Terms in this set (36)
The method used to ensure that all the water of crystallisation in a sample has been driven off is known as __________.
Heating to constant mass
Why is a lid put on a crucible?
To improve the accuracy of the experiment, as it prevents the loss of solid, but is loose-fitting enough to gases to escape.
Suggest a general safety precaution for handling irritant substances:
Suggest a general safety precaution for handling corrosive substances:
Suggest a general safety precaution for handling flammable substances:
Keep away from naked flames
Suggest general safety precautions for handling toxic substances:
Wear gloves to avoid skin contact, and wash hands after handling
Suggest general safety precautions for handling oxidising substances:
Keep away from flammable/easily oxidised materials.
Suggest sources of error when using a gas syringe:
- The gas escapes before the bung is inserted
- The syringe sticks
- Some gases such as carbon dioxide and sulphur dioxide are soluble in water
Suggest the steps involved in creating a solution of known concentration using a given sample of a solid:
- Weigh sample bottle containing solid using a (2dp) balance
- Transfer sample to beaker
- Reweigh empty bottle to find mass of solid added
- Dissolve in ~100cm³ of distilled water
- Stir with a glass rod until the solid has completely dissolved
- Use a funnel to transfer the solution into a 250cm³ volumetric flask
- Use a wash bottle with distilled water to wash glass rod, beaker and funnel into flask
- Make up to 250cm³ with water (use pipette when meniscus is close to mark)
- Stopper and invert the flask at least 20 times to mix
Why might there be a greater source of error when creating standard solutions of one compound than another?
Dark liquids (e.g. KMnO₄) can make it hard to see the meniscus.
Why shouldn't hot solutions be put in volumetric flasks?
The heat will cause the flask to expand, so it will have a slightly higher volume than normal.
Why is a conical flask preferred over a beaker when performing a titration?
It is easier to swirl the mixture without spilling the contents.
Describe how to carry out an acid-base titration:
- Rinse the equipment (conical flask = distilled water, burette = acid, vol. pipette = alkali)
- Use the volumetric pipette to add a known volume of alkali to the conical flask
- Touch surface of alkali with pipette to remove last drop
- Add acid to burette, and open tap until jet space is filled with acid
- Add indicator [NAME INDICATOR HERE, AND STATE COLOUR CHANGE]
- Use white tile to make colour change easier to observe
- Gradually add the acid to the alkali, whilst swirling, and add the acid dropwise towards the end point
- Note burette reading before and after addition of acid, and use to find titre at end point
- Repeat until at least 2 concordant results
Titres are concordant if...
...the two readings are within 0.1cm³ of each other.
"Titrate solution A with solution B" means that the burette contains solution ____ and the conical flask contains solution ____.
Burette: Solution B
Conical Flask: Solution A
When performing an acid-base titration, why should the burette be rinsed with acid?
The burette should be rinsed with the substance it is to be filled with as otherwise this may be diluted by residual water, or react with substances from a previous titration, which would reduce the concentration and hence increase the titre.
How much indicator should be used in an acid-base titration? Why?
A few drops, as indicators are typically weak acids themselves, so if too much is added, it will affect the titration result.
What does it mean for the results if titres are concordant?
It means that the results are accurate and repeatable, and that the titration technique is good and consistent.
In manganate titrations, the acid(s) that should be used is/are:
Dilute sulphuric acid
Manganate ions are titrated with ethanedioate ions. Suggest one modification to the typical titration procedure to make this titration more accurate, and suggest why it is needed.
The conical flask should be heated to ~60⁰C, as the initial reaction has a very high activation energy (as both ions are negatively charged).
What is the difference between a reading and a measurement?
A reading is found from a single judgement, whilst a measurement is the difference between the values of two readings.
What is the uncertainty of a burette?
Suggest sources of error when determining the enthalpy change when two solutions are mixed
- Energy transfer to/from the surroundings
- It is assumed that the mixture has the same SHC as water
- Ignoring the SHC of the calorimeter
- The reaction may be slow or incomplete
- The density of the solution is assumed to be the same as that of water
Why can't the enthalpy change involved in converting an anhydrous salt to a hydrated salt be measured directly?
- It's impossible to add the exact amount of water required without dissolving the salt
- It is difficult to measure the temperature change of a solid
Suggest sources of error when conducting a flame calorimetry experiment:
- Energy loss from calorimeter
- Incomplete combustion of fuel
- Incomplete energy transfer
- Evaporation of fuel after weighing
- Heat capacity of calorimeter not accounted for
- Measurements are taken under non-standard conditions (water is gaseous, rather than liquid, in the experiment)
Give the balanced symbol equation for the reaction of sodium thiosulphate with hydrochloric acid:
Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O
Why can 1/t approximate the rate of reaction in the reaction between sodium thiosulphate and hydrochloric acid?
It is assumed that the amount of sulphur required in order to block out the cross is constant, hence the change in sulphur does not change and so only the time to reach this concentration is relevant.
What is varied in the reaction when investigating the rate of reaction between sodium thiosulphate and hydrochloric acid? How is it controlled?
The temperature. This is varied by using a water bath and using the average temperature of the solution (average of start and end temperatures).
What is a stop bath? Why is it used?
A container of sodium carbonate solution that is used to neutralise the HCl and SO₂ in the 'disappearing cross' practical. It contains phenolphthalein to indicate when the Na₂CO₃ has been used up and needs replacing.
Suggest two reasons why the hazards regarding SO₂ in the rate experiment involving sodium thiosulphate and hydrochloric acid are considered to be low.
- The maximum amount of SO₂ produced is low (too low to be harmful, particularly in a well-ventilated area)
- SO₂ is soluble in water, so less is available to be inhaled than is produced
How can we test for ammonium ions?
- Add NaOH to the test solution
- Warm gently using a water bath
- Test the resultant fumes by holding damp red litmus paper in the mouth of the test tube (if it turns blue, NH₄⁺ is present (as NH₃ has been produced))
When should filtration under reduced pressure be used over gravitational filtration?
Gravitational: Small amounts of solid
Reduced Pressure: Large amount of solid
Why is water directed against gravity in a Liebig condenser?
It prevents back flow and makes the cooling more efficient.
Suggest the hazards associated with K₂Cr₂O₇
- Category 2 carcinogen
Suggest how acid impurities can be removed from an organic product:
Add NaHCO₃ solution, which will react with the acid and neutralise it. A separating funnel should be used to mix the solutions (venting regularly), then the aqueous layer can be removed.
Suggest two examples of drying agents:
Anhydrous sodium sulphate