CORE PRACTICALS

Question 1

name some hazards and the respective safety thing?

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Answer 1

Irritant - dilute acid and alkalis- wear googles

Corrosive- stronger acids and alkalis wear goggles

Flammable – keep away from naked flames

Toxic – wear gloves- avoid skin contact- wash hands after use

Oxidising- Keep away from flammable / easily oxidised materials

Question 2

when is a gas syringe used?

Answer 2

-gas syringe is used where the volume of gas is measured

Question 3

Potential errors when using gas syringe?

Answer 3

• gas escapes before bung inserted
• syringe sticks
• some gases are soluble in water so the true amount of gas is not measured.

Question 4

Explain in detail the making a standard solution practical?

Finding the concentration of NaOH?

Answer 4

1. weight approximately 2.5g of sulfamic acid into test tube (use weighing by difference).
2. dissolve the sulfamic acid in approximately 100 cm^3 of water in a beaker.
3. Transfer the solution including the washing into a 250 cm^3 volumetric flask make the solution up to the mark with deionised water.
4. Prepare titration apparatus . Burette will contain the acid and the conical flask will contain NaOH.
5. Pour a 25 cm aliquot sodium hydroxide of unknown conc into the conical flask.
6. Add 4 drops of methyl orange indicator to the conical flask.
7. Titrate the contents of the flask against the sulfamic acid solution.

Question 5

what are concordant results?

Answer 5

titres that are within 0.2 cm^3 of each other

Question 6

how would you make a diluted solution of hydrochloric acid?

Answer 6

• add 25 cm^3 of the HCl solution into the volumetric flask using a pipette.
• make the solution up to the line by adding distilled water.

Question 7

how to carry out titration ?

(finding conc of HCl) i.e-disabled one

Answer 7

• Use the pipette to transfer 25.0 cm3 of the hydrochloric acid solution into the volumetric flask. Make the solution up to the mark with deionised water.
• prepare titration apparatus
• use pipette to place HCl into conical flask, fill burette with NaOH (known conc). record inital volume
• add few drops of phenolphthalein indicator to the conical flask
• open burette allow NaOH to flow into the conical flask, swirling it to mix contents
• rough one to find out how much you need
• start adding drop by drop when you get close to the titre value
• stop when you see it go pale pink.(use a white tile to see colour change better)
• calc final volume (do math)
• repeat till concordant results are obtained.

Question 8

why are acid-base indicators used?

Answer 8

-to detect when a reaction reaches complete,usually by colour change.

Question 9

What is a hydrolysis reaction?

Answer 9

-reaction where water is used to break chemical bonds (split a reactant into two)

Question 10

What kind of a reaction is the hydrolysis of haloalkanes?

Answer 10

-nucleophilic substiution

Question 11

how do you test the rate of hydrolysis of different haloalkanes?

Answer 11

1.-set up water bath. 50 degrees.
2.-fill three test tubes with 5ml of ethanol. now add 4 drops 1-iodobutane, 1-bromobutane,1-chlorobutane to 3 seperate test tubes.
3. Loosely place a bung in each test tube and
place the test tubes in the water bath.
4. Pour 5 cm3 of silver nitrate solution into three clean test tubes. Now place the test tubes in the water bath.
5. When the halogenoalkane–ethanol solutions have reached the temperature of the water bath, add the silver nitrate solution to one of
the halogenoalkane–ethanol solutions and replace the bung. Start the stop clock as you do so.
6. Measure the time taken for the precipitate to appear. As soon as the solution becomes cloudy stop the stop clock.
7. Repeat steps 5 and 6 for the other two
halogenoalkanes.

Question 12

What is the weighing by difference technique?

Answer 12

-mass of substance = mass of weighing boat and substance - mass of weighing boat after substance has been transferred.

Question 13

how to reduce the uncertainty in the mass measurement?

Answer 13

• use a balance with a greater resolution

- use larger mass

Question 14

why can the enthalpy change of a thermal decomposition reaction not be measured directly?

Answer 14

• as thermal energy must be supplied for the reaction to occur, the temperature change measured is not only due to decomposition.

Question 15

how to carry out practical to find enthalpy change of the thermal decomposition of potassium hydrogencarbonate?

