3.1.12 - Acids and Bases Flashcards
1
Q
Define ‘Bronsted-Lowry Acid’
A
Proton donor
2
Q
Define ‘Bronsted-Lowry Base’
A
Proton Acceptor
3
Q
State the formula for pH?
A
<img></img>
4
Q
What is meant by the term strong when describing an acid?
A
Completely dissociates into H+ ions in water
5
Q
What is the value for Kw for all aqueous solutions at 25oC?
A
1 x 10-14 mol2dm-6
6
Q
How can you improve the accuracy of a titration curve experiment?
A
Maintaining constant T; calibrating pH meter (they can lose accuracy on storage)
7
Q
Describe a WA-WB Titration Curve.
A
<img></img>
8
Q
How do indicators work?
A
<div><img></img><br></br></div>
9
Q
What is a buffer solution?
A
One where the pH does not change significantly if small amounts of acid or alkali are added to it
10
Q
What is a basic buffer?
A
Made from a weak base and salt of that weak base.
e.g ammonia and ammonium chloride
11
Q
How do buffer solutions work if small amounts of acid are added?
A
<ul><li>Equilibrium shifts to the left</li><li>To use up all the H+ ions added</li><li>As there is a large [] of salt ion in the buffer, the ratio of [HX] / [X-] remains almost constant</li><li>pH remains almost constant</li></ul>
12
Q
How do buffer solutions work if small amounts of base are added?
A
<ul><li>OH- ions react with H+ to form water</li><li>Equilibrium shifts to the right to form more H+ ions</li><li>Some [HX] changes to [X-]</li><li>There is a large [X-] in the solution, so ratio [HX] / [X-] remains almost constant</li><li>pH remains almost constant</li></ul>
13
Q
Explain why [H2O] isn’t included in the Kw formula.
A
<ul><li>[H2O] is <u>very high </u>compared to [H+] and [OH-]</li><li>[H2O] is effectively constant</li></ul>
14
Q
When do you observe the buffer region for a weak acid and strong base?
A
below 7
15
Q
When do you observe the buffer region for a weak base and strong acid?
A
greater than 7
16
Q
Describe briefly the experimental procedure to form a titration curve.
A
<ul><li>Place a fixed amount of alkali solution into a beaker</li><li>Use a burette to add small portions of acid</li><li>Stir and measure pH after each addition of acid</li></ul>
17
Q
Suggest how a titration curve procedure could be refined to identify a suitable indicator.
A
<ul><li>Repeat with each proposed indicator</li><li>Select one that has the most <b>rapid colour change </b>within the steep/vertical region of the graph (quote these pH values)</li></ul>
18
Q
Where does equilibrium lie in a KW reaction?
A
Equilibrium lies really left
19
Q
Define Buffer solution:
A
Solution that resists changes in pH when small amounts of acid or alkali are added
20
Q
Draw the graph of a strong acid and strong base:
A
<img></img>
21
Q
Draw the graph of a strong acid and Weak base:
A
<img></img>
22
Q
Draw the graph of a Weak acid and strong base:
A
<img></img>
23
Q
Draw the graph of a Weak acid and Weak base:
A
<img></img>
24
Q
What is the half neutralization point?
A
<img></img>
25
Indicator used for strong acid / weak base
Methyl orange
26
Indicator used for strong Acid / weak Base
Phenolphthalein
27
Indicator used for Weak acid / weak base
No indicators used. Just pH meter
28
What is an alkali?
Any substance that produces hydroxide ions when in aqueous solution
29
What is a weak base?
- Slightly dissociate in aqueous solution
- Most common weak base is ammonia
- Weak bases are covalent substances which dissolve in water and dissociate slightly
30
Calculation OH From Kw formula:
31
What's the difference between the equivalence point and half neutralization point?
Equivalence point - No H+ ions left to react. Neutralization point - Equal amounts of H+ and OH- ions
32
What happens when we add H+ Ions?
Equilibrium shifts to the left where more weak acid is formed.
33
What is an acidic buffer made from?
A weak acid and its salt.
34
How do salts dissociate?
Salts dissociate fully - where the equilibrium lies well to the right
35
How are Buffers used in Shampoo?
Shampoos are marginally acidic (pH 5-6) where when the hair is exposed to alkaline conditions; it can become damaged. Buffer helps resist changes in the pH to keep the hair strong and soft.
36
Explain how buffers are used in washing powders:
- Biological washing powders contain enzymes
- Where Buffers resist the changes in pH allowing the enzymes to work at their optimum pH
37
Explain how buffers are used in human blood:
- Vital to make sure that the pH is maintained at pH 7.4 within the human blood
- Bodily enzymes rely on this so buffer helps maintain.
