Practicals Flashcards
Describe a (simplified) method for electrolysis
- Pour copper (II) sulfate solution into beaker (e.g. 50cm3)
- Add carbon rods (must not touch each other) + connect to power supply
- Turn on power supply
- Hold piece of blue litmus paper (with forceps) in solution next to the positive electrode
- After 5 minutes, switch off & examine negative electrode
Name safety precaution one should take when carrying out electrolysis
Wear goggles
When electrolysing copper (II) sulfate, what happens on the negative electrode (cathode)?
Metal (copper) coating forms
When electrolysing copper (II) sulfate, how do we know oxygen is produced?
If gas produced at positive electrode doesn’t bleach blue litmus paper
When electrolysing copper (II) sulfate, how do we know hydrogen is produced?
If there’s gas produced at negative electrode
What do pipettes measure?
Only one volume of a solution
What do burettes measure and let you do?
Different volumes + lets you add solution drop by drop
What does the scale on burette show?
Volume of acid/alkali used
Why don’t you use universal indicator for a titration?
Bc you want to see a sudden colour change, an end-point
Name a indicator suitable for titration
- Phenolphthalein
- Litmus
- Methyl orange
What colour is phenolphthalein in acids?
Colourless
What colour is phenolphthalein in alkalis?
Pink
What are titrations used for?
To find out exactly what volume of acid is needed to neutralise a measured volume of alkali (& vice versa)
Describe the method for a titration (6 marks)
- Use pipette + pipette filler to add acid to a conical flask & add indicator
- Use a funnel to fill burette with some alkai of known
concentration - Record initial volume of alkali in burette
- Use burette to add alkali to acid a bit at a time, giving conical flask a regular swirl
- Go slowly when you think end-point (colour change) is about to be reached (when colour lasts longer)
- Record final volume of alkai in burette (read at eye level) and use it + initial reading to calculate volume of alkai used to neutralise acid
Titration: When you’re filling the burette with an acid/alkai, how should you do it and why?
Do it BELOW EYE LEVEL - don’t want to be looking up if some acid spills over
What should your 1st titration be and why?
Rough titration to get an approximate idea of where the solution changes colour (the end-point)
Why should you repeat titrations and find a mean volume?
To increase the accuracy of your titration and to spot any anomalous results
How do you calculate the rate of a reaction?
Amount of reactant used or amount of product formed
Time
What colour is sodium thiosulfate and hydrochloric acid
Colourless
What is produced when you react sodium thiosulfate and hydrochloric acid together?
Yellow precipitate of sulfur
Describe how you can investigate how the concentration of a solution affects the rate of a chemical reaction (i.e. write a method)
- Add set volume of dilute sodium thiosulfate to conical flask
- Place flask on piece of paper with black cross drawn on it
- Add some dilute HCl to flask & start stopwatch
- Time how long it takes for black cross disappear in the cloudy sulfur
- Repeat reaction with different concentrations of HCl (e.g. 0.5, 1, 1.5 mol/dm3)
What is the effect of increasing the concentration of HCl on the rate of reaction - when added to excess of sodium thiosulfate?
Higher concentration = quicker reaction & ∴ less time it takes for mark to disappear
When investigating how the concentration of a solution affects the rate of a chemical reaction - give a hypothesis
As the conc. of acid increases, the solution will go more cloudy
Measuring Rates of Reaction: The faster the cross disappears…
the quicker the reaction
Measuring Rates of Reaction: If the reactants are coloured and the products are colourless, what could you do to find the rate of the reaction?
Time how long it takes for the solution to lose its colour
On a rate of reaction graph, what is normally the x-axis?
Time
On a rate of reaction graph, what is normally the y-axis?
Amount of product formed OR amount of reactant used up
How do you find a mean rate of a (whole) reaction on a graph?
(Overall) change in y
(Overall) change in x
How do you find the rate of reaction at a particular point in time?
You have to find the gradient of the curve at that point: Draw tangent to curve and find the gradient
What is the test for alkenes?
Add bromine water to alkene and it will go from orange to colourless
Why do alkenes go colourless when added to bromine water?
Bromine is added across double bond
What is the test for alkanes?
Add bromine water to alkane and it will remain bright orange
Describe a method for paper chromatography
- Draw line near bottom of filter paper in pencil
- Add spot of ink to line and place sheet in beaker of solvent
- Make sure ink isn’t touching solvent
- Place lid on top of container
- Solvent will seep up paper, carrying ink with it
- When solvent has nearly reached top of paper, take paper out of beaker & leave it to dry
During paper chromatography, why should you draw the line using pencil?
Pencil marks are insoluble (won’t dissolve in solvent)
During paper chromatography, why should you make sure ink isn’t touching the solvent?
Don’t want ink to dissolve into solvent
During paper chromatography, why should you place lid on top of container?
To stop solvent from evaporating
Why will the dyes in ink separate out in paper chromatography?
Because each different dye in ink moves up paper at different rate
What will happen if any dyes in the ink are insoluble (in the solvent used) in paper chromatography?
They’ll remain on the baseline
What is the end result (i.e. pattern of spots) called in paper chromatography?
Chromatogram
