Physical Chemistry Core Practicals (Edexcel A Level Chemistry): Flashcards
Exam code: 9CHO
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Define molar volume.
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Define molar volume.
Molar volume is the volume occupied by one mole of a gas at a specified temperature and pressure. At RTP it equals 24.0 dm3 mol-1.
In Core Practical 1, why must the gas syringe delivery tube be connected immediately after adding the solid sodium carbonate?
The delivery tube must be connected immediately to minimise the volume of gas lost before collection begins — any gas escaping before the syringe is sealed is not recorded.
True or False?
In Core Practical 1, a graph of mass of sodium carbonate against volume of CO2 produced gives a straight line while the acid is in excess.
True.
A straight line is produced while the acid is in excess (sodium carbonate is the limiting reagent). The line levels off once all the acid has been consumed.
What is a gas syringe?
A gas syringe is a piece of apparatus used to collect and measure the volume of gas produced in a reaction. It is connected to the reaction vessel via a delivery tube.
How is the molar volume of a gas calculated from the results of Core Practical 1?
Molar volume is calculated by dividing the volume of gas (in dm3) by the number of moles of gas produced: molar volume = volume ÷ moles.
In Core Practical 1, the volume of carbon dioxide is converted from cm3 to dm3 by dividing by ........... The moles of CO2 are then calculated using the ...........
In Core Practical 1, the volume of carbon dioxide is converted from cm3 to dm3 by dividing by 1000. The moles of CO2 are then calculated using the molar mass of sodium carbonate.
What safety precautions should be taken when handling dilute hydrochloric acid in Core Practical 1?
Eye protection (goggles) must be worn. Dilute acids are classified as harmful, so gloves are also advisable. More concentrated acids require additional precautions such as use of a fume cupboard.
How can the results of Core Practical 1 be used to identify an unknown Group 2 metal carbonate MCO3?
Calculate the moles of CO2 from its volume, then find the molar mass of MCO3 (mass ÷ moles). Subtract the mass of CO3 (60.0 g mol-1) to find the atomic mass of M, then match to the periodic table.
What is displacement of water?
Displacement of water is a method of collecting gas produced in a reaction by routing the gas into an inverted, water-filled container over a trough, where it displaces the water.
Define standard solution.
A standard solution is a solution of precisely known concentration, prepared as accurately as possible to minimise measurement uncertainty.
Why is a three decimal place balance used when preparing a standard solution?
A three decimal place balance is used to minimise the uncertainty in the mass measurement, ensuring the concentration of the resulting solution is as accurate as possible.
Define volumetric analysis.
Volumetric analysis is a technique that uses the volume and concentration of one reactant to determine the concentration of an unknown solution, most commonly via a titration.
To prepare a standard solution, the solute is first dissolved in a ........... The solution is then transferred to a ........... The flask is made up to the calibration mark using .......... water.
To prepare a standard solution, the solute is first dissolved in a beaker. The solution is then transferred to a volumetric flask. The flask is made up to the calibration mark using distilled water.
What is the formula used to calculate the number of moles of solute in a solution?
Number of moles = concentration (mol dm-3) x volume (dm3). The volume must be converted from cm3 to dm3 by dividing by 1000.
What is a volumetric flask?
A volumetric flask is a piece of glassware with a single calibration mark on its neck, used to prepare a solution of an exact, known volume with high precision.
True or False?
A concentrated solution contains a higher number of moles of solute per dm3 than a dilute solution.
True.
Concentration is defined as the amount of solute per unit volume of solution. A concentrated solution has a high concentration; a dilute solution has a low concentration.
What mass of NaOH is needed to prepare 250 cm3 of a 0.200 mol dm-3 solution? (Mr of NaOH = 40.00 g mol-1)
Moles = 0.200 x 0.250 = 0.0500 mol. Mass = 0.0500 x 40.00 = 2.00 g of NaOH required.
What is a burette?
A burette is a long, graduated glass tube with a tap at the bottom, used to deliver precise, variable volumes of solution during a titration.
Define titre.
A titre is the volume of solution delivered from a burette during a titration, calculated as the final burette reading minus the initial burette reading. It is recorded to an uncertainty of ±0.10 cm3.
What is the precision and uncertainty of a burette reading in a titration?
