In CP9, if the pH at the half-equivalence point is 4.75, what is the K a of the acid?

The K a = [H + ] = 10 −4.75 = 1.8 × 10 −5 mol dm −3 . At the half-equivalence point, pH = p K a , so K a is read directly from the pH.

What is the half-equivalence point ?

The half-equivalence point is the point in a weak acid titration at which half the acid has been neutralised, where pH = p K a and K a = [H + ].

In CP9, why is a second 25 cm 3 portion of ethanoic acid added after the titration end-point?

The second 25 cm 3 portion of acid is added to create a half-neutralised solution at the half-equivalence point, where the pH equals the p K a of the acid.

At the half-equivalence point, .......... = [H + ], so p K a = .......... .

At the half-equivalence point, K a = [H + ], so p K a = pH .

In CP9, what indicator is used and what colour change signals the end-point?

Phenolphthalein is used; it turns pink at the end-point when the acid is exactly neutralised by the sodium hydroxide.

p K a is the negative logarithm (base 10) of the acid dissociation constant K a : p K a = −log K a .

Why must the pH probe or meter be calibrated before use in CP9?

Calibration ensures the pH meter gives accurate readings; without it, systematic error in the pH measurement leads to an inaccurate value of K a .

True or False? In CP9, the burette reading introduces double the uncertainty of the pipette reading.

True. The pipette gives a single reading, contributing one uncertainty. The burette requires an initial and a final reading, so its uncertainty is doubled compared to the pipette.

State two sources of uncertainty in the CP9 determination of K a .

The pipette measurement (single reading uncertainty). The burette measurement (double uncertainty — initial and final readings). The judgement of the titration end-point is also a source of uncertainty.

What is a salt bridge in an electrochemical cell?

A salt bridge is a strip of filter paper soaked in saturated potassium nitrate solution that connects two half-cells, completing the circuit while preventing direct mixing of the half-cell solutions.

In CP10, why must metal strips be freshly cleaned with sandpaper before use?

Freshly cleaning removes oxide coatings from the metal surface; oxide layers would interfere with electron transfer and give inaccurate EMF readings.

True or False? Results from CP10 are expected to match theoretical standard electrode potential values exactly.

False. Theoretical values are obtained under standard conditions (298 K, 1 mol dm −3 solutions, 100 kPa), which are difficult to achieve in a school laboratory. The relative EMF values between cells should still match theory.

What is the EMF of an electrochemical cell?

The EMF (electromotive force) is the potential difference between the two half-cells measured under open-circuit conditions, recorded in volts using a high-resistance voltmeter .

In CP10, what does a positive voltmeter reading tell you about the polarity of the metals?

A positive reading confirms the relative polarity of the two metals: the terminal connected to the positive voltmeter lead is at higher potential, indicating which metal is the more positive electrode in the cell.

In CP10, a .......... -resistance voltmeter is used to measure EMF to prevent current from flowing, and the salt bridge is replaced each time to prevent .......... of ions between half-cells.

In CP10, a high -resistance voltmeter is used to measure EMF to prevent current from flowing, and the salt bridge is replaced each time to prevent cross-contamination of ions between half-cells.

In CP10, what concentration and type of solution is used for each metal ion half-cell?

Each half-cell uses a 1.0 mol dm −3 solution of the metal ion — typically a nitrate, chloride, or sulfate depending on the solubility of the salt.

True or False? A higher measured EMF in CP10 indicates a larger difference in reactivity between the two metals in the cell.

True. A larger EMF reflects a greater difference in reactivity between the two metals — i.e., a larger difference in their positions in the electrochemical series.

State two practical tips for obtaining a reliable EMF reading in CP10.

Wait for a steady reading on the voltmeter before recording. If the reading is negative, swap the terminals around rather than assuming the measurement is wrong.

What is a clock reaction ?

A clock reaction is a reaction used in kinetics studies that shows a sharp, dramatic colour change after a fixed period of time, allowing the time to a defined point to be measured precisely.

In CP13a, why is sodium hydrogencarbonate added to each withdrawn portion of the reaction mixture?

Sodium hydrogencarbonate stops the reaction (by neutralising the acid catalyst) in the withdrawn sample, so the concentration of iodine at that moment can be determined accurately by titration.

In CP13a, the concentration of iodine in each sample is determined by titration against .......... solution using .......... as indicator.

In CP13a, the concentration of iodine in each sample is determined by titration against sodium thiosulfate solution using starch as indicator.

