Paper 2 praticals Flashcards
(20 cards)
Rates of Reaction
- Use a measuring cylinder to put 10cm^3 of sodium thiosulfate solution into a conical flask
- Place conical flask onto a printed black cross
- Add 10cm^3 of hydrochloric acid into the conical flask
- Swirl the solution and start a stopwatch
- Look down through the top of the conical flask
- After a certain time the solution will turn cloudy
- Stop the clock when we can no longer see the cross
- Carry out experiments using lower concentrations of sodium thiosulfate solution
- Repeat the whole experiment and calculate the mean values for each concentration of sodium thiosulfate solution
- Do not include any anonymous results when calculating mean
- Issue with experiment is that different people have different eyesight, so some people can see the cross longer than others (not reproduceable)
- However since students use same sized printed cross the problem should not be too great
Volume of gas produced by a reaction
- Use a measuring cylinder to place 50cm^3 of hydrochloric acid into a conical flask
- Attach the conical flask to a bung and delivery tube
- Now place a delivery tube into a container filled with water
- Then placed a upturned measuring cylinder also filled with water over the delivery tube
- Add a 3cm strip of magnesium to the hydrochloric acid and start a stop watch
- The reaction produced hydrogen gas which is trapped in the measuring cylinder
- Every ten seconds measure the volume of gas in the measuring cylinder
- Continue until no more hydrogen is given off
- Repeat the experiment using different concentrations of hydrochloric acid
What do both rate of reaction experiments show
The greater the concentration of a chemical in a reaction, the faster the reaction takes place
Because shown by two different experiments we can say experiment reproducible
Chromotography steps
Chromatography
- Use a ruler to draw a horizontal pencil line on the chromatography paper
- Line should be 2cm from bottom of paper
- Mark five pencil spots at equal spaces across the line
- Leave at least 1cm clear at each side
- Use a capillary tube/pen to put small spot of the known chemical and unknown chemicals on the pencil spots
- A capillary tube is a very thin glass tube
- It is important we keep the spots relatively small. This prevents the colors from spreading into each other
- Pour water into a beaker to a depth of 1cm
- The water is the solvent in this case
- Attach paper to a glass rod using tape and lower the paper into the beaker
- The bottom of the paper should dip into the water
- The pencil line with spots of ink must be above the surface of the water
- Otherwise the water will wash the ink of the line
- The sides of the paper must not touch the side walls of the beaker
- If that happens, then it will interfere with the way the water moves
- We put a lid on the beaker to reduce evaporation of the solvent
- At this stage the water moves upward and carries the chemicals
- During this time be careful not to move the beaker
- Remove the paper when the water has traveled three quarters up
- Allow to dry
Analysis of chromotography
The unknown chemical may separate into several spots, showing it’s a mixture.
You can compare the spots of unknown chemicals with the known ones.
If the spots line up exactly, it’s likely the same substance.
How to Calculate Rf Value:
Rf = Distance moved by substance ÷ Distance moved by solvent
Step-by-step:
Measure (in mm or cm) from the pencil line to the centre of the spot.
Measure the distance from the pencil line to the solvent front (where the water reached).
Divide the distance moved by the spot by the distance moved by the solvent.
🧠 Note:
Rf values are always less than 1.
Substances with the same Rf value in the same solvent are likely to be the same chemical.
Hydrogen
Squek pop test
Collect the gas you suspect to be hydrogen in a test tube.
Remove the bung if there is one.
Light a wooden splint using a Bunsen burner.
While the splint is still burning, bring it near the mouth of the test tube (not inside).
Observe what happens.
If hydrogen is present, the gas burns rapidly with a ‘squeaky pop’ sound.
Oxegen test
Glowing splint test
Collect the gas you suspect to be oxygen in a test tube.
Light a wooden splint, then blow it out so it is just glowing (not burning).
Quickly insert the glowing splint into the mouth of the test tube containing the gas.
If oxygen is present, the glowing splint relights or bursts into flame.
Carbond di-oxide test
Limewater test
Bubble or pass the gas you suspect to be carbon dioxide through a solution of limewater.
Limewater is a dilute solution of calcium hydroxide.
This is usually done by using a delivery tube from the reaction into a test tube or beaker containing limewater.
If carbon dioxide is present, the limewater turns cloudy or milky white. from colorless.
Chlorine
Damp litmus paper test
Hold a piece of damp blue litmus paper (or universal indicator paper) near the mouth of the container with the suspected chlorine gas.
The paper must be damp so the gas can dissolve into it.
Chlorine bleaches the litmus paper, turning it white.
If using blue litmus paper, it may briefly turn red first (because chlorine is acidic) before turning white.
Flame test
- Place small amount of chemical onto a wire mounted in a handle
- We then place the end of this into a blue bunsen burner flame
- The colour of the fame can be used to work out the metal ion present
Lithum
Crimson flame
Sodium
Yellow
Potassium
Lilac
Calcium
Orange-red flame
Copper
Green flame
Issue with the flame test
The colour can be hard to distingush, especaly if it has a lower concentration of the metal compound
Also if it is a mixture of metal ions the colour can be masked
Flame emmison spectrospocy
Flame emission spectroscopy is an instrumental method used to identify metal ions and measure their concentrations in a solution.
A sample containing metal ions is placed in a flame.
The energy from the flame excites the electrons in the metal ions.
As the electrons fall back to their original energy levels, they release light.
This light is passed through a spectroscope, which separates it into a line spectrum.
Each metal ion produces a unique pattern of lines — like a fingerprint.
The metal ion: each ion gives a unique spectrum (can be used to identify the metal).
The concentration: the intensity (brightness) of the lines shows how much of the ion is present.
Potiable water
- Check the PH of the water, by placing a small amount of water onto a piece of universal indicator paper
- It is green if the PH is seven
- If it is not green, the sample contains dissolved acid or alkali so not pure
- Does not mean pure since could contain dissolved substences
- Use a scale to weigh a empty evaporation basin
- We need to record the mass
- Fill the evaporation basin with our water sample, and place this on a tripod and gauz
- Now use a bunsen burner to gently heat the water until all has evaporated
- At this point we allow the evaporation basin to cool and then weigh it again
- If the water sample contains any dissolved solids, then the mass of the empty evaporation basin will have increased because of unevaporated solids
How to distilate water
Place the water sample in a conical flask set on a tripod.
Attach a delivery tube to the top of the flask, directing the tube into a test tube.
The test tube should be placed in a beaker of ice and water to cool it.
Gently heat the water in the conical flask using a Bunsen burner until it boils gently.
As the water boils, it evaporates, forming water vapor.
The water vapor travels through the delivery tube into the test tube.
When the vapor enters the cool test tube, it condenses back into liquid water.
The liquid collected in the test tube is distilled water (pure water).
