Chemistry Combined Required Practicals P2

Only including the AQA chemistry combined (trilogy) course's required practicals

For PAPER TWO

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Concentration Affecting Reaction

Q: How does concentration affect the rate of a reaction?

Three methods = using a gas syringe, the 'disappearing cross', or a measuring cylinder in water to measure the volume of gas

Example method:

1. put 25cm3 of dilute hydrochloric acid in a conical flask, and connect a bung with a delivery tube to the gas syringe

2. drop a piece of magnesium ribbon into the flask and quickly place the bung over the top, setting a timer to start

3. wait until the fizzing has stopped (reaction is over) to read and record the volume of gas. Repeat using different concentrations of acid, and ensuring that the amount of magnesium ribbon is the same each time (you could use a mass balance to make sure)

Control Variables: temperature, amount of Mg, amount of HCl, etc

Possible Errors: not getting the bung in quickly enough, not starting the timer quickly ...

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Measuring Rates of Reaction

the higher the concentration, the faster the rate

Example: the disappearing cross

1. mark a large black cross on a piece of paper, and place a conical flask of 30cm3 of sodium thiosulfate solution directly over the top. You will be able to see the cross clearly at this point

2. add some dilute hydrochloric acid and start the timer

3. a cloudy precipitate will appear, and when the cross is completely obscured, stop the timer. This can be repeated with different concentrations of either HCl or sodium thiosulfate

Risks: this practical can be harmful as it produces sulfur dioxide, so make sure the area is well-ventilated ti reduce inhalation of the harmful gas

Issues: different people will say the cross has disappeared at different times, human errors (not starting the stopclock immediately), ...

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Identifying Substances Using Chromatography

chromatography = the process where a dissolved substance is separated by running a solvent through a material (eg water on paper)

Things to Remember:

  • always draw the start line in pencil (insoluble) so that the results aren't contaminated
  • ensure the start line and the dots of ink are above the water level
  • you may wish to put a lid on the beaker, to prevent the solvent from evaporating

Rf value = distance moved by substance / distance moved by solvent (solvent front)

example: for a pen dot you'd do this - distance of ink from the start line / solvent front from start

  • paper = stationary phase
  • solvent = mobile phase

the further up the paper the solute goes, the more soluble it is. So if ink does not move (eg a permanent pen) then it is insoluble

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Purifying and Testing Water

Key Word: Potable Water = water safe to drink (eg tap water in our kitchens)

You can use distillation to test water (eg mineral water, tap water, seawater/brine, or rainwater)

  • set up the apparatus, with a Bunsen Burner or appropriate heater below (a Bunsen may not be the best choice as you may find it difficult to accurately control the temp)
  • the substance with the lower boiling point will evaporate and go through the tubes, where cold water (outdie the tubes) will cause it to condense and run into the beaker on the end

Note: it is not economical to do this process on a large scale as it uses so much energy so is expensive

You may not get all of the water from the solution as some will stay in the tubes, and some may not evaporate (only a tiny amount though)

Remember - water boils at 100 degrees C, and it has a pH of 7 (neutral)

If you are comparing different types of water you may want to test the pH before and after distillation

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