practical skills Flashcards
Methods of collecting gases
There are four ways to collect gases produced in a chemical process.
Each method is different and depends on the nature of the gas and experiment.
Downward delivery is used when the gas being collected is denser than air.
The opposite method is called upward delivery and is used when the gas is less dense than air.
A gas syringe can be used to collect and measure the volume of a gas.
Collection over water is used when the gas being collected is insoluble in water.
Methods of drying gases
and common drying agents
In many cases the gas we are collecting needs to be dried.
This can be done by passing the gas through a drying agent which removes any water present.
Depending on the gas being collected, a different drying agent may be used.
Three common drying agents are:
Concentrated sulfuric acid which can dry all gases except ammonia as neutralisation occurs.
Anhydrous calcium chloride can dry all gases except for ammonia as it forms calcium chloride.
For ammonia and neutral gases, calcium oxide is the preferred drying agent.
To dry a substance in Chemistry means to…
remove any water molecules that may be present.
Water can be removed from liquids in this way, and even though in liquid form, they are referred to as dried.
Mixtures of solids
Methods of Separation and Purification based on
Differences in density, magnetic properties, sublimation and solubility can be used.
For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities.
Methods of Separation and Purification
Mixtures of liquids
mmiscible liquids can be separated using a
Examples include when an
separating funnel or by decanting (pouring carefully).
organic product is formed in aqueous conditions.
separating funnel drawing
Filtration
method and drawing
Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water).
Crystallisation
method and drawing
The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind.
The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow.
Crystals are collected by filtering the solution.
They are then washed with cold, distilled water to remove impurities and allowed to dry.
Simple Distillation
The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask.
The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H2O that is collected in a beaker.
Fractional distillation
The solution is heated to the temperature of the substance with the lowest boiling point.
This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker.
For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker.
When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated.
For the distillation of volatile liquids, it is often recommended to use an…. rather than the naked flame of a bunsen burner as volatile organic liquids tend to be….
This is a typical exam question on safety and hazards that often comes up.
electric heater
flammable.
Retention factor (Rf) values
The Rf value of a particular compound is always the same.
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
Calculation
Retention factor = distance moved by compound ÷ distance moved by solvent.
The Rf value is a ratio and therefore has no units.
Locating agents
Locating agents are substances which react with colourless samples and produce a coloured product which is then visible.
The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye.
The agent is sprayed on and gently heated for a few minutes to take effect.
