Chemistry: Chapter 12 - Experimental techniques and chemical analysis

what apparatus is used for measuring time?

stopwatch

what apparatus is used for measuring temperature

thermometer/temperature probe

what apparatus is used for measuring mass

• electronic balance

• triple beam balance

what apparatus is used for measuring volume?

• measuring cylinder

• burette

• volumetric pipette with pipette filler

what are the advantages and disadvantages of using a stopwatch?

advantages: high level of precision to the nearest 0.01s

disadvantages: dependent on the reaction time of the user, which can can human reaction time error

what are the advantages and disadvantages of using a thermometer?

advantages: easy to use and cheap

disadvantages: limited temperature ranges and can only measure to the nearest 0.5° which makes it less precise than a digital temperature probe.

what are the advantages and disadvantages of using a measuring balance?

advantages: speed readings to a high level of accuracy (nearest 0.001g for digital, 0.1 for triple beam)

disadvantages: can be affected by air currents and other environmental factors

what are the advantages and disadvantages of using a burette?

advantages: wide ranges of burettes so it can measure accurate volumes (to the nearest 0.1cm³)

disadvantages: parallax error can occur if reading of the meniscus is taken incorrectly

what are the advantages and disadvantages of using a volumetric pipette?

advantages: can accurately measure a fixed volume consistently

disadvantages: limited to measuring only one fixed volume

what apparatus is used to measure the volumes of gases?

gas syringe

what are the advantages and disadvantages of using a gas syringe?

advantages: measures volumes of a gas to a high level of accuracy

disadvantages: sensitive to environmental factors such as temperature or pressure changes

define solvent

a substance that dissolves a solute

define solute

a substance that is dissolved in a solvent

define solution

a mixture of one or more solutes dissolved in a solvent

define saturated solution

a solution containing the maximum concentration of a solute dissolved in the solvent at a specific temperature

define residue

a substance that remains after evaporation, distillation, filtration or any similar process

define filtrate

a liquid or solution that has passed through a filter

what are the apparatus needed for acid-base titrations?

• burette

• volumetric pipette

• pipette filler

• conical flask

• suitable indicator

• alkali of known concentration

• acid of unknown concentration

• clamp stand

• white tile

• funnel

how would an acid-base titration be carried out?

1. use the pipette filler and volumetric pipette to add 25cm³ of NaOH solution to the conical flask

2. fill the burette with sulfuric acid

3. place an empty beaker underneath the tap and run a small volume of acid to prevent any air bubbles

4. record the initial volume on the burette to the nearest 0.05cm³

5. place the conical flask underneath the tap on top of a white tile

6. add 3 drops of the indicator into the conical flask and swirl gently to mix

7. run the acid from the burette into alkali, whilst swirling the solution, until endpoint is reached

8. record the final volume on the burette in your table to the nearest 0.05cm³

9. calculate the rough titre

10. repeat the experiment until 2 concordant titres are found

why is a white tile used in titrations?

a white tile is placed underneath the conical flask to allow any colour changes in the indicator (the endpoint) to be more visible

how is the titre volume calculated?

final volume - initial volume = titre volume

what are concordant results?

results that are 0.10cm³ of each other excluding the rough titration, at least 2 of the results are used to calculate the mean titre

describe how to identify the end-point of a titration using an indicator

a sudden colour change will be visible when the solution goes from acid to alkali

what is paper chromatography used for?

separate mixtures of soluble substances, using a suitable solvent

how does separation occur in paper chromatography?

there is a stationary phase (chromatography paper) and a mobile phase (solvent)

substances have different solubilities in the mobile phase so will travel at different rates causing separation. more soluble substances will travel further

how could paper chromatography be used to separate an unknown substance by comparison with known substances?

• draw a pencil line 2cm from the bottom of chromatography paper

• place one dot of 3 known substances and one dot of the unknown substance. label each dot

• place the chromatography paper in a beaker containing 1cm of water

• wait for the water to travel up most of the paper and then remove the paper from the beaker and mark the height reached by the solvent. dry the paper

• observe the chromatogram and record results

why should pencil be used to draw the line along the bottom of the chromatography paper?

it is insoluble in the solvent so it will not affect the experiment. pen ink would dissolve along the substance being tested and disrupt the chromatogram

why should the solvent in the beaker be no deeper than 1cm for paper chromatography?

if it is deeper, it will wash away the substances on the chromatography paper

in paper chromatography, what is the stationary phase?

the chromatography paper

in paper chromatography, what is the mobile phase?

the solvent

what 2 things affect how long the molecules spend in each phase in paper chromatography?

