Topic 8: Acids and Bases Flashcards
What is the Bronsted-Lowry theory of Acids and Bases?
Acids are proton/H+ donors. Bases are proton/H+ acceptors.
What charges do the conjugate pairs gain?
The conjugate acid gains a positive charge.
The conjugate base gains a negative charge.
What colour is Litmus in acids and bases?
Red in acids.
Blue in bases.
What colour is Methyl orange in acids and bases?
Red in acids.
Yellow in bases.
What colour is phenolphthalein in acids and bases?
Colourless in acids.
Pink in bases.
Give the formula for an acid reacting with a base
Acid + base —> Salt + water
Give the formula for an acid reacting with a metal oxide
Acid + metal oxide —> Salt + water
Give the formula for an acid reacting with a metal
Acid + metal —> Salt + hydrogen
Give the formula for an acid reacting with a carbonate
Acid + carbonate —> Salt + water + carbon dioxide
What does a change of one pH unit represent?
And if the pH increases how does this affect [H+]?
A ten-fold change in [H+].
Increase in pH = decrease in [H+]
Dilute acids vs concentrated acids (+ what ion does so)
Dilute acids demonstrate the typical properties of acids.
i.e. they turn litmus red
Concentrated acids do not demonstrate the typical properties of acids.
*IT IS THE HYDRONIUM ION THAT GIVES ACIDS THEIR ACID PROPERTIES
Define strong acids
Strong acids ionize/dissociate almost completely in water. “—>”
Define weak acids
Weak acids will transfer a few protons to water. They mostly remain as molecules. “⇌”
Give three examples of strong acids and three examples of weak acids
Hydrochloric acid (HCl)
Nitric acid (HNO₃)
Sulfuric acid (H₂SO₄)
——
Phosphoric acid (H₃PO₄)
Ethanoic acid (CH₃COOH)
Carbonic acid (H₂CO₃)
Give four examples of strong bases and two examples of weak bases
Lithium hydroxide LiOH
Sodium hydroxide NaOH
Potassium hydroxide KOH
Barium hydroxide Ba(OH)₂
——
Ammonia NH₃
Ethylamine CH₃CH₂NH₂
Electrical conductivity, rate of reaction and pH for strong and weak acids / bases
Strong acids and bases
- Good electrical conductors as lots of ions (charged particles)
- Fast rate of reaction
- pH
Acids: Low as many H+ ions
Bases: High as many OH- ions
Weak acids and bases
- Poor electrical conductors as few ions
- Slow rate of reaction
- pH
Acids: Weakly acidic as less H+ ions
Bases: Weakly basic as less OH- ions
A weak acid has a …
A strong acid has a …
strong conjugate base.
weak conjugate base (and vice versa)
Normal rainwater vs acid rain
Normal rainwater is naturally acidic due to the presence of dissolved carbon dioxide which forms weak carbonic acid. Normal rain has a pH above 5.6.
Acidic rain is caused by sulfur and nitrogen oxides and has a pH below 5.6.
Describe how sulfur oxides contribute to acid rain
Sulfur dioxide is produced by the burning of fossil fuels.
It can then produce sulfurous acid (H₂SO₃), sulfur trioxide (SO₃) or sulfuric acid (H₂SO₄).
Describe how nitrogen oxides contribute to acid rain
Nitrogen monoxide is produced by internal combustion engines in vehicles.
It can produce nitrogen dioxide (NO₂), nitrous acid (HNO₂) and nitric acid (HNO₃).
What are the effects of acid deposition?
- Impact on materials. Corrodes metals and erodes structures. (Nitrous acid and sulfurous acid)
- Is toxic to plant life as it stops plants’ ability to undergo photosynthesis and damages the roots.
- Is toxic to marine life/affects marine ecosystems. Contributes to eutrophication (algae growth) and lake acidification. (Carbonic acid)
- Can cause respiratory issues in humans.
How can SO₂ emissions be reduced?
Pre-combustion methods
- Crushing coal and washing with water
- Hydrodesulfurization (reacts sulfur with hydrogen to form hydrogen sulfide)
Post-combustion methods
- Flue gas desulfurization
How can NO₂ emissions be reduced?
Pre-combustion methods
- Catalytic converters in vehicles
- Lower temperature of combustion (this reduces the amount of nitrogen monoxide produced)
What is the Lewis theory of Acids and Bases?
A Lewis acid is an electron acceptor (it is electron deficient) i.e. an electrophile
A Lewis base is an electron donor (it is electron-rich) i.e. a nucleophile.
(REMEMBER BASE MAKES BANK BC THEY’RE RICH)
What is the value of pKw?
14
What is the value of Kw?
10^-14
What type of relationship does K of acids or bases have with pK of acids or bases? Demonstrate with strong acids.
Inverse relationship. i.e. in strong acids Ka is high (bc lots of products are formed) and the pKa is low.
What are the assumptions we make when calculating the pH of weak acids or bases?
The concentration of the products are equal.
The concentration of the reactant stays the same because so little has broken down.
Define amphiprotic
Able to both donate or accept a proton (act as a Bronsted-Lowry acid or base).
Remember ‘h’ in amphiprotic as H+ is the basis of the Bronsted-Lowry definition.
Define amphoteric
Able to act as an acid or a base/display acidic or basic properties.
Define a buffer solution
An aqueous solution consisting of a weak acid/base and its conjugate pair that resists changes to pH when small amounts of acid or base are added. This is because the solution is in equilibrium (Le Chatelier’s principle - the system will act to minimise the effect of the change).
Preparations of a buffer solution
1) Mixing roughly equal proportions of a weak acid/base and its conjugate pair
2) Partially neutralising a weak acid with a strong base
At the equivalence point, a strong acid and a strong base give what pH? (pH curves)
pH of 7
At the equivalence point, a strong acid and a weak base give what pH? (pH curves)
pH<7 (Buffer region is present. This is where the pH is hardly changing.)
At the equivalence point, a weak acid and a strong base give what pH? (pH curves)
pH>7 (Buffer region is present. This is where the pH is hardly changing.)
At the equivalence point, a weak acid and a weak base give what pH? (pH curves)
pH of 7
Where is the equivalence point on a pH curve?
Halfway up the point of inflexion (the steep line).
On a pH curve where is the pKa found?
Half of the equivalence point.
Why with a weak acid and strong base or strong acid and weak base is the pH NOT 7?
Because one of the products of the neutralisation reaction will react further in hydrolysis (proton transfer).
How do we select an indicator for titrations?
If the pKa is + or - 1 of the equivalence point.
i.e. it changes colour at pH values close to the equivalence point and does not interfere with the reaction.
What are acid-base indicators?
Acid-base indicators are weak acids or weak bases. The species differs in colour from its conjugates.
Hln ⇌ H⁺ + ln⁻
Where Hln is colour 1, ln⁻ is colour 2.
