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Back to AI Flashcard MakerChemistry /A-level Chemistry: 3.1.10 Acids and Bases Part 2

A-level Chemistry: 3.1.10 Acids and Bases Part 2

Chemistry46 CardsCreated 2 months ago

This flashcard set introduces the Brønsted–Lowry definitions of acids and bases, emphasizing their behavior in water. It covers key concepts such as proton donation and acceptance, along with the corresponding chemical equations for acid-base interactions in aqueous solutions.

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How do you deicide which indicator to use?

Need to pick one that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve

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Key Terms

Term
Definition

How do you deicide which indicator to use?

Need to pick one that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve

Name 2 indicators

  • Methyl orange

  • Phenolphthalein

State the colour of methyl orange at low pH (in acid)

Red

State the colour of methyl orange at high pH (in base)

Yellow

State the approx. pH of colour change for methyl orange

3.1-4.4

State the colour of phenolphthalein at low pH (in acid)

Colourless

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A-level Chemistry: 3.1.10 Acids and Bases Part 2
TermDefinition

How do you deicide which indicator to use?

Need to pick one that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve

Name 2 indicators

  • Methyl orange

  • Phenolphthalein

State the colour of methyl orange at low pH (in acid)

Red

State the colour of methyl orange at high pH (in base)

Yellow

State the approx. pH of colour change for methyl orange

3.1-4.4

State the colour of phenolphthalein at low pH (in acid)

Colourless

State the colour of phenolphthalein at high pH (in base)

Pink

State the approx. pH of colour change for phenolphthalein

8.3-10

State an indicator you can use for a strong acid/strong base titration

Methyl orange or phenolphthalein

(Rapid pH change over range for both indicators)

State an indicator you can use for a strong acid/weak base titration

Methyl orange

State an indicator you can use for a weak acid/strong base titration

Phenolphthalein

State an indicator you can use for a weak acid/weak base titration

Can't use indicator

Why can't you use a indicator for weak acid/weak base titrations?

Don't get sharp change in weak acid/weak base titration

What should you use instead of an indicator for weak acid/weak base titrations?

pH meter

Name 3 things you can do to make your titration results as accurate as possible

  1. Measure neutralisation volume as precisely as possible

    • (i.e. to nearest 0.05 cm3)

  2. Repeat titration at least 3 times and take mean titre value

    • Makes result reliable

  3. Don't use anomalous results

    • Results should be within 0.1 cm3 of each other

If you use a pH meter, describe how you can work out how much acid or base is needed for neutralisation.

When a diprotic acid reacts with a base, the reaction occurs in __ ____

2 stages

Why is it that when you react a diprotic acid with a base, the reaction occurs in 2 stages?

∵ 2 protons are removed from acid separately

Diprotic acid (e.g. ethanedioic acid) + strong base results in a pH curve with __ equivalence points

2

Write the equation for when hydrogen ions react with hydroxide ions

H+ + OH- → H2O

Write the equation for when hydrogen ions react with carbonate ions

2 H+ + CO32- → H2O + CO2

Write the equation for when hydrogen ions react with bicarbonate ion

H+ + HCO3- → H2O + CO2

Write the equation for when hydrogen ions react with ammonia

H+ + NH3 → NH4+

What is a buffer?

Solution that resists changes in pH when small amounts of acid or base are added, or when it's diluted

How are acidic buffers made?

Made by mixing weak acid with one of its salts

e.g. ethanoic acid and sodium ethanoate

Explain what happens when add a small amount of acid to this acidic buffer solution

  • H+ conc. ↑

  • Most of extra H+ ions combine with CH3COO- ions to form CH3COOH

  • Shifts equilibrium to left = reducing H+ conc. close to its original value

  • ∴ pH doesn't change

Explain what happens when add a small amount of base to this acidic buffer solution

  • OH- conc. ↑

  • Most of extra OH- ions react with H+ ions to form water = removing H+ ions from solution

  • Causes more CH3COOH to dissociate to form H+ ions = shifting equilibrium to the right

  • H+ conc. increases until it's close to original value ∴ pH doesn't change

How are basic buffers made?

Made by mixing weak base with one of its salt

e.g. solution of ammonia and ammonium chloride acts as a basic buffer

In a solution of ammonia and ammonium chloride acts a ____ ___, the salt …

basic buffer, the salt fully dissociates in solution

In a solution of ammonia and ammonium chloride, some of NH3 will…

react with water molecules

Explain what happens when you add a small amount of acid to this basic buffer solution

  • H+ conc. ↑

  • Some of H+ ions react with OH- ions to make H2O

  • ∴ equilibrium position moves to right to replace OH- ions that have been used up

  • Some of H+ ions react with NH3 = NH4+

  • These reactions remove most of extra H+ ions ∴ pH won't change much

Explain what happens when you add a small amount of base to this basic buffer solution

  • OH- conc. ↑

  • Most of extra OH- ions will react with NH4+ ions to form NH3 and H2O

  • Equilibrium will shift to left, removing OH- ions from solution

  • Stops pH from changing much

Making a buffer by adding a salt solution

Calculate the pH of a buffer made from 45cm3 of 0.1 mol dm-3 ethanoic acid and 50cm3 of 0.15 mol dm-3 sodium ethanoate (Ka = 1.7 x 10-5)

