T&P Measurement

Question 1

Describe how you would investigate the flame colours produced by the metal ions in solutions of lithium / sodium / potassium / barium / calcium / copper (II) chloride.

Answer 1

• Soak wooden splints in 0.1 mol dm^-3 solutions of each metal chloride
• Wear eye protection
• Hold each splint successively in blue bunsen flame, then remove when they begin burning
• Record flame colours

Question 2

Describe how you would investigate the flame colours produced by solid ionic compounds containing lithium / sodium / potassium / barium / calcium / copper (II) ions.

Answer 2

• Dip wire loop into HCl (cleaning + adhesion) then into solid
• Wear eye protection
• Hold loop in blue bunsen flame
• Record flame colour
• Repeat for each solid

Question 3

Describe how you would weigh a solid accurately.

Answer 3

• Zero mass balance
• Place weighing boat onto balance
• Add approximate mass of solid required
• Accurately record mass of boat + solid
• Empty solid into glassware
• Accurately record mass of boat
• Mass of solid used = mass of boat + solid - mass of empty boat

Question 4

Describe how you would accurately dispense 25 cm³ of a liquid.

Answer 4

• Rinse 25 cm³ pipette with water then liquid to be dispensed
• Attach filler to pipette, then draw in solution until bottom of meniscus is level with line on neck at eye level
• Dry exterior with paper towel
• Run liquid into glassware until it stops dripping
• Touch tip against inside of glassware, then remove pipette

Question 5

Name 2 pieces of apparatus used to measure the volume of gas produced in a reaction.

Answer 5

1. Gas syringe
2. Inverted burette / measuring cylinder (collecting over water)

Question 6

Draw a diagram of the apparatus, including a gas syringe, which could be used to measure the volume of gas produced by a reaction.

Answer 6

Tube should be labelled “delivery tube”

Question 7

Draw a diagram of the apparatus, including an inverted measuring cylinder, which could be used to measure the volume of gas produced by a reaction.

Answer 7

Question 8

State and explain the piece of apparatus you would use to collect a water-soluble gas.

Answer 8

A gas syringe. If you collected it over water, some would dissolve before reaching the inverted burette / measuring cylinder.

Question 9

Calcuim carbonate decomposes as follows:

CaCO₃(s) → CaO(s) + CO₂(g)

1. Draw a diagram of the apparatus you could use to measure the volume of gas produced when a fixed mass of solid decomposes fully, and the mass of barium carbonate needed to produce the same volume of gas.
2. Explain how the apparatus could be used to show that barium carbonate is thermally stabler than calcium carbonate.

Answer 9

1. Diagram:

• Tube connected to gas syringe or measuring cylinder over water
• Labels (heat, syringe/measuring cylinder)
• Proper cross-section with passage for gases and no leaks

2. Heat samples of both carbonates, using methods as similar as possible. First to produce a certain amount of gas is less thermally stable or heat for a fixed time; whichever produces less gas is more stable

Question 10

Describe how you would measure the enthalpy change, per mole of solute, for an exothermic reaction between a solution and a solid.

(Also applicable for 2 solutions reacting, and for endothermic reactions)

Answer 10

• Use measuring cylinder to add known volume of solution to polystyrene cup
• Measure initial temperature
• Add excess solid
• Stir
• Top with lid with hole + put thermometer through it
• Record temperature every 10 seconds until no further change
• Plot temperature against time and extrapolate theoretical T_max as shown below (accounts for heat loss)
• Calculate ΔT
• Calculate ΔE using ΔE = mcΔT, where m = volume of solution and c = 4.18
• Calculate ∆H in kJ mol^-1 = ∆E / (n x 1000)

Question 11

Sodium bicarbonate and citric acid react endothermically:

C₆H₈O₇(aq) + 3NaHCO₃(s) → Na₃C₆H₅O₇ + 3CO₂(g) + 3H₂O(l)

Describe how the enthalpy change of this reaction would be accurately measured, and how the ΔH per mole of sodium bicarbonate would be calculated.