38
Suggest why this pure water at 10 Degrees is Not alkaline?
[H+] = [OH-]
39
Suggest why the pH probe is washed with distilled water between each of the calibration measurements?
Different solutions must not contaminate each other.
40
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached?
To avoid missing the end point.
41
What acts as a buffer in blood?
Carbonic acid - Hydrogen carbonate equilibrium acts as a buffer in the control of blood pH
42
What does Carbonic Acid - Hydrogen carbonate equilibrium do?
Buffer is present in blood plasma; maintaining a pH between 7.35-7.45
43
Equilibrium of Carbonic Acid - Hydrogen carbonate equilibrium:
44
Draw a weak acid and weak base curve:
45
What is the Half-Equivalence point?
When half of the acid has been neutralised.
46
Outline a simple way of calibrating a pH probe:
- Use a buffer
- Rinse Probe with distilled water between measurements
- Measure the pH of more than one Buffer solution
- Plot a graph of pH of buffer against pH of probe
47
Why would the end point of a reaction be hard to judge accurately using an indicator?
- The change in pH is gradual
- Where an indicator would change colour over time.
48
Suggest why calibrating a pH meter just before improves its accuracy of the pH measurement:
Over time meter does not give accurate readings.
49
Describe how you would obtain the pH for the curve for the titration:
- Measure pH
- Add alkali
- Stir mixture
- Measure pH
- Repeat until alkali is in excess
- Add smaller increments nearer to end point.
50
Explain why data books do not usually contain values of Ka for strong acids:
- Strong acids completely dissociate
- Ka value is usually very large.
51
Propanoic acid with sodium carbonate:
52
Acid?
Proton Donor
53
Base?
Proton acceptor
54
Define a Bronsted-Lowry acid and a Bronsted-Lowry base.
A Bronsted-Lowry acid is a substance that can donate a proton (H+), while a Bronsted-Lowry base is a substance that can accept a proton.
55
How do you calculate the pH of a strong acid like HCl?
For strong acids that completely dissociate, the pH is calculated using the formula pH = -log[H+], where [H+] is the concentration of hydrogen ions, equal to the initial concentration of the acid.
56
What is the pH of pure water at 25°C, and how is it calculated?
The pH of pure water at 25°C is 7.0, calculated using the autoionization constant of water (Kw = 1 x 10^-14 mol²/dm⁶) and finding [H+], which equals √Kw = 1 x 10^-7 M, then pH = -log[H+].
57
How does an increase in temperature affect the pH of pure water?
Increasing temperature increases the autoionization of water (an endothermic process), resulting in more H+ ions and a lower pH. For example, at 50°C, the pH drops as Kw increases due to increased ionization.
58
How do you calculate the pH of a strong base like NaOH?
For a strong base like NaOH, which completely dissociates, calculate the [OH-], then use the Kw expression to find [H+]: [H+] = Kw / [OH-], and pH = -log[H+].
59
What do Ka and pKa represent in acid-base chemistry?
Ka is the acid dissociation constant, which quantifies the strength of a weak acid. pKa is the negative logarithm of Ka (pKa = -log Ka), providing a more convenient measure of acid strength.
60
What is a buffer solution, and how does it work?
A buffer solution resists changes in pH upon the addition of small amounts of acid or base. It usually consists of a weak acid and its conjugate base or a weak base and its conjugate acid. The buffer maintains pH by shifting its equilibrium to counteract added H+ or OH- ions.
61
How do you prepare a buffer solution, and calculate its pH?
To prepare a buffer, mix a weak acid with a salt of its conjugate base. Calculate the pH using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
62
Define pH
Measure of acidity by measuring concentration of H⁺ ions.
63
Define Buffer
Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base.
64
Explain the process in which a pH titration is carried out.
- Calibration curve of pH meter and electrode using buffer solutions.
- Prepare graph of pH against volume of alkali added.
- Pipette 25cm³ of alkali solution into a flask.
65
What is a measure of acidity?
Measure of acidity by measuring concentration of H⁺ ions.
66
Define Buffer.
Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base.
67
Explain the process in which a pH titration is carried out.
1. Calibration curve of pH meter and electrode using buffer solutions.
2. Prepare graph of pH against volume of alkali added.
3. Pipette 25cm³ 0.1M acid into 250cm³ beaker. Measure pH and record as initial pH.
4. Add 0.1M alkali from burette in appropriate portions, measuring and plotting pH after each addition.
5. Repeat up to 50cm³.
6. Plot curve of best fit.
68
Why does the value of Kw increase with temperature?
The dissociation of water is endothermic, meaning it absorbs heat. As temperature increases, the equilibrium shifts towards more product formation (more H+ and OH- ions), increasing the value of Kw.