In CP13b, what causes the sudden blue-black colour change in the iodine clock reaction?

When all the sodium thiosulfate has been consumed, iodine is no longer removed from solution and builds up in excess. The excess iodine then reacts with the starch indicator to produce the sudden blue-black colour.

True or False? In CP13a, the propanone–iodine reaction is found to be first order with respect to iodine.

False. The concentration–time graph for iodine is a straight line with constant gradient, meaning the rate is independent of iodine concentration. The reaction is zero order with respect to iodine.

In CP13b, how is the rate of reaction calculated from the clock reaction time data?

Rate is calculated as the reciprocal of the time taken for the colour change: rate ∝ 1/ t . A graph of rate versus concentration is then plotted to determine the order of reaction.

In CP13b, the hydrogen peroxide–iodide reaction: H 2 O 2 (aq) + 2I − (aq) + 2H + (aq) → I 2 (aq) + 2H 2 O (l). Adding sodium thiosulfate acts as a .......... timer by reacting with the .......... produced.

In CP13b, the hydrogen peroxide–iodide reaction: H 2 O 2 (aq) + 2I − (aq) + 2H + (aq) → I 2 (aq) + 2H 2 O (l). Adding sodium thiosulfate acts as a reaction timer by reacting with the iodine produced.

In CP13b, what is the order of reaction with respect to potassium iodide, and how is this deduced from the graph?

The reaction is first order with respect to potassium iodide. The rate–concentration graph is a straight line through the origin, showing that rate is directly proportional to [KI] — doubling concentration doubles the rate.

True or False? In CP13a, the stop clock should be paused each time a sample is withdrawn from the reaction mixture.

False. The stop clock should be kept running throughout the practical. The time at which sodium hydrogencarbonate is added to each withdrawn sample is noted, as this is the precise moment the reaction is stopped in that portion.

In CP13a, state the rate equation for the propanone–iodine reaction and explain why iodine does not appear.

Rate = k [CH 3 COCH 3 ][H + ]. Iodine does not appear because the reaction is zero order with respect to iodine — its concentration does not affect the rate.

What is activation energy ?

Activation energy ( E a ) is the minimum energy required for a reaction to occur — the energy that colliding particles must possess for a successful collision leading to products.

In CP14, how is rate of reaction calculated from the experimental time data?

Rate is taken as 1/*t * (the reciprocal of the time for the methyl red indicator to disappear), since all experiments use the same amount of phenol and the indicator bleaches when phenol is consumed.

True or False? In CP14, the two boiling tubes are mixed before they reach the water bath temperature.

False. Both boiling tubes are placed in the water bath and allowed to reach the water temperature before mixing. Mixing too early means the reaction does not occur at the intended temperature, giving inaccurate rate data.

What is the Arrhenius equation ?

The Arrhenius equation relates the rate constant k to temperature: k = A e (− E a /R T ) , or in linear form ln k = − E a /R T + ln A , where A is the pre-exponential factor and R = 8.314 J mol −1 K −1 .

In CP14, why is the experiment repeated at several different temperatures?

Multiple temperatures provide multiple (1/ T , ln k ) data points, allowing an Arrhenius plot (ln k vs 1/ T ) to be drawn and the gradient (− E a /R) to be determined accurately.

In CP14, a graph of ln .......... against .......... gives a straight line with gradient equal to − E a /R.

In CP14, a graph of ln k against 1/*T gives a straight line with gradient equal to − E* a /R.

In CP14, what role does methyl red play in the bromate–bromide reaction?

Methyl red acts as the indicator : it is present throughout the reaction and is bleached by excess bromine as soon as all the phenol has been consumed, marking the end of the reaction and stopping the clock.

True or False? Ice can be used in CP14 to achieve the lowest temperature required for the experiment.

True. The procedure states that ice is used to achieve the lowest required temperature, allowing the experiment to be carried out across a wider temperature range and giving a more reliable gradient on the Arrhenius plot.

In CP14, how is E a calculated once the Arrhenius plot has been drawn?

E a = −gradient × R, where the gradient is the slope of the ln k vs 1/ T graph (in K) and R = 8.314 J mol −1 K −1 . The result gives E a in J mol −1 , which can be converted to kJ mol −1 .

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Advanced Physical Chemistry Core Practicals (Edexcel A Level Chemistry): Flashcards

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FrontFinding Ka Values
In CP9, if the pH at the half-equivalence point is 4.75, what is the K_a of the acid?

