Balanced equations and associated calculations

What are the different types of yield?

• percentage yield

• theoretical yield

• actual yield

what is the percentage yield?

how much product was actually made compared with the amount of product that was expected

what is the theoretical yield?

the maximum mass of product expected from the reaction, using moles

what is the actual yield?

the mass of the product that is actually obtained from the real chemical reaction

what is the equation for percentage yield?

(actual yield / theoretical yield) x 100

can the percentage yield be more than or equal to 100% ?

no

why is percentage yield never 100% ?

• reaction may be reversible

• some of the product may be lost when it is separated from the reaction mixture

• side reactions may occur and give bye-products which may be unwanted

what is atom economy?

the amount of starting materials that end up as useful products

why is a high atom economy important in the chemical industry?

• to minimise waste of non-renewable reactants

• to make as much useful product as possible

• to reduce pollution from waste products

• to minimise energy use in heating chemical reactions and running of factories (saves money)

• to produce chemicals cheaper making them available to more people

• reactions are more sustainable (use less of the Earth’s resources)

what is the equation for atom economy?

(mr of desired products / sum of mr of all reactants) x 100

what is concentration?

• the number of moles of solute per decimetre cubed of solution

• the amount of solute in a given volume of solution

what is the unit for concentration?

mol dm^-3

what is volumetric analysis?

finding the concentration of a solution by measuring the volume of a solution that will react with a known volume of standard solution

what is a standard solution?

a solution for which the concentration is known

what can standard solutions be used for?

to find the concentration of solutions of other reagents

what is the purpose of an acid-base titrations?

to calculate the concentration of sodium hydroxide using the known concentration of hydrochloric acid

describe how to carry out a titration?

• rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)

• pipette 25 cm3 of alkali into conical flask

• add acid solution from burette

• make sure the jet space in the burette is filled with acid

• add a few drops of indicator and refer to colour change at end point

• use a white tile underneath the flask to help observe the colour change

• add acid to alkali whilst swirling the mixture and add acid dropwise at end point

• note burette reading before and after addition of acid

• repeat titration until at least 2 concordant results are obtained

when is methyl orange used?

if HCl is used

what is the colour change with methyl orange?

• yellow in alkali

• red in acid

when is phenolphthalein used?

if NaOH is used

what is the colour change with phenolphthalein?

• colourless in a neutral solution and acid

• pink in an alkaline solution

what unit must volume be in?

decimetres cube

what are the safety precautions used in titrations?

• acids and alkalis are corrosive (at low concentrations acids are irritants)

• wear eye protection and gloves

• if spilled then immediately wash affected parts

• if the substance is unknown treat it as potentially toxic and wear gloves

what happens if the jet space is not filled properly prior to commencing the titration?

it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading

why is a conical flask used in preference to a beaker?

it is easier to swirl the mixture in a conical flask without spilling the contents

why do we only add a few drops of indicator?

indicators are generally weak acids so if too much is added it will affect the titration results

what values are used to work out an average titre result?

never use the rough titration and only make an average of the concordant titre results

what does it mean about the titration if 2 or more results are concordant?

results are accurate and repeatable and the titration technique is good/ consistent

how should the results of a titration be recorded?

• results should be clearly recorded in a table

• result should be recorded in full (i.e. both initial and final readings)

• record titre volumes to 2dp

why is distilled water added to the conical flask during a titration?

to wash the sides of the flask so that all the acid on the side is washed into the reaction mixture to react with the alkali

does adding distilled water to the conical flask during a titration affect the titration reading? why?

no, as water does not react with the reagents or change the number of moles of acid added

what compounds are good for treating excess acid in the stomach?

• sodium hydrogen carbonate (NaHCO3) and calcium carbonate (CaCO3) - they are not corrosive and will not cause a hazard if used in excess

• they have no toxicity if used for indigestion remedies. however the co2 produced can cause wind

• magnesium hydroxide (Mg(OH)₂) - has low solubility in water and is only weakly alkaline so not corrosive or dangerous to drink. it will also not produce any co2 gas

what readings are taken in a titration to calculate uncertainty?

the values found from a single judgement when using a piece of equipment

what measurements are used from a titration to calculate uncertainty?

the values taken as the difference between the judgements of two values

what value is the uncertainty of a reading (one judgement) at least of the smallest scale reading?

±0.5

what value is the uncertainty of a measurement (2 judgements) at least of the smallest scale reading?

±1

what is the equation for % uncertainty?

± (uncertainty/measurement made on apparatus) x100

how do you calculate the maximum total percentage apparatus uncertainty in the final result of a titration?

add all the individual equipment uncertainties together

how do you decrease the apparatus uncertainties?

• decrease the sensitivity uncertainty by using apparatus with a greater resolution (finer scale divisions)

• increase the size of the measurement made

what happens if the burette used in the titration had an uncertainty for each reading of ±0.05 cm3?

during a titration two readings would be taken so the uncertainty on the titre volume would be ±0.10 cm3. then often another 0.05 is added on because of uncertainty identifying the end point colour change

how do you reduce uncertainties in a titration?

replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty

how do you reduce the % uncertainty in a burette reading?

make the titre a larger volume. this could be done by:

• increasing the volume and concentration of the substance in the conical flask

• decreasing the concentration of the substance in the burette

how do you reduce uncertainties in measuring mass?

• use a balance that measures to more dp or use a larger mass - reduces the % uncertainty in weighing a solid

• weigh the sample before and after addition and then calculate the difference - ensures a more accurate measurement of the mass added

what does it mean if the % uncertainty due to the apparatus < percentage difference between the actual value and the calculated value?

there is a discrepancy in the result due to other errors

what does it mean if the % uncertainty due to the apparatus > percentage difference between the actual value and the calculated value?

there is no discrepancy and all the differences between values can be explained by the sensitivity of the equipment