Answer 15

1. Place approximately 3 g of solid potassium carbonate into a test tube. Accurately weigh the test tube and its contents.
2. Use a burette to dispense 30 cm3 of 2 mol dm–3 hydrochloric acid into a polystyrene cup, which is supported in a beaker.
3. Measure the temperature of the acid.
4. Continue measuring the temperature whilst adding potassium carbonate to the acid and stirring. Record the highest temperature
   reached.
5. Reweigh the empty test tube.
6. Repeat steps 1 to 5 using
   approximately 3.5 g of potassium hydrogencarbonate instead of potassium carbonate. This time record the lowest temperature reached

Question 16

Core practical 1 - molar volume of gas steps?

Answer 16

1. Place 30 cm3 of 1 mol dm−3 ethanoic acid in the boiling tube.
2. Place approximately 0.05 g of calcium carbonate in a test tube.
3. Weigh the test tube and its contents accurately.
4. Remove the bung from the boiling tube and tip the calcium carbonate into the boiling tube. Quickly replace the bung in the boiling tube.
5. Once the reaction is over, measure the volume of gas collected in the measuring cylinder.
6. Reweigh the test tube that had contained the calcium carbonate.
7. Repeat the experiment six more times, increasing the mass of calcium carbonate by about 0.05 g each time. Do not exceed 0.40 g of calcium carbonate.

Question 17

Oxidation of ethanol?

Core practical 5?

Answer 17

1. Carefully add 20 cm3 of acidified sodium/potassium dichromate solution to a 50 ml pear-shaped flask. Cool the flask in an ice-water bath.
2. Set the flask up for reflux keeping it in the ice-water bath.
3. Place a few anti-bumping granules into the pear- shaped flask.
4. Measure out 1 cm3 of ethanol.
5. Using a pipette, add the ethanol a few drops at a time down the reflux condenser. This must be done slowly. Allow for the reaction to subside after each addition before adding more.
6. When all of the ethanol has been added, remove the ice-water bath and allow to warm to room temperature (approximately 5 minutes).
7. Position the flask in a hot water bath using water from a kettle. Light a Bunsen burner and maintain a boiling water bath for 20 minutes. Allow the apparatus to cool.
8. Distil your product using the apparatus shown.Collect 3-4cm3 of clear, colourless liquid.

Question 18

How to do analysis of results from core practical 5? Oxidation of ethanol,one

Answer 18

Split the distillate into four portions and perform the following tests on each portion:

• Measure the pH of the distillate using universal indicator paper.
• Add a few drops of acidified potassium dichromate solution to 1 cm3 of the distillate. Warm the mixture in a 60 °C water bath.
• Add a quarter of a spatula of calcium carbonate powder to 1 cm3 of the distillate.
• Add a 1 cm long length of magnesium ribbon to 1 cm3 of the distillate.
• Add 1 cm3 of Fehling’s solution to 1 cm3 of the distillate. Warm the mixture gently using a water bath.

Question 19

Procedure for the investigation of chlorination of 2-methylpropan-2-ol?

Answer 19

1. Pour 10 cm3 of 2-methylpropan-2-ol and 35 cm3 of concentrated hydrochloric acid into a large conical flask. Swirl the contents of the flask very gently.
2. Place the bung in the mouth of the flask. Gently swirl again, then remove the bung to release the pressure.
3. Continue swirling the mixture with the bung fitted and then releasing the pressure for around 20 minutes. There should be two layers in the flask. The upper layer is the crude product.
4. Add approximately 6 g of powdered anhydrous calcium chloride to the flask and swirl until it has dissolved. This ensures that any unreacted alcohol is in the lower aqueous layer.
5. Transfer the reaction mixture to a separating funnel. Allow the mixture to settle into the two layers. Run off and discard the lower layer. Retain the upper organic layer in the separating funnel.
6. Add approximately 20 cm3 of sodium hydrogencarbonate solution to the separating funnel. Swirl the funnel and remove the bung to release the pressure, caused by the production of carbon dioxide, at frequent intervals. Run off and discard the lower aqueous layer.
7. Repeat the washing with sodium hydrogencarbonate, shake the separating funnel and release the carbon dioxide gas produced, at frequent intervals.
8. Run off and discard the lower layer. Ensure none of the aqueous layer remains in the tap.
9. Run off the organic layer into a small conical flask. Add a spatula full of anhydrous sodium sulfate. Place the bung in the flask and swirl the contents to mix. Leave the mixture until the liquid looks completely clear, swirling occasionally.
10. Decant the organic liquid into a 50 cm3 pear-shaped (or round-bottomed) flask.
11. Set the flask up for distillation
12. Collect the fraction boiling between 50 °C and 52 °C.
13. Place your pure product in a labelled sample tube

Question 20

Why does phenolphthalein become colourless again after the end point has been reached?

Answer 20

-because it reacts with the carbon dioxide in the air.