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In CP9, if the pH at the half-equivalence point is 4.75, what is the K_a of the acid?
The K_a = [H⁺] = 10^−4.75 = 1.8 × 10⁻⁵ mol dm⁻³. At the half-equivalence point, pH = pK_a, so K_a is read directly from the pH.
What is the half-equivalence point?
The half-equivalence point is the point in a weak acid titration at which half the acid has been neutralised, where pH = pK_a and K_a = [H⁺].
In CP9, why is a second 25 cm³ portion of ethanoic acid added after the titration end-point?
The second 25 cm³ portion of acid is added to create a half-neutralised solution at the half-equivalence point, where the pH equals the pK_a of the acid.
At the half-equivalence point, .......... = [H⁺], so pK_a = .......... .
At the half-equivalence point, K_a = [H⁺], so pK_a = pH.
In CP9, what indicator is used and what colour change signals the end-point?
Phenolphthalein is used; it turns pink at the end-point when the acid is exactly neutralised by the sodium hydroxide.
Define pK_a.
pK_a is the negative logarithm (base 10) of the acid dissociation constant K_a: pK_a = −log K_a.
Why must the pH probe or meter be calibrated before use in CP9?
Calibration ensures the pH meter gives accurate readings; without it, systematic error in the pH measurement leads to an inaccurate value of K_a.
True or False?
In CP9, the burette reading introduces double the uncertainty of the pipette reading.
True.
The pipette gives a single reading, contributing one uncertainty. The burette requires an initial and a final reading, so its uncertainty is doubled compared to the pipette.
State two sources of uncertainty in the CP9 determination of K_a.
1. The pipette measurement (single reading uncertainty).
2. The burette measurement (double uncertainty — initial and final readings).
The judgement of the titration end-point is also a source of uncertainty.
What is a salt bridge in an electrochemical cell?
A salt bridge is a strip of filter paper soaked in saturated potassium nitrate solution that connects two half-cells, completing the circuit while preventing direct mixing of the half-cell solutions.
In CP10, why must metal strips be freshly cleaned with sandpaper before use?
Freshly cleaning removes oxide coatings from the metal surface; oxide layers would interfere with electron transfer and give inaccurate EMF readings.
True or False?
Results from CP10 are expected to match theoretical standard electrode potential values exactly.
False.
Theoretical values are obtained under standard conditions (298 K, 1 mol dm⁻³ solutions, 100 kPa), which are difficult to achieve in a school laboratory. The relative EMF values between cells should still match theory.
What is the EMF of an electrochemical cell?
The EMF (electromotive force) is the potential difference between the two half-cells measured under open-circuit conditions, recorded in volts using a high-resistance voltmeter.
In CP10, what does a positive voltmeter reading tell you about the polarity of the metals?
A positive reading confirms the relative polarity of the two metals: the terminal connected to the positive voltmeter lead is at higher potential, indicating which metal is the more positive electrode in the cell.
In CP10, a ..........-resistance voltmeter is used to measure EMF to prevent current from flowing, and the salt bridge is replaced each time to prevent .......... of ions between half-cells.
In CP10, a high-resistance voltmeter is used to measure EMF to prevent current from flowing, and the salt bridge is replaced each time to prevent cross-contamination of ions between half-cells.
In CP10, what concentration and type of solution is used for each metal ion half-cell?
Each half-cell uses a 1.0 mol dm⁻³ solution of the metal ion — typically a nitrate, chloride, or sulfate depending on the solubility of the salt.
True or False?
A higher measured EMF in CP10 indicates a larger difference in reactivity between the two metals in the cell.
True.
A larger EMF reflects a greater difference in reactivity between the two metals — i.e., a larger difference in their positions in the electrochemical series.
State two practical tips for obtaining a reliable EMF reading in CP10.
1. Wait for a steady reading on the voltmeter before recording.
2. If the reading is negative, swap the terminals around rather than assuming the measurement is wrong.
What is a clock reaction?
A clock reaction is a reaction used in kinetics studies that shows a sharp, dramatic colour change after a fixed period of time, allowing the time to a defined point to be measured precisely.
In CP13a, why is sodium hydrogencarbonate added to each withdrawn portion of the reaction mixture?
Sodium hydrogencarbonate stops the reaction (by neutralising the acid catalyst) in the withdrawn sample, so the concentration of iodine at that moment can be determined accurately by titration.
