Physical practicals Flashcards
Which test is used for CO3(2-) ions and what is the observation?
HNO3, effervesence
Which test is used for SO4(2-) ions and what is the observation?
Ba(NO3)2, white ppt.
Which test is used for halide ions and what is the observation?
AgNO3, white ppt.
Which white precipitates dissolve in NH3: AgCl, AgBr, AgI (ion tests)?
AgCl dissolves in DILUTE NH3
AgBr dissolves in CONC. NH3
AgI doesn’t dissolve
Which test is used for NH4(+) ions and what is the observation?
Warm with NaOH and test the gas produced with damp red litmus paper, red litmus paper turns blue
What is the order of the ion tests?
- CO3(2-)
- SO4(2-)
- Halide
Why must the ion tests be done in a specific order?
- CO3(2-) can also react with Ba(NO3)2 to form another white ppt. so false positives with SO4(2-) test
- Both CO3(2-) and SO4(2-) can react with AgNO3 to make white ppt. so false positives with halide test
What are the observations when acidified AgNO3 reacts with Cl-, Br-, and I-?
Cl-: White ppt.
Br-: Cream ppt.
I-: Yellow ppt.
Why is the hydrated substance weighed to a constant mass when determining water of crystallisation?
Ensures all water has been removed
What effect does forgetting to take the lid off when heating in the crucible have on the value of x (water of crystallisation)?
Mass of water LOWER so moles of water LOWER so x appears to be LOWER
What effect does some of the hydrated solid spitting out during initial heating have on the value of x (water of crystallisation)?
Mass of water HIGHER so moles of water HIGHER so x appears to be HIGHER
What is the 5 step method for preparing a standard solution?
- Weigh solid by weighing by difference
- Dissolve solid in water in volume less than 250cm3
- Transfer to 250cm3 volumetric flask and add washings from funnel, glass rod, and beaker
- Make up to the mark using a pipette to get bottom of meniscus on the line
- Invert several times to ensure conc. is evenly distributed throughout solution
Why is a trial titration carried out and why are titrations repeated several times?
Trial titration: To get an idea of where titre is which makes following titrations more accurate
Repeated: To achieve concordant results
Is the acid or alkali usually measured in the burette?
Acid
How would titre be affected if the pipette was only washed with distilled water prior to the titration?
KOH less concentrated and no change in HCl conc. so less HCl needed to neutralise KOH so smaller titre
How would titre be affected if the top of the meniscus was used when measuring with the PIPETTE?
Smaller VOLUME of KOH so fewer moles of KOH and less moles of HCl needed to reach end point so smaller titre
How would titre be affected if the top of the meniscus was used when measuring with the BURETTE?
Same difference between both readings so no effect on titre
How would titre be affected if the conical flask was washed with distilled water only between each titration?
Doesn’t affect moles of KOH in beaker so no effect on titre
How can % uncertainty be reduced whilst keeping the same volume of KOH and same procedure?
Increase titre by decreasing conc. of HCl
Why are pipettes used in titrations rather than measuring cylinders?
More precise with lower % uncertainty
Why is it important to make sure there is no air in the tap of the burette before starting a titration?
Volume in tap is part of titre so titre would be LOWER than it should be if tap filled with air
What is the TITRATION colour change when manganate reacts with Fe2+ ions?
Colourless –> pale pink
What is the colour change in the thiosulfate titration?
Brown –> orange –> red –> yellow –> blue-black –> colourless
What measurements need to be taken when determining enthalpy?
- Vol. of water in polystyrene cup
- Mass of substance combusted/dissolved
- Temp. change
What are 2 procedural errors in calorimetry (determining enthalpy)?
- Heat lost to the surroundings
- Not done under standard conditions
What are 2 errors for spirit burner calorimetry specifically (determining enthalpy)?
- Heat capacity of beaker not taken into account
- Incomplete combustion
What effect would doubling the volume of water being heated in the beaker have on the value of q (determining enthalpy)?
Increase in m cancelled out by decrease in ▲T so no effect on q
What effect would some of the liquid fuel evaporating (spirit burner) have on the value of ▲H (determining enthalpy)?
Contrbutes to mass change without being burnt so moles of fuel burnt (n) HIGHER than should be so ▲H decreases
What is a limitation of using the collection of gas over water and collection of gas using a gas syringe methods?
Gas can be lost between mixing reagents and placing the bung
What is an advantage and disadvantage of measuring mass lost over time on a balance compared to gas collection?
Advantage: No error from gas lost
Disadvantage: Mass lost is very small so can’t be measured using 2dp balance
What are the 2 ways rate can be determined using continuous monitoring and what graph does continuous monitoring produce?
- Loss of mass over time
- Vol. of gas produced over time
Conc-time graph
What is the 7 step method for determining rate and the orders of reactants using the clock reaction/initial rates method?
- Time how long it takes for a visual change (ppt./colour change)
- Run experiment with set conc.s of A and B
- Repeat but change [A], keeping [B] the same
- Repeat again but change [B], keeping [A] the same
- Keep the total vol. and temp. the same
- Rate = 1/t
- Plot a rate-conc graph and deduce orders from graph shapes
What is an evaluation of the clock reaction/initial rates method?
Assumes that average rate of reaction is similar to initial rate - doesn’t work with visual changes that happen slowly
What does the esterification experiment used to determine Kc require and why does it also need a control?
Requires an acid catalyst, control needed to calculate moles of catalyst present
Why are the flasks left for a week (determining Kc by esterification)?
To allow dynamic equilibrium to be reached
How could the determining Kc by esterification method be modified to ensure that equilibrium had been reached?
- Leave for longer
- Do titration with NaOH again
- Check eqm. moles haven’t changed
What is the 5 step method for determining enthalpy change of a solution?
- Weigh out sample of ionic compound
- Place known volume of distilled water into polystyrene cup
- Measure initial temp.
- Add solid to water and stir until all solid dissolved
- Record final temp.
In the manganate titration, which substances go in the burette/conical flask, what is the end point, and why is no indicator needed?
Burette: MnO4(-)
Conical flask: Reducing agent (Fe2+)
End point: Colourless to pale pink
No indicator because there is a colour change associated with different oxidation states of Mn
In the thiosulfate titration, which substances go in the burette/conical flask and what is the end point?
Burette: S2O3(2-)
Conical flask: I2
End point: Blue-black to colourless (I2 has ran out)
