Physical practicals

Question 1

Which test is used for CO3(2-) ions and what is the observation?

Answer 1

HNO3, effervesence

Question 2

Which test is used for SO4(2-) ions and what is the observation?

Answer 2

Ba(NO3)2, white ppt.

Question 3

Which test is used for halide ions and what is the observation?

Answer 3

AgNO3, white ppt.

Question 4

Which white precipitates dissolve in NH3: AgCl, AgBr, AgI (ion tests)?

Answer 4

AgCl dissolves in DILUTE NH3
AgBr dissolves in CONC. NH3
AgI doesn’t dissolve

Question 5

Which test is used for NH4(+) ions and what is the observation?

Answer 5

Warm with NaOH and test the gas produced with damp red litmus paper, red litmus paper turns blue

Question 6

What is the order of the ion tests?

Answer 6

1. CO3(2-)
2. SO4(2-)
3. Halide

Question 7

Why must the ion tests be done in a specific order?

Answer 7

1. CO3(2-) can also react with Ba(NO3)2 to form another white ppt. so false positives with SO4(2-) test
2. Both CO3(2-) and SO4(2-) can react with AgNO3 to make white ppt. so false positives with halide test

Question 8

What are the observations when acidified AgNO3 reacts with Cl-, Br-, and I-?

Answer 8

Cl-: White ppt.
Br-: Cream ppt.
I-: Yellow ppt.

Question 9

Why is the hydrated substance weighed to a constant mass when determining water of crystallisation?

Answer 9

Ensures all water has been removed

Question 10

What effect does forgetting to take the lid off when heating in the crucible have on the value of x (water of crystallisation)?

Answer 10

Mass of water LOWER so moles of water LOWER so x appears to be LOWER

Question 11

What effect does some of the hydrated solid spitting out during initial heating have on the value of x (water of crystallisation)?

Answer 11

Mass of water HIGHER so moles of water HIGHER so x appears to be HIGHER

Question 12

What is the 5 step method for preparing a standard solution?

Answer 12

1. Weigh solid by weighing by difference
2. Dissolve solid in water in volume less than 250cm3
3. Transfer to 250cm3 volumetric flask and add washings from funnel, glass rod, and beaker
4. Make up to the mark using a pipette to get bottom of meniscus on the line
5. Invert several times to ensure conc. is evenly distributed throughout solution

Question 13

Why is a trial titration carried out and why are titrations repeated several times?

Answer 13

Trial titration: To get an idea of where titre is which makes following titrations more accurate

Repeated: To achieve concordant results

Question 14

Is the acid or alkali usually measured in the burette?

Answer 14

Acid

Question 15

How would titre be affected if the pipette was only washed with distilled water prior to the titration?

Answer 15

KOH less concentrated and no change in HCl conc. so less HCl needed to neutralise KOH so smaller titre

Question 16

How would titre be affected if the top of the meniscus was used when measuring with the PIPETTE?

Answer 16

Smaller VOLUME of KOH so fewer moles of KOH and less moles of HCl needed to reach end point so smaller titre

Question 17

How would titre be affected if the top of the meniscus was used when measuring with the BURETTE?

Answer 17

Same difference between both readings so no effect on titre

Question 18

How would titre be affected if the conical flask was washed with distilled water only between each titration?

Answer 18

Doesn’t affect moles of KOH in beaker so no effect on titre

Question 19

How can % uncertainty be reduced whilst keeping the same volume of KOH and same procedure?

Answer 19

Increase titre by decreasing conc. of HCl

Question 20

Why are pipettes used in titrations rather than measuring cylinders?

Answer 20

More precise with lower % uncertainty

Question 21

Why is it important to make sure there is no air in the tap of the burette before starting a titration?

Answer 21

Volume in tap is part of titre so titre would be LOWER than it should be if tap filled with air

Question 22

What is the TITRATION colour change when manganate reacts with Fe2+ ions?

Answer 22

Colourless –> pale pink

Question 23

What is the colour change in the thiosulfate titration?

Answer 23

Brown –> orange –> red –> yellow –> blue-black –> colourless

Question 24

What measurements need to be taken when determining enthalpy?

Answer 24

1. Vol. of water in polystyrene cup
2. Mass of substance combusted/dissolved
3. Temp. change

Question 25

What are 2 procedural errors in calorimetry (determining enthalpy)?

Answer 25

1. Heat lost to the surroundings 2. Not done under standard conditions

Question 26

What are 2 errors for spirit burner calorimetry specifically (determining enthalpy)?

Answer 26

1. Heat capacity of beaker not taken into account 2. Incomplete combustion

Question 27

What effect would doubling the volume of water being heated in the beaker have on the value of q (determining enthalpy)?

Answer 27

Increase in m cancelled out by decrease in ▲T so no effect on q

Question 28

What effect would some of the liquid fuel evaporating (spirit burner) have on the value of ▲H (determining enthalpy)?

Answer 28

Contrbutes to mass change without being burnt so moles of fuel burnt (n) HIGHER than should be so ▲H decreases

Question 29

What is a limitation of using the collection of gas over water and collection of gas using a gas syringe methods?

Answer 29

Gas can be lost between mixing reagents and placing the bung

Question 30

What is an advantage and disadvantage of measuring mass lost over time on a balance compared to gas collection?

Answer 30

Advantage: No error from gas lost Disadvantage: Mass lost is very small so can't be measured using 2dp balance

Question 31

What are the 2 ways rate can be determined using continuous monitoring and what graph does continuous monitoring produce?

Answer 31

1. Loss of mass over time 2. Vol. of gas produced over time Conc-time graph

Question 32

What is the 7 step method for determining rate and the orders of reactants using the clock reaction/initial rates method?

Answer 32

1. Time how long it takes for a visual change (ppt./colour change) 2. Run experiment with set conc.s of A and B 3. Repeat but change [A], keeping [B] the same 4. Repeat again but change [B], keeping [A] the same 5. Keep the total vol. and temp. the same 6. Rate = 1/t 7. Plot a rate-conc graph and deduce orders from graph shapes

Question 33

What is an evaluation of the clock reaction/initial rates method?

Answer 33

Assumes that average rate of reaction is similar to initial rate - doesn't work with visual changes that happen slowly

Question 34

What does the esterification experiment used to determine Kc require and why does it also need a control?

Answer 34

Requires an acid catalyst, control needed to calculate moles of catalyst present

Question 35

Why are the flasks left for a week (determining Kc by esterification)?

Answer 35

To allow dynamic equilibrium to be reached

Question 36

How could the determining Kc by esterification method be modified to ensure that equilibrium had been reached?

Answer 36

1. Leave for longer 2. Do titration with NaOH again 3. Check eqm. moles haven't changed

Question 37

What is the 5 step method for determining enthalpy change of a solution?

Answer 37

1. Weigh out sample of ionic compound 2. Place known volume of distilled water into polystyrene cup 3. Measure initial temp. 4. Add solid to water and stir until all solid dissolved 5. Record final temp.

Question 38

In the manganate titration, which substances go in the burette/conical flask, what is the end point, and why is no indicator needed?

Answer 38

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

Question 39

In the thiosulfate titration, which substances go in the burette/conical flask and what is the end point?

Answer 39

Burette: S2O3(2-) Conical flask: I2 End point: Blue-black to colourless (I2 has ran out)