In CP13a, the concentration of iodine in each sample is determined by titration against .......... solution using .......... as indicator.
In CP13a, the concentration of iodine in each sample is determined by titration against sodium thiosulfate solution using starch as indicator.
In CP13b, what causes the sudden blue-black colour change in the iodine clock reaction?
When all the sodium thiosulfate has been consumed, iodine is no longer removed from solution and builds up in excess. The excess iodine then reacts with the starch indicator to produce the sudden blue-black colour.
True or False?
In CP13a, the propanone–iodine reaction is found to be first order with respect to iodine.
False.
The concentration–time graph for iodine is a straight line with constant gradient, meaning the rate is independent of iodine concentration. The reaction is zero order with respect to iodine.
In CP13b, how is the rate of reaction calculated from the clock reaction time data?
Rate is calculated as the reciprocal of the time taken for the colour change: rate ∝ 1/t. A graph of rate versus concentration is then plotted to determine the order of reaction.
In CP13b, the hydrogen peroxide–iodide reaction: H₂O₂ (aq) + 2I⁻ (aq) + 2H⁺(aq) → I₂ (aq) + 2H₂O (l). Adding sodium thiosulfate acts as a .......... timer by reacting with the .......... produced.
In CP13b, the hydrogen peroxide–iodide reaction: H₂O₂ (aq) + 2I⁻ (aq) + 2H⁺(aq) → I₂ (aq) + 2H₂O (l). Adding sodium thiosulfate acts as a reaction timer by reacting with the iodine produced.
In CP13b, what is the order of reaction with respect to potassium iodide, and how is this deduced from the graph?
The reaction is first order with respect to potassium iodide. The rate–concentration graph is a straight line through the origin, showing that rate is directly proportional to [KI] — doubling concentration doubles the rate.
True or False?
In CP13a, the stop clock should be paused each time a sample is withdrawn from the reaction mixture.
False.
The stop clock should be kept running throughout the practical. The time at which sodium hydrogencarbonate is added to each withdrawn sample is noted, as this is the precise moment the reaction is stopped in that portion.
In CP13a, state the rate equation for the propanone–iodine reaction and explain why iodine does not appear.
Rate = k[CH₃COCH₃][H⁺]. Iodine does not appear because the reaction is zero order with respect to iodine — its concentration does not affect the rate.
What is activation energy?
Activation energy (E_a) is the minimum energy required for a reaction to occur — the energy that colliding particles must possess for a successful collision leading to products.
In CP14, how is rate of reaction calculated from the experimental time data?
Rate is taken as 1/*t* (the reciprocal of the time for the methyl red indicator to disappear), since all experiments use the same amount of phenol and the indicator bleaches when phenol is consumed.
True or False?
In CP14, the two boiling tubes are mixed before they reach the water bath temperature.
False.
Both boiling tubes are placed in the water bath and allowed to reach the water temperature before mixing. Mixing too early means the reaction does not occur at the intended temperature, giving inaccurate rate data.
What is the Arrhenius equation?
The Arrhenius equation relates the rate constant k to temperature: k = Ae⁽⁻^E_^a^/R^T⁾, or in linear form ln k = −E_a/RT + ln A, where A is the pre-exponential factor and R = 8.314 J mol⁻¹ K⁻¹.
In CP14, why is the experiment repeated at several different temperatures?
Multiple temperatures provide multiple (1/T, ln k) data points, allowing an Arrhenius plot (ln k vs 1/T) to be drawn and the gradient (−E_a/R) to be determined accurately.
In CP14, a graph of ln .......... against .......... gives a straight line with gradient equal to −E_a/R.
In CP14, a graph of ln k against 1/*T gives a straight line with gradient equal to −E*_a/R.
In CP14, what role does methyl red play in the bromate–bromide reaction?
Methyl red acts as the indicator: it is present throughout the reaction and is bleached by excess bromine as soon as all the phenol has been consumed, marking the end of the reaction and stopping the clock.
True or False?
Ice can be used in CP14 to achieve the lowest temperature required for the experiment.
True.
The procedure states that ice is used to achieve the lowest required temperature, allowing the experiment to be carried out across a wider temperature range and giving a more reliable gradient on the Arrhenius plot.
In CP14, how is E_a calculated once the Arrhenius plot has been drawn?
E_a = −gradient × R, where the gradient is the slope of the ln k vs 1/T graph (in K) and R = 8.314 J mol⁻¹ K⁻¹. The result gives E_a in J mol⁻¹, which can be converted to kJ mol⁻¹.

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