Physical practicals

Give method for preparing a 250cm3 standard solution

• Accurately weigh out dry sample of X using high precision balance

• Transfer to beaker and re-weigh boat to determine mass transferred-weigh by difference

• Dissolve X in small volume of distilled water ,glass stirring rod

• Transfer to volumetric flask and rinse all equipment,funnel,beaker,stirring rod all rinsing ends up in flask

• Add distilled/deionised water up to calibration line 250cm3

• Use dropping pipette at end dropwise

• Eye level to see meniscus

• Stopper bottle and invert 10 times

Explain why you invert flask multiple times

• ensure concentration is same throughout

Give a method for carrying out a volumetric titration

• Rinse Burette with reagent known concentration (NaOH)

• Overfill buirette sodium hydroxide and pour some out

• Use volumetric pipette to transfer 25cm3 of hcl to conical flask

• Add 2 drops of indication/phenolhtalein-small amount otherwise react

• Slowly add NaOH to acid swirling constantly

• Rinse buirette and flask with distilled water in wash bottle

• Stop at first permanent colour change,record volume

• Repeat until get concordant data 0.1

• Calculate mean titre from results

• Moles=cxv to find moles of NaOH ,use ratio to find moles of other substance /hcl

Explain why you rinse the Burette with solution

Remove impurities

• water left over  would dilute NaOH so concentration is still known

Why must fully fill the tap of Burette

• so jet space/spout is filled -first few cm3 not in conical flask would be used to fill spout -inaccurate reading

Ensure accurate volume measurement

Air bubbles mean volume added is less than measured

Why do we use a conical flask for a titration

Prevent splashing from swirling

Why do we add small amount of indicator

React with reactant impact results

Indicator does not affect pH

Why do we wait for permanent colour change

Know end point reached

Ensures reaction complete

What value do results have to be to be concordant

0.1

Why do we rinse conical flask with deionised water

Does not affect number of moles present

Dilutes the solution no effect on titre

Why do we remove the funnel

Prevent extra solution dropping into Burette which would change titre volume

How to decrease uncertainty of Burette

More dilute solution in the conical flask/burette

So larger titre volume needed

Describe how to measure Enthalpy change (solution) and describe how analyse results

• Measure 50cm3 of water using a Burette

• Transfer to polystyrene cup with lid

• Measure 4g copper sulfate using accurate/high precision 2dp balance

• Use precise thermometer to take temeprature of water each minute for 4 minutes -accurate initial temperature

• Put copper sulfate into water at minute 5-no recorded value

• Not rinse weighing boat

• Re weigh weighing boat to establish accurate mass tansfered

• Replace lid and stir

• Continue to record temp at minute interval up to 15 minutes

• Plot temp on Y time on X

• Two lines of best fit and extrapolate values to point of addition find change in temp

Analysis

• Q=mcdT

• Q in joules to kilojoules divide by 1000

• Divide by moles

• Accurate mass transferred from weighing divided by mr to find moles

Improvements which can be used for RP2 and explanations

• Plot graph of temperature against time and extrapolate to point of addition-more accurate measurement

• Re weigh weighing boat-accurate mass

• Burette -volume of solution added more accurate less uncertainty

• Polystyrene cup-reduce heat loss to surroundings

• Take temp of solution before addition-accurate initial temperature

Give a reason that the Enthalpy of combustion of liquid alkane is different from value obtained

Alkane not gaseous

How to reduce percentage uncertainty in temperature change

Higher concentration of solutions

so temperature reading larger

How calculate x when two solutions rather than combustion experiment

Work out moles of each solution-usually same moles ,nothing limiting-moles that is used in Q/N

Q=MC🔼T

Combined volume =mass of solution that is M