69
How to calculate the pH of water at 50°C?
Use the Kw value at 50°C to calculate [H+] as (Square root Kw). Then, use pH = -log10[H+] to find the pH. For example, if Kw at 50°C is 5.48×10−14, then Square root of [H+] = 5.48×10−14 and pH can be calculated accordingly.
70
Why is it important to wash the pH probe with distilled water between calibration measurements?
Washing the pH probe prevents contamination between solutions, ensuring accurate and reliable pH measurements without interference from previous solutions.
71
Why do you add smaller volumes of titrant as you approach the endpoint in a titration?
Adding smaller volumes near the endpoint ensures more precise measurement of the point where the reaction completes, allowing for a sharper transition and more accurate determination of the equivalence point.
72
Why are all indicators suitable for a titration involving hydrochloric acid and sodium hydroxide?
These indicators cover the pH range that includes the sharp pH change observed near the equivalence point of the titration between a strong acid and a strong base, ensuring visible color change at the correct moment.
73
Explain why adding sodium sulfate to a solution of NaHSO4 increases the pH.
Sodium sulfate provides additional sulfate ions, which combine with H+ to form HSO4-, reducing the concentration of H+ ions and thus increasing the pH.
74
Why does the pH of a buffer solution remain relatively constant upon dilution?
A buffer solution resists changes in pH upon dilution because the ratio of the concentration of the weak acid to its conjugate base remains constant, maintaining the pH according to the Henderson-Hasselbalch equation.
75
What pH curve is this?
Strong acid and a Strong base.
76
Define a weak acid.
A weak acid is one that does not completely dissociate in solution, meaning only a small percentage of the acid molecules donate protons (H+) in water, resulting in an equilibrium between the undissociated acid and the ions.
77
Which indicator is most suitable for titrating a weak acid with a strong base?
Choose an indicator whose pH range falls within the rapid pH change at the equivalence point of the titration. For a weak acid and strong base, an indicator that changes color in a basic range, like phenolphthalein, is suitable.
78
How to calculate the mass of sodium methanoate needed to make a buffer solution with a specific pH?
Use the Henderson-Hasselbalch equation, pH = pKa + log([base]/[acid]), rearrange to find the ratio of base to acid needed for the desired pH, calculate the moles of acid present, and from the ratio find the moles (and subsequently mass) of the base required.
79
Which acid-base pair results in the highest pH at the equivalence point?
CH3COOH and NaOH will have the highest pH at the equivalence point because the reaction produces a weakly acidic solution due to the presence of CH3COO- ions, which are weakly basic.
80
Suggest a strong acid and alkali combination that could produce a typical pH titration curve.
Hydrochloric acid (HCl) and sodium hydroxide (NaOH) are a common strong acid and strong alkali pair used in titrations, resulting in a sharp pH change at the equivalence point.
81
Explain the steps to carry out a pH curve.
1. Measure pH of acid.
2. Add acid in 1-2 cm³ small portions.
3. Stir mixture.
4. Measure pH.
5. Repeat until all alkali in excess.
6. Add in smaller increments near endpoint.
82
Describe how to prepare a buffer solution.
To prepare a buffer solution, mix a weak acid with a salt of its conjugate base in a specific ratio to achieve the desired pH. Adjust the volume with a solvent (usually water) to the desired concentration while maintaining the acid-base ratio.
83
What are the different methods for storing hydrogen in fuel cells?
Hydrogen can be stored as a compressed gas in high-pressure tanks, adsorbed onto the surface of materials, or absorbed within the structure of solid materials.
84
List the environmental benefits of using fuel cells compared to conventional combustion engines.
Fuel cells produce less pollution and CO2 emissions since the only byproduct of a hydrogen fuel cell is water. Additionally, they are more efficient at converting fuel into energy.
85
Calculate the pH of the strong base 0.1 mol dm-3 NaOH.
Assume complete dissociation.
Kw = [H+ (aq)]/[OH- (aq)] = 1x10-14
[H+ (aq)] = Kw / [OH- (aq)] = 1x10-14 / 0.1 = 1x10-13 mol dm-3
pH = - log[1x10-13] = 13.00.
86
Calculate the pH of the resulting solution when 25cm3 of 0.1 mol dm-3 NaOH is added to 50cm3 of 0.1 mol dm-3 CH3COOH.
From the volumes and concentrations spot it is half neutralisation (or calculate).
pH = pKa = -log(1.7 x 10-5) = 4.77.
87
What is a basic buffer solution made from?
A basic buffer solution is made from a weak base and a salt of that weak base (made from reacting the weak base with a strong acid).
88
Draw the weak acid - Weak base graph.
Refer to the provided image.