A burette is marked to a precision of ±0.10 cm3. Each individual reading has an uncertainty of ±0.05 cm3 (half the smallest division). The calculated titre has an uncertainty of ±0.10 cm3 because two readings are subtracted.
In a titration, the solution of known concentration is placed in the ........... A known volume of the other solution is measured into a ..........using a .......... pipette.
In a titration, the solution of known concentration is placed in the burette. A known volume of the other solution is measured into a conical flask using a volumetric pipette.
What steps should be taken to process raw titration data to find an average titre?
Calculate each individual titre (final reading minus initial reading). 2. Identify concordant results (within 0.10 cm3 of each other). 3. Discard non-concordant results. 4. Average the concordant titres.
Define concordant results.
Concordant results are titration results (titres) that are within 0.10 cm3 of each other. Only concordant results are averaged; non-concordant results are discarded.
Why is a white tile placed under the conical flask during a titration?
A white tile provides a pale background that makes it easier to detect the colour change of the indicator at the end point, improving the accuracy of the titration.
True or False?
As you approach the end point in a titration, the burette tap should be opened fully to add solution quickly.
False.
As the end point approaches, the burette tap should be nearly closed so that solution is added dropwise. This allows the colour change to be detected after exactly one drop, improving accuracy.
In a titration, 22.80 cm3 of 0.100 mol dm-3 Na2CO3 reacts with 25.0 cm3 of HCl in a 1:2 ratio. What is the concentration of the HCl?
Moles of Na2CO3 = (22.80 / 1000) × 0.100 = 2.28 × 10-3 mol. Moles of HCl = 2 × 2.28 × 10-3 = 4.56 × 10-3 mol (1:2 ratio). Concentration of HCl = 4.56 × 10-3 / 0.0250 = 0.182 mol dm-3.
What is the end point of a titration?
The end point (or equivalence point) is the point in a titration at which the two solutions have reacted completely, shown by a permanent colour change of the indicator.
Define calorimetry.
Calorimetry is a technique used to measure the enthalpy change of a chemical reaction by recording the temperature change of a known mass of liquid in an insulated container.
Write the equation used to calculate the energy transferred in a calorimetry experiment, defining each variable.
q = mcΔT, where q = energy transferred (J), m = mass of solution (g), c = specific heat capacity (J g-1 K-1), and ΔT = temperature change (K).
In Core Practical 8, why is one reactant used in excess?
One reactant is used in excess to ensure the other reactant is the limiting reagent and is fully consumed. This makes it possible to calculate the enthalpy change per mole of the limiting reagent.
In Core Practical 8, the enthalpy change is calculated using ΔH = .......... / .........., where the moles used are those of the .......... reagent.
In Core Practical 8, the enthalpy change is calculated using ΔH = q / n, where the moles used are those of the limiting reagent.
What is the specific heat capacity of water?
Specific heat capacity is the energy required to raise the temperature of 1 g of a substance by 1 K. For water (and dilute aqueous solutions assumed to behave like water), c = 4.18 J g-1 K-1.
State three assumptions made in Core Practical 8 calorimetry calculations.
The specific heat capacity of the solution equals that of pure water (4.18 J g-1 K-1). 2. The density of the solution equals that of pure water (1 g cm-3). 3. Heat losses to the surroundings are negligible.
True or False?
In Core Practical 8, the specific heat capacity of the polystyrene cup is included when calculating the energy transferred.
False.
One of the standard assumptions of the experiment is that the heat capacity of the calorimeter (polystyrene cup) is negligible and can be ignored in calculations.
Why does a temperature rise in Core Practical 8 indicate an exothermic reaction, and how is this shown in the sign of ΔH?
A temperature rise means the reaction releases heat to the surroundings, so it is exothermic. The enthalpy change ΔH is given a negative sign to indicate energy is released by the system.
What is a temperature correction graph?
A temperature correction graph is a plot of temperature against time used to determine the true maximum temperature change in a calorimetry experiment. The cooling portion of the graph is extrapolated back to the moment the reactant was added.
Why is temperature recorded for several minutes before adding the second reactant in Core Practical 8?
Recording the initial temperature over several minutes establishes a steady baseline. When the cooling curve is later extrapolated back to the mixing point, an accurate ΔT can be determined, accounting for heat loss.
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