• their solubility in the mobile phase

• their attraction to the chromatography paper

What is an Rf value?

Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent

how do you calculate Rf values?

in paper chromatography, what affects the Rf value of a substance?

the solvent, repeating the experiment with a different solvent will change the Rf value

when measuring the distance moved by a substance on the chromatography paper, where should you measure between?

from the pencil baseline to the middle of the spot of the substance

how many spots will be observed on a chromatogram of a pure substance

one

how can you identify that two mixtures contain a substance which is the same using a chromatogram?

both mixtures will produce different chromatograms but the position of one spot will match exactly

how does solubility affect the distance a substance travels in paper chromatography?

a substance that is more soluble in the mobile phase will travel further up the chromatography paper

how can paper chromatography be used if a mixture contains colourless substance?

using locating agents. after the chromatogram has been produced, it is treated with a locating agent to make the spots visible

how can a pure substance be identified using melting or boiling points?

pure substances have a sharp, exact melting and boiling point whereas impure substances will melt/boil over a range of temperatures

what method can be used to separate an insoluble substance from a solution?

filtration:

• put a piece of filter paper into a funnel and place over a conical flask

• pour the mixture into the funnel so that the liquid collects in the beaker. the insoluble salt is left on the filter paper

• pour deionised water into the funnel to wash any of the solution from the salt

• leave the salt to dry on the filter paper

how can a soluble substance be separated from a solution?

crystallisation:

• place the solution in an evaporating basin

• warm the solution gently so that the solvent starts to evaporate and the concentration of the solution increases

• remove from the heat and allow the mixtures to cool before all the solvent evaporates

• leave the evaporate without heating. dry and collect the crystals

when is simple distillation used as a separating technique?

to separate a solvent from a solution

describe how to separate a mixture using simple distillation

• pour the mixture into a round bottomed flask and connected to a condenser and place a beaker at the outlet

• slowly heat the flask until the solvent starts to vaporise. the solvent will evaporate first then condenses in the condenser before being collected in the beaker

what mixture is fractional distillation commonly used to separate?

a mixture of miscible liquids

describe how fractional distillation separates a mixture of miscible liquids

• the mixture is heated until it evaporates

• the vapours enter a fractionating column. the column has a temperature gradient (hottest at the bottom).

• the vapours rise up the column and substances condense at different fractions depending on their boiling points.

how can you identify aluminium cation (Al+3) using aqueous sodium hydroxide

a white precipitate is formed, which is soluble in excess giving a colourless solution

how can you identify aluminium cation (Al+3) using aqueous ammonia

white precipitate is formed which is insoluble in excess

how can you identify ammonium cation (NH+4) using aqueous sodium hydroxide

ammonia produced on warming

how can you identify calcium cation (Ca+2) using aqueous sodium hydroxide

white precipitate is formed which is insoluble in excess

how can you identify calcium cation (Ca+2) using aqueous ammonia?

no precipitate or slight precipitate is formed

how can you identify chromium (iii) cation (Cr+3) in aqueous sodium hydroxide

green precipitate is formed that is soluble in excess

how can you identify chromium (iii) cation (Cr+3) in aqueous ammonia

green precipitate is formed that is insoluble in excess

how can you identify copper (ii) cation (Cu+2) in aqueous sodium hydroxide?

light blue precipitate is formed that is insoluble in excess

how can you identify copper (ii) cation (Cu+2) in aqueous ammonia?

light blue precipitate is formed that is soluble in excess, giving a dark blue solution

how can you identify iron (ii) cation (Fe+2) in aqueous sodium hydroxide?

green precipitate is formed that is insoluble in excess, the precipitate turns brown near the surface on standing

how can you identify iron (ii) cation (Fe+2) in aqueous ammonia

green precipitate is formed that is insoluble in excess, precipitate turns brown near surface on standing

how can you identify iron (iii) cation (Fe+3) in aqueous sodium hydroxide?

red-brown precipitate is formed that is insoluble in excess

how can you identify iron (iii) cation (Fe+3) in aqueous ammonia?

red-brown precipitate is formed that is insoluble in excess

how can you identify zinc cation (Zn+2) in aqueous sodium hydroxide?

white precipitate is formed that is soluble in excess, giving a colourless solution

how can you identify zinc cation (Zn+2) in aqueous ammonia?

white precipitate is formed that is soluble in excess, giving a colourless solution