practicals

Question 1

heating in a crucible

mass gain by reacting Mg with O₂

method

Answer 1

• weigh crucible and lid
• add Mg (that has been cleaned with emery paper) and reweigh
• heat with **bunsen burner ** then allow to cool
• weigh contents, then repeat until mass is constant

Question 2

heating in a crucible

why is a lid added to oxidation of Mg reaction?

Answer 2

• improves the accuracy of the experiment, by preventing loss of solid
• should be loose fitting to ensure gases can enter and escape

Question 3

heating in a crucible

why must you repeat the heating in the oxidation of Mg reaction?

Answer 3

• when the mass is constant, signifies the reaction is fully complete

Question 4

heating in a crucible

decomposition reaction- what does it do

same method as oxidation reaction, just different reaction

Answer 4

• removes water of crystallisation as water vapour by heating

Question 5

heating in a crucible

why must the crucible be dry in a decomposition reaction?

Answer 5

• a wet crucible would cause a large mass loss, as the water would be lost when heating
• would give inaccurate results as the mass change isn’t from the water of crystallisation only

Question 6

measuring gas volumes

two methods for measuring gas volumes

Answer 6

• using a gas syringe
• measuring cylinder over water

Question 7

measuring gas volumes

potential errors of using a gas syringe

Answer 7

• gas may escape before the bung is inserted
• the syringe could stick

Question 8

measuring gas volumes

measuring cylinder over water diagram

components required labels

Answer 8

• measuring cylinder upside down in a trough of water
• markings on measuring cylinder to show measurements can be taken
• tubing that goes under the water and into the measuring cylinder

Question 9

making a standard solution

method for making a standard solution from a solid

Answer 9

• weigh required mass of solid (in bottle) then transfer to beaker
  (weigh empty bottle to ensure mass of solid is correct)
• add 100 cm³ of distilled water to beaker, stir with glass rod to dissolve
• pour into volumetric flask with a funnel, rinse beaker/funnel/glass rod with distilled water and add washings to flask
• make up to the mark with distilled water
• add stopper and invert flask multiple times to mix

Question 10

what other method can a volumetric flask be used for?

apart from standard solution

Answer 10

• dilution
• pipette 25cm³ of original solution to 250cm³ flask, make up to mark with distilled water

Question 11

acid-base titration

acid-base titration

method

Answer 11

• pipette 25 cm³ of alkali into conical flask, touching surface with pipette
• fill burette with acid, run down into ‘jet space’ and record initial volume
• add indicator to conical flask and place on a white tile
• add acid to alkali while swirling conical flask, drop-by-drop until end point reached (indicated by colour change)
• record final volume of acid and calculate titre
• repeat until concordant results obtained (within 0.1 cm³)

Question 12

acid-base titration

why is a conical flask used over a beaker?

Answer 12

• easier to swirl
• less likely to spill contents

Question 13

acid-base titration

measuring using a pipette

how do you accurately take a reading (badly explained soz)

Answer 13

• make sure the bottom of the meniscus lies on the line

Question 14

acid-base titration

before using a burette, how do you ensure no contamination occurs?

Answer 14

• rinse out with the substance you’re about to put in it
• if residual water left over, the acid or alkali may be diluted
• if substances left over from a past titration, conc may be lowered

Question 15

acid-base titrations

what indicators are used when?

i don’t think we actually need to know this?? idk

Answer 15

• phenolphthalein- if acid is added from the burette, and strong alkali in the conical flask- pink to colourless
• methyl orange- neutralisation reactions where strong acid is used- red in acid, yellow in alkali, orange at end point

Question 16

manganate redox titrations

equation

there’s like nothing on this in the pag things so idek

Answer 16

MnO₄⁻₍ₐ₎+ 8H⁺₍ₐ₎+ 5Fe ²⁺₍ₐ₎–> Mn²⁺₍ₐ₎+ 4H₂O₍ₗ₎ + 5Fe³⁺₍ₐ₎

Question 17

manganate redox titration

colour change

Answer 17

• self-indicating, MnO₄⁻ is purple and Mn²⁺ is colourless
• if manganate in burette, colourless to first permanent purple colour indicates end point

Question 18

percentage uncertainty calculation

Answer 18

%= (uncertainty/measurement taken) x 100
note that if two measurements are taken, must multiply uncertainty by 2

Question 19

decreasing % uncertainties

Answer 19

• use apparatus with a greater resolution
• or increase the size of the measurement made

Question 20

enthalpy change of reaction

equipment used

calorimetric method

Answer 20

• use a polystyrene cup inside beaker (insulation AND support)
• thermometer to monitor temp change
• glass rod to stir

Question 21

enthalpy change of reaction

method

calorimetric method

Answer 21

• measure out desired volumes of solutions with pipettes and transfer to insulated cup (or could used solid dissolving in one solution)
• clamp thermometer into place with bulb immersed in liquid
• measure initial temperature, then every 30 seconds
• at 3 minutes add second solution and stir, keep recording temp every 30 secs/1 min

Question 22

enthalpy change of reaction

errors from this method

calorimetric method

Answer 22

• will be loss of heat energy to the surroundings
• reaction (or dissolving) may be slow or incomplete
• SHC assumed to be same as water for all solutions

Question 23

enthalpy change of combustion

apparatus and brief method

Answer 23

• weigh empty spirit burner and lid, then reweigh with alcohol added
• add water to beaker/ calorimeter and clamp above spirit burner, with lid and thermometer
• measure temp change of water for reaction
• weigh spirit burner after experiment
  calculate mass change, temp change

