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PQs Flashcards

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1
Q

Why do you rinse the burette with the solution it will contain (not water)?

A

To ensure any water droplets left inside don’t dilute the solution, which would affect the concentration and give inaccurate volume readings.

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2
Q

Why do you rinse the pipette with the solution it will contain?

A

To ensure the correct concentration is delivered into the conical flask — rinsing with water would dilute the solution and affect the number of moles added.

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3
Q

Why do you not rinse the conical flask with the solution?

A

Rinsing the flask with solution would introduce an unknown amount of extra moles, affecting the result. Only rinse with distilled water (which doesn’t affect moles).

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4
Q

Why is it important to remove the funnel from the burette during titration?

A

To prevent extra drops of solution from entering the burette, which would change the volume recorded and make your titre inaccurate.

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5
Q

Why is a white tile used under the conical flask?

A

To make the colour change at the endpoint easier to see.

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6
Q

What is meant by “concordant titres”?

A

Titres that are within ±0.10 cm³ of each other — this increases reliability and reduces random error.

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7
Q

How could you make your titration results more accurate?

A

• Use a white tile
• Remove funnel after filling burette
• Use concordant results only
• Add dropwise near endpoint
• Repeat titration for consistency

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8
Q

Why are rough titrations done first?

A

To get a quick estimate of the endpoint — helps plan more accurate repeat titrations by avoiding overshooting.

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9
Q

Why is it better to use a pipette instead of a measuring cylinder?

A

Pipettes are more accurate and have a lower uncertainty than measuring cylinders.

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10
Q

Why is phenolphthalein used in a titration between a strong acid and strong base?

A

Actually, phenolphthalein is best for strong base + weak acid, because it has a sharp colour change at the equivalence point.

For strong acid + strong base, methyl orange or phenolphthalein could both be used depending on what’s clearer.

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11
Q

Describe how to make a standard solution of sodium carbonate (Na₂CO₃)

A
  1. Accurately weigh the required mass of Na₂CO₃ using a balance and a weighing boat.
    1. Transfer the solid into a beaker and dissolve it in a small volume of distilled water.
    2. Rinse the weighing boat into the beaker with distilled water to ensure all solid is transferred.
    3. Pour the solution into a 250 cm³ volumetric flask using a funnel.
    4. Rinse the beaker and funnel into the flask to ensure all solution is transferred.
    5. Add distilled water to the flask until the bottom of the meniscus sits on the mark.
    6. Stopper the flask and invert several times to mix thoroughly.
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12
Q

Why do you rinse the beaker, funnel, and stirring rod into the volumetric flask?

A

To ensure all of the solute is transferred, so the concentration is accurate — you don’t want to leave any of the substance behind.

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13
Q

Why do you invert the volumetric flask several times after making up to the mark?

A

To ensure the solution is fully mixed and has a uniform concentration throughout.

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14
Q

Why should you not blow air into the pipette filler or pipette tip?

A

Blowing introduces extra volume or contaminants, making the volume delivered inaccurate.

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15
Q

What error would be caused if the solution was made up above the calibration mark?

A

The solution would be more dilute, so the concentration would be lower than intended.

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16
Q

Why is a volumetric flask better than a measuring cylinder for making up a standard solution?

A

Volumetric flasks are more precise, with a much lower uncertainty in volume measurement

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17
Q

How does using a wet beaker (not dried after rinsing) affect the concentration?

A

It would dilute the solution slightly, giving a lower concentration than intended.

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18
Q

What does “weighing by difference” mean?

A

Weigh the full container (e.g. weighing boat with solid), then reweigh after tipping out the solid. Subtract the second value from the first to get accurate mass transferred.

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19
Q

What type of balance should be used for making a standard solution?

A

A 2 decimal place (0.01 g) or 4 decimal place (0.0001 g) balance for higher precision.

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20
Q

What is the meniscus and how do you read it correctly?

