Physical Chemistry Practicals (AQA A Level Chemistry): Flashcards
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What is a standard (volumetric) solution?
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What is a standard (volumetric) solution?
A primary amine is an organic compound in which one hydrogen of ammonia has been replaced by an alkyl or aryl group, giving the structure R–NH2.
When making a standard solution, after dissolving the solid in a beaker, the solution is transferred to a .......... flask, the beaker is rinsed and the .......... are added, and the solution is made up to the graduation mark with .......... water.
When making a standard solution, after dissolving the solid in a beaker, the solution is transferred to a volumetric flask, the beaker is rinsed and the washings are added, and the solution is made up to the graduation mark with distilled (or deionised) water.
True or False?
After adding water to the graduation mark on a volumetric flask, the flask should be stoppered and inverted several times to ensure thorough mixing.
True.
Stoppering and inverting mixes the solution fully. Forgetting this step is a common exam error that costs marks.
Calculate the mass of NaOH needed to prepare 250 cm3 of a 0.200 mol dm-3 solution. (Mr of NaOH = 40.00 g mol-1)
n = 0.200 × 0.250 = 0.0500 mol
mass = 0.0500 × 40.00 = 2.00 g
The solid should be weighed on a balance reading to at least .......... decimal places; a .......... balance is used to minimise measurement uncertainty.
The solid should be weighed on a balance reading to at least 2 decimal places; a three-decimal-place balance is used to minimise measurement uncertainty.
True or False?
It is acceptable to dissolve the solid directly in a volumetric flask rather than in a beaker first.
False.
The solid should be dissolved in a beaker first (stirring until fully dissolved), then transferred to the volumetric flask with all washings. Dissolving directly in the flask risks cracking it if heat is produced and makes thorough mixing more difficult.
Why must the beaker and glass rod be rinsed and the washings added to the volumetric flask?
To ensure all the dissolved solute is transferred to the flask, so the final solution has exactly the intended concentration. Leaving any solute in the beaker would make the solution less concentrated than calculated.
What are concordant titration results?
A thin-layer chromatography (TLC) is a chromatographic technique used to separate small amounts of substances. A solid stationary phase (e.g. silica) is coated on a plate, and a solvent mobile phase carries components up the plate at different rates.
Burettes are graduated in 0.10 cm3 divisions, so the reading uncertainty is ±.......... cm3, and the uncertainty in a calculated titre (using two burette readings) is ±.......... cm3.
Burettes are graduated in 0.10 cm3 divisions, so the reading uncertainty is ±0.05 cm3, and the uncertainty in a calculated titre (using two burette readings) is ±0.10 cm3.
True or False?
A white tile is placed under the conical flask during a titration to make the colour change of the indicator easier to see.
True.
The white background provides contrast so that even a faint colour change at the endpoint is clearly visible.
What is the difference between the equivalence point and the endpoint in a titration?
The equivalence point is where the moles of titrant and analyte are stoichiometrically equal.
The endpoint is where the indicator changes colour, signalling the reaction is complete. Ideally these coincide, but a small discrepancy can occur.
Percentage uncertainty = (.......... ÷ measured value) × 100%.
This is different from percentage .......... , which compares a result to a known literature value.
Percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%.
This is different from percentage error, which compares a result to a known literature value.
True or False?
When two measurements are subtracted (e.g. final burette reading minus initial), their absolute uncertainties are also subtracted.
False.
When adding or subtracting measurements, absolute uncertainties are added together, not subtracted. The total uncertainty in the titre is ±0.05 + ±0.05 = ±0.10 cm3.
Why is the solution added dropwise from the burette as the endpoint is approached?
To avoid overshooting the endpoint. Adding drop by drop allows the colour change caused by a single drop to be observed, so the tap can be closed immediately and an accurate endpoint recorded.
What is calorimetry?
A zwitterion is a form of an amino acid that carries both a positive charge (on the –NH3+ group) and a negative charge (on the –COO- group) simultaneously. It is the predominant form at the isoelectric point.
The heat transferred in a calorimetry experiment is calculated using q = .......... × .......... × .......... , where the specific heat capacity of water is 4.18 J g-1 K-1.
The heat transferred in a calorimetry experiment is calculated using q = m × c × ΔT, where the specific heat capacity of water is 4.18 J g-1 K-1.
True or False?
In a combustion calorimetry experiment, the mass used in q = mcΔT is the mass of the fuel burned.
False.
The mass used is the mass of the water (or solution) being heated, not the fuel. The mass of fuel burned is used separately to calculate the moles of fuel combusted.
State two main sources of error in a simple combustion calorimetry experiment.
Heat losses to the surroundings (and to the calorimeter itself)
Incomplete combustion of the fuel, which reduces the heat released
In a temperature correction graph, the .......... section of the graph is extrapolated back to the time the second reactant was added, to find the .......... temperature change corrected for heat loss.
In a temperature correction graph, the cooling section of the graph is extrapolated back to the time the second reactant was added, to find the true maximum temperature change corrected for heat loss.
True or False?
A temperature change of 1 °C is equal to a temperature change of 1 K, so no conversion is needed when using ΔT in the q = mcΔT formula.
True.
The Celsius and Kelvin scales differ only in their zero point; a change of 1 °C is identical in magnitude to a change of 1 K. Adding 273 to a temperature change is a common error.
Why is a polystyrene cup preferred over a metal beaker as a calorimeter for reactions in solution?
Polystyrene is a poor conductor of heat, so less heat is lost to the surroundings. This makes the temperature change measured more representative of the actual enthalpy change.
What does rate of reaction measure, and what is the formula?
Rate of reaction measures how quickly reactants are consumed or products are formed. Rate = change in amount of reactants or products (mol dm-3) ÷ time (s).
In the disappearing cross experiment, sodium thiosulfate reacts with HCl to produce .......... as a solid precipitate, which makes the solution .......... and obscures the cross.
In the disappearing cross experiment, sodium thiosulfate reacts with HCl to produce sulfur (S) as a solid precipitate, which makes the solution opaque (cloudy) and obscures the cross.
True or False?
The rate of a reaction remains constant throughout the reaction because the concentrations of reactants decrease steadily.
False.
The rate changes throughout the reaction: as reactant concentrations decrease, the rate generally decreases too. The instantaneous rate at any point is found from the gradient of a tangent drawn on a concentration–time graph.
In the disappearing cross experiment, what graph is plotted to investigate the effect of temperature on rate, and why?
A graph of 1/t (where t = time for the cross to disappear) against temperature. 1/t is proportional to the rate of reaction, so higher values indicate a faster reaction.
In the disappearing cross experiment, the .......... variable is temperature, the .......... variable is the time for the cross to disappear, and the .......... variables include volume, concentration and depth of solution.
In the disappearing cross experiment, the independent variable is temperature, the dependent variable is the time for the cross to disappear, and the control variables include volume, concentration and depth of solution.
True or False?
The rate of reaction at a specific point on a concentration–time graph is found by drawing a tangent at that point and calculating its gradient.
True.
The gradient of the tangent equals the instantaneous rate at that moment. Since concentration is decreasing, the tangent gradient will be negative; rate is quoted as a positive value.
State a safety precaution specific to the disappearing cross experiment and the reason for it.
Work in a well-ventilated room or fume cupboard because sulfur dioxide (SO2) is produced, which is a toxic and irritant gas.
Rate of reaction
A peptide bond is the amide linkage (–CO–NH–) formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water.
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