used to further dissociate ethanoic acid completely.
So, the enthalpy change of neutralisation between KOH and HCl is more exo than that of KOH and CH3COOH
Show by calculations, why a 0.2C thermometer would be more suitable that a 1C thermomenter
1C thermometer: 0.5/(35C-29C=6C) x 100% x 2 =16.7%
0.2C thermometer: 0.1(35C-29C =6C) x 100% x 2 =3.33%
due to a smaller temperature rise observed, a 1C would lead to a larger percentage error in the measured temperature rise than that of a o.2C thermometer