A

The meniscus is the curved surface of the liquid. Always read the volume at the bottom of the meniscus at eye level to avoid parallax error.

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21
Q

Why is a polystyrene cup used in calorimetry?

A

Because it’s a good insulator, which helps reduce heat loss to the surroundings and improves accuracy

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22
Q

What are common sources of error in a simple calorimetry experiment?

A

• Heat loss to the surroundings
• Incomplete combustion (if using a fuel)
• Evaporation of fuel or liquid
• Assuming solution has same density/specific heat capacity as water
• Not stirring evenly → uneven temperature

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23
Q

What effect would heat loss to the surroundings have on the calculated ΔH?

A

It would cause a lower temperature change, so the calculated enthalpy change (ΔH) would be less exothermic than it should be.

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24
Q

How can you reduce errors in a calorimetry experiment?

A

• Use a lid on the calorimeter
• Use a draught shield or insulation
• Use a digital thermometer or temperature probe
• Stir continuously for even heating

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25
A student calculated q correctly but forgot to divide by moles. What mistake does this cause?
The value for ΔH will be too large — ΔH must be calculated per mole of limiting reagent: Delta H = q/n
26
Why is the mass used in the q = mcΔT calculation usually the mass of the solution, not the solute?
Because the solution is what’s absorbing/releasing heat — the solid’s mass is usually small and its heat capacity negligible.
27
Why is the temperature change sometimes found by plotting a graph?
Because heat loss might begin before max temperature is recorded. By extrapolating a temperature vs. time graph, you can estimate the temperature at the point of reaction, improving accuracy.
28
Is ΔH positive or negative for an exothermic reaction in calorimetry?
Negative — because the system releases heat, so ΔH < 0.
29
What is the test for carbonate ions (CO₃²⁻)?
Answer: Add dilute hydrochloric acid (HCl). If carbonate ions are present, you’ll see effervescence (bubbling), and the gas produced is carbon dioxide. Confirm CO₂: Bubble it through limewater, which turns cloudy/milky.
30
What is the test for sulfate ions (SO₄²⁻)?
Answer: Add dilute hydrochloric acid (HCl) followed by barium chloride solution (BaCl₂). A white precipitate of barium sulfate (BaSO₄) indicates sulfate ions.
31
What is the test for halide ions (Cl⁻, Br⁻, I⁻)?
Answer: Add dilute nitric acid (HNO₃) and then silver nitrate (AgNO₃): • Cl⁻ → white precipitate (AgCl) • Br⁻ → cream precipitate (AgBr) • I⁻ → yellow precipitate (AgI) Follow-up test (with ammonia): • AgCl dissolves in dilute NH₃ • AgBr dissolves in concentrated NH₃ • AgI does not dissolve
32
Why is nitric acid used before adding silver nitrate (not HCl)?
Answer: Because HCl contains Cl⁻ ions, which would interfere and give a false positive (white ppt) — nitric acid doesn’t interfere with halide tests.
33
What’s the purpose of adding dilute HCl before the sulfate test?
Answer: To remove carbonate ions, which would also form a white ppt (barium carbonate), causing a false positive.
34
What’s the ionic equation for the reaction between sulfate ions and barium chloride?
Ba2+(aq) + SO4 2- (aq) -> BaSO4(s)
35
What order should the ion tests be carried out in and why?
Answer: 1. Carbonate 2. Sulfate 3. Halide Because: • Carbonates also give white ppt with Ba²⁺ and Ag⁺, so they must be ruled out first • Sulfates give ppt with Ag⁺, so test them before halide
36
What would happen if you added silver nitrate directly to a solution with carbonate ions?
Answer: A white precipitate of silver carbonate would form — gives a false positive for chloride.
37
How can you distinguish between white precipitates of BaSO₄ and AgCl?
Answer: Add dilute ammonia: • AgCl dissolves, BaSO₄ does not dissolve
38
What’s the flame colour for calcium and barium ions (Group 2)?
Answer: • Calcium (Ca²⁺) → brick red • Barium (Ba²⁺) → green
39
Why is a pipette more accurate than a measuring cylinder?
Answer: A pipette has a lower uncertainty and delivers a precise fixed volume. Measuring cylinders are less accurate and have greater uncertainty.