Question 24

enthalpy change of combustion

errors in this method

Answer 24

• heat loss to the surroundings from spirit burner, wick and calorimeter
• evaporation of water
• evaporation of alcohol
• incomplete combustion of alcohol
• reaction is unlikely to occur under standard conditions

Question 25

# enthalpy change of combustion improvements to reduce error

Answer 25

* improving draught exclusion by **adding a draught shield** * minimising the distance between the flame and the calorimeter * placing a **cap on the calorimeter** * **insulating the spirit burner and calorimeter**

Question 26

# anions test for ammonium ions

Answer 26

* add sample and **NaOH** to a test tube, and shake * gently warm (using a water bath) * test the fumes by holding **damp red litmus paper** over the test tube * alkaline ammonia gas is produced which turns the red litmus paper **blue**

Question 27

# cations test for carbonates

Answer 27

* add any **dilute acid** * **effervescence** is observed

Question 28

# cations test for sulfates

Answer 28

* **barium chloride** acidified with HCl is used as s reagent * **white precipitate** of barium sulfate forms * HCl used to react with carbonate impurities, which would form a white barium carbonate precipitate which is a false positive

Question 29

# cations test for halides- precipitates

Answer 29

* add **silver nitrate** (and HNO₃ to remove carbonates) * **chloride** ions form a **white precipitate** * **bromide** ions form a **cream precipitate** * **iodide** ions form a **yellow precipitate**

Question 30

# cations test for halides- solubility

Answer 30

* silver **chloride** dissolves in **dilute ammonia** * silver **bromide** dissolves in **concentrated ammonia** * silver **iodide** is **insoluble in ammonia**

Question 31

# purifying an organic liquid method - always in the mark scheme

Answer 31

* put the impure product into a **separating funnel** and allow the layers to separate * **upper layer is least dense**, run off the lower (usually aqueous) layer * run the organic layer into a conical flask and **add anhydrous magnesium sulfate** * decant into distillation flask and **distill to further purify (collect fraction at given boiling point)**

Question 32

# purifying an organic liquid why might a substance be refluxed before being put in a separating funnel?

Answer 32

reactant vapours of volatile compound are condensed and returned to the reaction mixture

Question 33

when measuring a boiling point, what is the indication that the substance is impure?

Answer 33

* if the liquid is pure it will have the boiling point referred to in data books * if impure the boiling point tends to be higher than the pure liquid

Question 34

# purifying an organic solid recrystallisation method

Answer 34

* dissolve the impure compound in a **minimum volume of hot solvent** * **filter** hot solution to **remove insoluble impurities** * **cool** the solution * **filter** when cold (using a Buchner flask so reduced pressure) and **wash to remove soluble impurities** * **dry** crystals

Question 35

# purifying an organic solid why must a **minimum** volume of hot solvent be used?

Answer 35

* lower yield (less product collected) as some of the product would remain dissolved in the hot water

Question 36

# purifying an organic solid during filtration, why is it important to only wash your solid with ice-cold solvent?

Answer 36

* the solid would dissolve in hot water and so the yield would decrease

Question 37

# thin-layer chromatography what must the baseline be drawn in and why?

Answer 37

* pencil line * will not dissolve in the solvent

Question 38

# thin-layer chromatography general method

Answer 38

* draw a baseline on TLC plate and draw a spot for each sample * use a capillary tube to add a small drop to each spot * add 1cm of solvent to a beaker and add TLC plate, making sure the solvent is below the baseline * add a lid! * remove from solvent just before the solvent front reaches the top of the plate * allow to dry and place under UV light to develop the spots * calculate Rf values

Question 39

# thin-layer chromatography how do stationary phases separate?

Answer 39

* solid stationary phase- by adsorption * liquid stationary phase- by relative solubility

Question 40

# electrochemical cells diagram of zinc and copper electrodes

Answer 40

* Zn electrode in solution containing 1 moldm⁻³ of Zn²⁺ ions * copper electrode in solution containing 1 moldm⁻³ of Cu²⁺ ions * connected with a voltmeter * salt bridge required * 298K and 100KPa

Question 41

# electrochemical cells what is the use of a salt bridge? how is it made?

Answer 41

* salt bridge is used to connect the circuit- the free moving ions conduct the charge * usually made from a piece of filter paper soaked in a salt solution, which is usually potassium nitrate

Question 42

# measuring the rate of a reaction what is the typical continuous monitoring method?

Answer 42

measurement of the change in volume of a gas- using a gas syringe

Question 43

# measuring the rate of reaction method for volume of gas collected in gas syringe

Answer 43

* measure 50 cm³ of the 1.0 mol dm⁻³ hydrochloric acid and add to conical flask * set up the gas syringe in the stand * weigh 0.20 g of magnesium * add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the timer * record the volume of hydrogen gas collected every 15 seconds for 3 minutes

Question 44

# measuring the rate of a reaction what is the typical method for initial rate

Answer 44

clock reaction- normally with hydrogen peroxide and iodine * start the timer and stop when the solution turns blue/black * repeat with different concentrations of potassium iodide

Question 45

# pH curves titration/pH method

Answer 45

* transfer 25cm³ of acid to a conical flask with a volumetric pipette * measure initial pH of the acid with a pH meter * add alkali in small amounts (2cm³) noting the volume added each time (using a burette and titration method) * stir mixture to equalise the pH as you go along * measure and record the pH to 1 d.p. * repeat but when approaching endpoint add in smaller volumes of alkali * add until alkali in excess (or burette used up)