Making buffer by mixing weak acids and strong bases

55cm3 of 0.5 mol dm-3 CH3CO2H is reacted with 25cm3 of 0.35 mol dm-3 NaOH. Calculate the pH of the resulting buffer solution. Ka = 1.7 x 10-5 mol dm-3

CH3CO2H + NaOH → CH3CO2Na + H2O

pH ≈ 4.74

Calculating change in pH of buffer on addition of acid/alkali

2cm3 of 0.10 mol dm-3 NaOH is added to 100cm3 of a buffer solution containing 0.15 mol dm-3 ethanoic acid and 0.10 mol dm-3 sodium ethanoate (Ka ethanoic acid = 1.74 x 10-5 mol dm-3). Calculate the change in pH of the buffer solution.

Change in pH ≈ +0.05

Draw pH curve when…

Flask: 25 cm3 0.10 mol dm-3 HNO3

Burette: 50 cm3 0.20 mol dm-3 NaOH

Dilution of a Strong Acid

Calculate the pH of the solution formed when 250 cm3 of 0.300 mol dm-3 H2SO4 is made up to 1000 cm3 solution with water

  • [H+] in original H2SO4 solution

    • 2 x 0.300 = 0.600

  • [H+] in diluted solution

    • 0.600 x 250/1000 = 0.150

  • pH = -log(0.150)

= 0.82

Reaction between strong acid & strong base

Calculate the pH of the solution formed when 50 cm3 of 0.100 mol dm-3 H2SO4 is added to 25 cm3 of 0.150 mol dm-3 NaOH

pH ≈ 1.00

Reaction between strong acid & strong base

Calculate the pH of the solution formed when 25 cm3 of 0.250 mol dm-3 H2SO4 is added to 100 cm3 of 0.2 mol dm-3 NaOH

  • Mol H+ added: 5 x 10-3 x 0.1 = 5 × 10–4

  • Mol HCOOH: 2.35 x 10-2 + 5 x 10-4 = 2.40 × 10–2

  • Mol HCOO–: 1.84 x 10-2 – 5 x 10-4 = 1.79 × 10–2

  • [H+] = Ka × [HA] / [A-]

    • 1.78 × 10-4 × 2.40 × 10-2 / (1.79 × 10-2 ) = 2.39 × 10-4

  • ​pH = 3.62

5 cm3 of 0.10 mol dm-3 hydrochloric acid is added to 1 dm3 of a buffer solution containing 2.35 x 10-2 mol of methanoic acid and 1.84x10-2 mol of sodium methanoate (Ka methanoic acid = 1.78 x 10-4 mol dm-3). Calculate the pH of the buffer solution after this addition. (4)

  • Mol H+ added: 5 x 10-3 x 0.1 = 5 × 10–4

  • Mol HCOOH: 2.35 x 10-2 + 5 x 10-4 = 2.40 × 10–2

  • Mol HCOO–: 1.84 x 10-2 – 5 x 10-4 = 1.79 × 10–2

  • [H+] = Ka × [HA] / [A-]

    • 1.78 × 10-4 × 2.40 × 10-2 / (1.79 × 10-2 ) = 2.39 × 10-4

  • ​pH = 3.62

Describe how to investigate of how the pH of a solution of ethanoic acid changes as sodium hydroxide solution is added

  1. Calibrate the pH meter

    1. Place pH probe in standard buffer solutions (e.g. pH 4, 7, 9) & record pH readings

    2. Plot graph of pH reading (y-axis) against the pH of the buffer solution

  2. Rinse pipette with ethanoic acid & fill it with this

    • Transfer 20cm3 of ethanoic acid to beaker

  3. Rinse burette with NaOH & then fill it with this

  4. Rinse pH probe with distilled water and clamp it so bulb is immersed in ethanoic acid

    • Use to rod to stir solution & record pH

  5. Add NaOH to ethanoic acid in small regular intervals

    • e.g. 2cm3 and then change to smaller intervals around 22cm3 e.g. 0.2cm3

    • Repeat until alkali in excess

    • Add in smaller increments near endpoint

  6. Stir solution and take a reading after each addition

Explain briefly why a pH meter should be calibrated before use (1)

Over time/after storage meter doesn't give accurate readings

State why water at 50°C is neutral (1)

[H+] = [OH−]

Describe breifly how you would ensure that a reading from a pH meter is accurate (2)

  • Calibrate meter with solution(s) of known pH/buffer(s)

  • Adjust meter/plot calibration curve

Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air. The pH of the pH 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution changes. (3)

  • CO2

  • pH decreases

  • acidic (gas) reacts with alkali / OH−

    • ​or CO2 + 2OH− → CO32− + H2O

    • CO2 + OH− → HCO3−

Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator. (2)

  • The change in pH is gradual at the end point

  • An indicator would change colour over a range of volumes of sodium hydroxide / indicator would not change colour rapidly with a few drops of NaOH