Answer 11

Measuring Δ​E of an endothermic reaction in solution

• Add excess volume of acid to polystyrene cup
• Measure initial temperature
• Add known mass of bicarbonate
• Stir
• Top with lid with hole + put thermometer through it
• Record temperature every 10 seconds until no further change
• Calculate maximum ΔT by subtracting minimum temperature from initial temperature
• Calculate n, mol bicarbonate = mass/84
• ΔE per n mol bicarbonate = ΔE = mcΔT, where m = volume of acid and c = 4.18
• Calculate ΔH per mol bicarbonate = ΔE / n
• ∆H in kJ mol^-1 = ∆E / (n x 1000). Write with + sign

Question asked for ΔH for bicarbonate, which is why the acid is used in excess

Question 12

Draw a diagram of the apparatus used to determine the enthalpy change of combustion of a fuel.

Answer 12

Question 13

Describe how the enthalpy change of combustion of a fuel would be measured.

Answer 13

• Clamp a copper calorimeter above a spirit burner containing fuel
• Use measuring cylinder to transfer known volume of water to calorimeter
• Record initial temperature of water with thermometer
• Record initial mass of burner with mass balance
• Burn fuel
• Record final temperature of water. Calculate ∆T
• Record final mass of burner. Calculate ∆mass
• Calculate moles of fuel burned, n = mass/Mr
• ∆E per n mol fuel = mc∆T, where m = mass of water and c = 4.18
• ∆_cH in kJ mol^-1 = ∆E / (n x 1000)

Question 14

Some students investigate the properties of ALA, which is an oil at room temperature.

Describe a simple experiment that the students could carry out to measure the ∆_cH for ALA. State the measurements they would make and how they would calculate the result.

Answer 14

• Clamp a copper calorimeter above a spirit burner containing ALA
• Use measuring cylinder to transfer known volume of water to calorimeter
• Record initial temperature of water with thermometer
• Record initial mass of burner with mass balance
• Burn fuel
• Record final temperature of water. Calculate ∆T
• Record final mass of burner. Calculate ∆mass
• Calculate moles of fuel burned, n = mass/Mr
• ∆E per n mol fuel = mc∆T, where m = mass of water and c = 4.18
• ∆_cH in kJ mol^-1 = ∆E / (n x 1000)

Question 15

What are standard solutions and what are they used for?

Answer 15

• Solutions whose concentrations are accurately known
• Used to determine the concentration of a solution / the purity of a solid

Question 16

Describe a method which could be used to make a 250 cm³ standard solution of a solid compound.

Answer 16

• Zero mass balance
• Place weighing bottle (see below) onto balance
• Add approximate required mass of solid into bottle
• Accurately weigh bottle + solid
• Stir solid into 250 cm³ beaker containing 100 cm³ deionized water
• Reweigh weighing bottle
• Mass of solid transferred = change in mass
• Transfer solution to 250 cm³ volumetric flask
• Rinse beaker + stirring rod. Transfer washings to flask
• Swirling regularly, add deionized water up to 1 cm³ below mark
• Use pipette to add water until bottom of meniscus is level with mark at eye level
• Stopper flask + invert several times

Question 17

Describe how a standard solution would be prepared by dilution of a stock solution which has too high a concentration.

Answer 17

• Rinse dry beaker with, then half-fill with, stock solution
• Rinse then fill 25 cm³ pipette with stock solution so that bottom of meniscus is level with with 25 cm³ mark
• Transfer solution to 250 cm³ volumetric flask
• Swirling regularly, add deionized water up to 1 cm³ below mark
• Use pipette to add water until bottom of meniscus is level with mark at eye level
• Stopper flask + invert several times

Question 18

What volume, in cm³, of stock solution of concentration 2.0 mol dm^-3 would be required to prepare 50 cm³ of 1.0 mol dm^-3 solution?

Answer 18

Mol required = conc x vol = 50 x 10^-3 x 1 = 0.05 mol

Vol required = mol / conc = 0.05 / 2 = 0.025 dm³ = 25 cm³