A2 physical practicals 7-9

Rates,pH,electrode potential

Explain how you can work out pKa from graph

Go to half equivalence

HA=A- conc equal

pH=pKa

State why calibrating pH meter just before it is used improves accuracy of measurement

Over time probe readings become less accurate

Describe how would would obtain pH curve for the titration

• Measure pH of acid

• Small amounts of known volumes of alkali

• Stir

• Measure pH after each addition -washing im between

• Add until hydroxide in excess

• Add in smaller increments near end point

Suggest an improvement a student could make to increase the accuracy of the end point when using an indicator instead of pH probe

Add dropwise near end point

Outline simple method for calibrating a pH probe

• Measure pH of series of bufffers with range of known pH

• Wash distilled water between each use-prevent contamination

• Plot calibration curve with measured pH on x and actual on y

• Draw line of best fit

• For each reading in the titration, refer to the line of best fit and adjust reading accordingly
  

Explain how you can use curve to choose suitable indicator

Steep vertical line where colour change occurs

Suggest suitable piece of apparatus could be used to measure out sodium hydroxide

Explain why this apparatus more suitable than pipette

Burette

Can deliver variable volumes

Explain why end point would be difficult to judge accurately-no steep

Ph change gradual

Indicator colour change not rapid

Suggest why 1cm3 portion of reaction mixture (propanone,HCl and iodine) to an excess of NaHCO3

NaHCO3 neutralise the acid

Stop reaction as it is a I catalysed

Suggest why the order with respect to x can be ignored when testing with y

Significantly larger conc of x than y

Assume x is constant any change is neglicable

Explain why reaction rate is fastest at start

Higher conc of reactants

Greater frequency of successful collisions

State the order of straight line graph through origin with conc of reactant against rate

First

Straight line

Directionally proportional

Explain why using a large excess of x means reaction can depend only on conc of y

Concentrations almost constant

Changes have negligible impact

ZERO ODER

Samples of reaction mixture removed at time interval to determine con

State and explain what must be done to each sample

Quench stop reaction

Ice slow ror/Dilute/

How dos a conc of reactant vs time graph with straight line \

Gradient constant

As reactant used up

So has no effect on rate

Explain how you could use a series of experiments to determine order with respect to A

A+B—>D+E

X react rapidly with E which is dark blue

Measure volumes of reagent

Measure known volume of x

Into separate containers

Start clock at point of mixing

Time recorded for appearance of blue until cross disappears

Keep b conc and c conc constant keep volume of x constant

Control temp-water bath

Repeat with different conc of A

1/t as measurement. Of rate

Plot rate against A use graphs to determine order

Purpose of salt bridge

Connect the circuit

Allow ions to move between electrodes

KNO3

Give essential conditions for SHE

1 mol dm3 HCL

H2(g) 100kpa

298K pt

State change made to apparatus to allow cell reaction to go to voltmeter

Replace voltmeter with bulb