40
How can accuracy be improved in a titration/calorimetry experiment?
Answer: • Use equipment with lower uncertainty • Repeat readings and calculate a mean • Add acid dropwise near endpoint (titration) • Use insulation or a lid (calorimetry) • Stir the mixture for even heating
41
What is a control variable? Why are control variables important?
Answer: A control variable is one you keep constant so that any change in results is only due to the independent variable. Without controls, the experiment becomes unreliable.
42
Why are experiments repeated?
Answer: To identify and reduce random errors and allow for a more reliable mean value.
43
What is a systematic error? Give an example.
Answer: An error that affects results in the same direction every time. Example: Using an uncalibrated balance that reads 0.05 g too high.
44
What is a random error? Give an example.
Answer: An error that causes unpredictable variations. Example: Reading the meniscus slightly too high or low each time.
45
What is parallax error and how do you avoid it?
Answer: Error from not reading measurements at eye level, causing misreading of the meniscus or scale. Avoid it by always reading at eye level.
46
How do you identify the limiting reagent in a reaction?
Answer: Calculate moles of each reactant, divide by the mole ratio from the balanced equation. The smallest value = limiting reagent.
47
When drawing a graph, what key features are needed for full marks?
Answer: • Axes labelled with units • Appropriate scale • Points plotted accurately • Best-fit line or curve drawn • Use of extrapolation or interpolation if asked
48
What makes a measurement precise? What makes it accurate?
• Precise = results are close to each other (repeatable) • Accurate = result is close to the true/expected value
49
Why is reflux used in organic synthesis?
To allow a reaction mixture to be heated for a long time without losing volatile substances — vapours condense and return to the reaction flask.
50
Describe how to set up a reflux apparatus.
Answer: • Use a pear-shaped flask with the reaction mixture • Attach a vertical condenser above it • Add anti-bumping granules • No stopper at the top — to prevent pressure build-up • Heat with a water bath or heating mantle (not Bunsen)
51
Why are anti-bumping granules used?
Answer: To ensure smooth boiling and prevent sudden bursts or splashing of the mixture.
52
Why is the top of the condenser left open during reflux?
Answer: To avoid pressure build-up, which could cause the apparatus to explode.
53
Why is it dangerous to use a Bunsen burner for heating during reflux?
Answer: Because many organic solvents are flammable, so an open flame could cause a fire. Use an electric heating mantle instead.
54
What does reflux prevent?
Answer: It prevents the loss of volatile substances that would otherwise escape during boiling.
55
Why is distillation used after reflux?
Answer: To separate the desired product from the mixture by boiling and condensing it based on its boiling point.
56
Describe how to set up a distillation apparatus.
Answer: • Pear-shaped flask with the mixture • Attach a thermometer at the top of the flask (bulb at the junction) • Connect to a condenser sloping downwards • Collect distillate in a receiving flask
57
Why must the thermometer bulb be level with the condenser entrance?
Answer: To measure the accurate boiling point of the vapour that is actually being distilled.
58
Why is water always entered at the bottom of the condenser?
Answer: To ensure the condenser is filled completely, giving more efficient cooling.
59
How can you tell when to collect the desired fraction during distillation?
Answer: When the thermometer reading matches the boiling point of the product.
60
Why is distillation not suitable for separating liquids with very similar boiling points?
Answer: Because they will co-distil, making the separation inefficient. Fractional distillation is needed instead.
61
What safety precautions should be taken during distillation?
Answer: • Avoid using open flames • Ensure apparatus is sealed properly but not air-tight • Use a clamp stand to secure apparatus • Use anti-bumping granules

Physical Chem (8 decks)

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