| what is a volumetric titration | titrating with an alkali or acid of known concentration using a suitable indicator to show when the end point is reached |
| what are the volumes of reactant accurately measured by in a titration? | a pipette or burette |
| what is a standard solution | a solution of which the concentration is accurately known |
| describe how to make up a standard solution | calculate the quantity of solute required by using n=cV and n=m/gfm
weigh out the solute by difference using an electronic balance and transfer the solid to a beaker or conical flask
dissolve the solute in a small volume of water, smaller than the final volume to be made up.
transfer the the solution quantitatively using a filter funnel to a volumetric flask of the required size.
add water to the volumetric flask so that it is approximately 1cm below the mark on the neck of the standard flask, and make it up to the mark with a dropper so that the meniscus sits on the mark
stopper the flask and invert it to mix. |
| what is meant by a quantitative transfer? | transfer of solution without any loss.
this is done by rinsing the beaker/conical flask, filter funnel and stirring rod after the initial transfer with a small volume of water, and then transferring the washings to the standard flask.
this can be repeated several times as long as the volume of the standard flask is not exceeded. |
| what is meant by a self-indicating reaction | when during a titration, a colour change involving one of the reactants indicates the end point of the reaction. |
| what is meant by chromatography? | a technique which allows chemists to separate substances in complex mixtures, analysing what they are made up of.
these substances are separated as they travel in a mobile phase which passes through a stationary phase. |
| what factors affect the distance a substance travels during chromatography? | the size of the moleculelarger molecules take longer to move whereas smaller molecules are more mobile.polarity of the moleculepolar molecules will be more strongly attracted to polar solvents and so will move faster when a polar is solvent is used as opposed to a non polar solvent. |
| what is the stationary phase in paper chromatography? | a sheet of chromatography apper |
| what is the mobile phase in paper chromatography | an aqueous (water based) liquid or a non aqueous (carbon based )organic solvent |
| what is the first step in paper chromatography? | a small sample of the mixture is spotted on the base line (this is usually a line drawn in pencil) and the paper is placed in a solvent so the spots are just above the level of the solvent |
| what does overall separation of the components of the mixture depend on as the solvent moves up the paper during chromatography? | overall separation of the components of the mixture depends on how strongly attracted the chemicals are to the mobile and the staionary phase |
| how can the components of a mixture be identified during chromatography? | by comparing them to reference samples
by working out an Rf value |
| how do you calculate an Rf value? | by measuring with a ruler, the distance the spots have moved, and dividing that by the distance of the solvent front. |
| what is meant by solvent front during chromatography | how far up the paper the solvent has moved. |
| where can you find the Rf values for experiments carried out with the same solvent? | they can be looked up in a data book to identify the unknown substances in the mixture |
| in what case during paper chromatography would you use 2 way (2 dimensional) chromatography? | if it is hard to identify components as the spots overlap |
| how would you identify the components of a mixture using two way chromatography? | carry out the chromatography experiment, then flip your paper 90 degrees and repeat with reference samples and then place on sheet on top of another on a light box to compare the positions of the reference and the unknown spots |
| how is paper chromatography used to analyse colourless mixtures such as amino acids? | by breaking down (hydrolysing) the protein into its constituent amino acids
the hydrolysed mixture is then spotted onto chromatography paper with reference samples. Since none of the amino acids have any colour the paper is removed from the solvent when the solvent front is almost at the top of the paper
it is then hung in a fume cupboard to dry and sprayed with a disclosing agent which causes the amino acids to show up as coloured spots
after the chromatogram is sprayed, the paper is placed in a warm oven and the amino acids show up as blue pink or brown spots
OR
the chromatogram is looked at under uv light since most colourless organic molecules are visible under uv |
| what is thin layer chromatography (TLC)? | it is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert substance such as silica supported on a flat unreactive surface like a glass plate. |
| what is the advantage of TLC over paper chromatography | TLC is faster and there is no paper to become soggy |
| describe gas-liquid chromatography (or gas chromatography/GLC) | In GLC the mobile phase is an inert gas like helium which will not react with the chemicals being analysed
the stationary phase is a very thin layer of a high boiling point inert liquid absorbed onto an inert solid support like beads of silica packed into a thin long tube |
| what are the advantages of GLC compared to TLC and paper chromatography? | GLC is better at separating complex mixtures than TLC or paper chromatography since it is more sensitive, allowing the determination of what chemicals and how much of each chemical there is in the mixture |
| describe a GLC experiment | The mixture to be analysed is injected into the stream of gas. As it passes along the column (thin long tube) it separates into the different substances. Substances with a greater attraction for the mobile phase reach the detector at the end of the column more quickly. Substances with more attraction for the stationary phase move more slowly through the column. |
| describe a gas liquid chromatogram | a gas liquid chromatogram usually shows the retention time along the x axis and the strength of response or the quantity of the component along the y axis. |
| what is retention time | the time that a substance takes to pass through the column in GLC |
| what pieces of information can be gathered from a gas liquid chromatography? | the number of compounds in the mixture - represented by the number of peaks
how much of each compound is present - represented by the height of the peak (higher=more)
retention time - indicated by the position of the beak |
| how can GLC be used by forensic scientists | to try to determine the cause of a house fire where arson using petrol is suspected
A sample of the material is placed in a sealed bag which is then heated in an oven. Any volatile components on the material will turn to gas and then can be removed using a hypodermic syringe. These are then injected into the injection port of the GLC.
Samples of petrol are then analysed in the same way and the chromatograms are compared. If the peaks match (retention times of some of the peaks are the same) this would be evidence that petrol may have been used to start the fire |
