Further Physical Chemistry Practicals (OCR A Level Chemistry A): Flashcards

Taring at the start means the balance directly reads mass lost, giving a curve that rises as the reaction proceeds — making it easier to read and directly comparable to a gas volume graph.

A curve of best fit is a smooth line drawn through a set of plotted data points that follows the overall trend of the data, without necessarily passing through every point.

True.

Keeping mass constant ensures a fair comparison; only surface area varies between experiments, so any difference in rate can be attributed to particle size.

1. Draw a tangent at time = 0.

2. Calculate the gradient using Δy / Δx.

3. The rate has units of g s^-1.

Note that the gradient will be negative because mass decreases over time.

Hydrogen gas has a very low density, so the mass change as it escapes is too small to register reliably on a standard laboratory balance.

A student mixes 20 cm³ of 2.0 mol dm^-3 acid with 20 cm³ of distilled water to produce 40 cm³ of 1.0 mol dm^-3 acid.

The total volume is kept constant so that concentration is the only variable changed.

True.

If the plunger is not fully inserted, a volume of trapped air will be recorded as gas produced, introducing a systematic error into the results.

Magnesium readily forms a layer of magnesium oxide on its surface; cleaning this off ensures consistent reactivity and reproducible results between repeat experiments.

A serial dilution is a stepwise process of diluting a stock solution by a fixed factor each time to produce a series of solutions of known, decreasing concentration.

True.

Drawing tangents at the origin for each curve on a single graph allows the initial rates at different concentrations to be compared directly from their gradients.

If the apparatus leaks, collected gas will escape and the recorded volume will be lower than the true volume, making rate calculations inaccurate.

The colour change shows that iodine is now in excess over the sodium thiosulfate; once all the thiosulfate is consumed, free iodine reacts with starch to produce the dark blue-black colour.

A clock reaction is a reaction that shows a sudden, dramatic colour change after a fixed time interval, making it useful for investigating the effect of concentration on reaction rate.

The rate is determined by calculating the reciprocal (1/t) of the time taken for the colour change to appear.

This converts time into a measure of rate: a shorter time gives a higher rate.

True.

Sodium thiosulfate reacts with iodine as it is produced, consuming it and preventing a colour change until all the thiosulfate has been used up.

A straight line through the origin indicates the reaction is first order with respect to that reactant, meaning rate is directly proportional to concentration.

True.

Burettes provide the precision needed for the small volumes of reagents whose concentration is being varied; the acid volume does not need the same precision so a measuring cylinder is adequate.

As the concentration is increased, the time for the colour change decreases and the calculated rate increases.

This inverse relationship between time and rate is why rate is expressed as 1/t.

The end point is when the cross drawn beneath the conical flask can no longer be seen through the solution, indicating sufficient precipitate has formed to make the mixture opaque.

The disappearing cross experiment is a method for investigating reaction rate in which a cross is placed beneath a reaction vessel and the time is recorded until a forming precipitate obscures the cross from view.

The total volume of solution must remain constant in each run.

This ensures that only concentration changes between runs and any effect on rate can be attributed to the change in concentration alone.

True.

A higher concentration increases the rate of reaction, so the precipitate forms more quickly and the cross disappears in a shorter time.

The concentrations of the reagents are kept constant while the temperature of the sodium thiosulfate solution is varied; the time for the cross to disappear is recorded at each temperature.

False.

A straight-line relationship between rate (1/t) and concentration indicates first order; a graph of time against concentration is curved and does not directly reveal reaction order.

The solid product is S (s) (sulfur) and the gas also produced is SO₂ (g).

The solid sulfur scatters light, making the solution cloudy and eventually obscuring the cross.

An activation energy (E_a) is the minimum energy that colliding reactant particles must possess for a reaction to occur, measured in kJ mol^-1.

A water bath provides uniform, stable heating so that the temperature of all reactant solutions is the same before mixing, ensuring the temperature is the only variable that changes between runs.

1. Convert temperature to Kelvin (K) by adding 273.

2. Calculate 1/T as the x-axis variable.

3. Calculate ln(rate) as the y-axis variable.

The gradient of the resulting straight line equals –E_a / R.

True.

Rearranging the Arrhenius equation gives ln k = ln A – E_a/RT, so the gradient of the straight-line graph is –E_a/R.

The time is recorded when the colour change is judged complete; inconsistent judgment introduces random error into the time measurement, affecting the calculated rate and ultimately the value of E_a obtained.

True.

Since the time for a fixed amount of reaction to occur is inversely proportional to rate, 1/t gives a relative measure of rate that can be used in the Arrhenius calculation.

E_a = –gradient × R

The answer must be converted from J mol^-1 to kJ mol^-1 by dividing by 1000.

A negative gradient combined with the negative sign gives a positive value for E_a.

A pH probe calibration is the process of adjusting a pH probe to give accurate readings by immersing it in buffer solutions of known pH and correcting any drift before use.

True.

Universal indicator matches a colour chart to an approximate pH, whereas a calibrated pH probe gives a continuous numerical reading with greater precision.

To avoid cross-contamination between solutions, which would alter the recorded pH values.

When calibrating a pH probe, the probe is first placed in a pH 4 buffer solution, then in a pH 9 buffer solution after rinsing with distilled water.

Phenolphthalein turns pink in alkaline solutions and remains colourless in acidic or neutral solutions.

True.

After extended use the glass electrode can drift, so recalibration between measurements maintains accuracy.

A universal indicator is a mixture of indicators that produces a range of colours corresponding to different pH values, used to give an approximate pH of a solution.

The equivalence point is the point in a titration where stoichiometrically equal amounts of acid and alkali have reacted, located at the centre of the steep vertical section of a pH curve.

A calibration method in which the probe is adjusted using two buffer solutions of known pH, typically pH 4 and pH 9, to ensure accurate readings across a range.

False.

When a strong acid reacts with a weak base, the resulting salt is slightly acidic, so the equivalence point lies below 7.

In a titration curve experiment, the acid is placed in a beaker and the base is added from a burette, with pH recorded after each 5 cm³ addition.

In a strong acid–strong base titration the equivalence point is at pH 7, whereas in a weak acid–strong base titration it is above 7 due to hydrolysis of the salt formed.

True.

Smaller additions around the equivalence point produce more data points in the steep region, improving accuracy when locating the exact equivalence point.

The end point is the volume of titrant at which an indicator changes colour, which approximates but may not exactly coincide with the equivalence point.

A buffer capacity is the amount of acid or base a buffer solution can absorb before its pH changes significantly, which increases with higher concentrations of the weak acid and its conjugate base.

A technique in which a solution is diluted by the same factor in each successive step, producing a series of solutions with known decreasing concentrations.

True.

For a strong acid, [H⁺] decreases by a factor of 10 with each ten-fold dilution, and since pH = −log[H⁺], the pH increases by exactly 1.

To prepare a buffer solution, a weak acid and its conjugate base are dissolved together. The ratio of the conjugate base concentration to the acid concentration is adjusted to reach the target pH.

A small excess is weighed to account for transfer losses when moving the solid to the beaker, ensuring the final amount dissolved reaches the required quantity.

True.

Higher concentrations of both the weak acid and its conjugate base mean the buffer can neutralise larger amounts of added acid or base before the pH changes significantly.

The Henderson–Hasselbalch equation is an equation used to calculate the pH of a buffer solution: pH = pKa + log([A^–]/[HA]), relating pH to the pKa and the ratio of conjugate base to weak acid concentrations.