Topic 8: Acids and Bases Flashcards
Bronsted-Lowry acid
A substance that behaves as an acid when it donates a proton.
Bronsted-Lowry base
A substance that behaves as a base when it accepts a proton.
Conjugate acid-base pair definiton
A pair of species differing by a single proton
An acid becomes into …
conjugate base
A base becomes into a …
conjugate acid
Amphiprotic definition
Substance capable of accepting and donating H+, thus able to act as a Bronsted-Lowry acid and base
Example of amphiprotic substance
H2O (H3O+ / OH-)
Amphoteric definiton
A species that can act as an acid or base, including reactions that do not involve H+ transfer
Relationship between amphoteric and amphiprotic reactions
All amphiprotic species are also amphoteric but not all amphoteric species are amphiprotic.
Equation of reaction of an acid with a metal
Metal + Acid => Salt + H2(g)
Example: Zn(s) + 2HCl(aq) => ZnCl2(aq) + H2(g)
Which type of metals do acids react with?
- Reactive metals (e.g., Ca / Mg / K / Zn)
- No unreactive metals (e.g., Cu / Au / Ag)
Neutralization reaction
Chemical reaction where a base and an acid react to form a salt and water
Why are neutralization reactions exothermic?
- When a strong acid and a strong base are dissolved in water, they are completely dissociated into their ions.
- The anions from the acid and the cations from the base remain in solution as spectator ions.
- The only reaction that takes place is the reaction between hydronium and hydroxide ions to form water, and this reaction is exothermic.
Equation of reaction of an acid with metal oxides and metal hydroxides
Acid + Base => Salt + Water
Example: HCl(aq) + NaOH(aq) => NaCl(aq) + H2O(l)
Equation of reaction of an acid with carbonate and hydrogen carbonate
Acid + Carbonate => Salt + H2O(l) + CO2(g)
Example: Na2CO3(aq) + H2SO4(aq) => Na2SO4(aq) + H2O(l) + CO2(g)
Indicator definition
- Substances that have different color according to the concentration of H+ ions in the solution
- Weak acids whose conjugates have different colors
Common indicators (Acid => Base)
Methyl orange (Red => Yellow)
Litmus (Pink => Blue)
Phenolphthalein (Colorless => Pink)
Why does water self-ionize?
Due to its amphoteric nature
Chemical equation of ionization of water
2H2O(l) ⇌ H3O+(aq) + OH-(aq)
Ionization constant of water (Kw)
Kw = [H+][OH-] = 1.0 x 10^-14
Based on the relative concentrations of H+ and OH-, a solution is:
a) Neutral if…
b) Acidic if…
c) Basic if…
a) [H+] = [OH-]
b) [H+] > [OH-]
c) [H+] < [OH-]
Formulas for calculating pH and pOH from their concentrations
- pH = -log[H+]
- pOH = -log[OH-]
Formula for calculating pKw at 298 K
pKw = pH + pOH = 14.00
Acidity of a solution when pH is:
a) = 7.00
b) < 7.00
c) > 7.00
a) Neutral
b) Acidic
c) Basic
1 unit change in pH equals … in the H+ ion concentration
tenfold change
Methods to measure pH
- Universal indicator
- pH meter
Functioning of universal indicator
The substance tested gives a distinct color, which is compared with a color chart supplied with the indicator
Functioning of pH meter
The probe directly reads the [H+} concentration through a special electrode
What is the main difference between strong and weak acids/bases?
Extent of ionization/dissociation
- Strong fully dissociate in solution (=>)
- Weak partially dissociate in solution (<=>)
Which type of substance produces more OH-/H+ ions at the same concentration?
Strong acids/bases
7 strong acids
- HCL
- HNO3
- H2SO4
- HBr
- HI
- HClO4
- HClO3
Examples of weak acids
- CH3COOH
- H2CO3
- C₃H₆O₂
- H3PO4
Examples of strong bases
- Group 1 Hydroxides
- Ba(OH)2
Examples of weak bases (2)
- NH3 (Ammonia)
- C2H5NH2
Definition of a strong acid
It is a good proton donor and has a weak conjugate base
Definition of a strong base
It is a good proton acceptor and has a weak conjugate base
Experimental techniques used to determine acid / base strengths
a) Conductivity
b) pH
c) Rates of reaction
Hypothesis using conductivity to determine strength of an acid / base
Strong acids and bases will be better conductors than weak acids and bases at the same concentration
Explanation of difference in conductivity between strong and weak acids/bases
- The concentration of H+ will be higher in a strong acid than in a weak acid, and the concentration of OH- will be higher than in a weak base
- The more ions present, the more conductive a solution.
Hypothesis using pH to determine strength of an acid / base
At the same concentration, strong acids have lower pH than weak acids / strong bases have a higher pH than weak bases
Explanation of difference in pH between strong and weak acids/bases
- Because pH is a measure of H+ concentration, the pH scale can be used directly to compare the strengths of acids (provided they are of equal molar concentration).
- The higher the H+ concentration, the lower the pH value.
Hypothesis using rates of reaction to determine strength of an acid / base
Strong acids will react more vigorously with metals/carbonates
Explanation of difference in pH between strong and weak acids/bases
- The reaction of acids depend on the concentration of H+ ions.
- Reaction rate will be sped up with stronger acids.
Difference between strength and concentration
- How much an acid/basedisassociates in aqueous solution
- Number of moles of acid/base in a certain volume.
Definition of acid deposition
Process by which acid forming pollutants are deposited on the Earth’s surface
What is the natural pH of rain? Why does it have that pH?
Rain is naturally acidic due to dissolved CO2 and has a pH of 5.6
What is the pH of acid deposition? How is it formed?
- It has a pH below 5.6
- Acid deposition occurs when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3.
What are the sources of sulfur oxides?
- Extracting metals from their ores
- Burning of fossil fuels (e.g., coal / heavy oil)
Chemical equation for the formation of sulfur oxides from combustion reactions
- S(s) + O2(g) => SO2(g)
- 2SO2(g) + 02(g) => 2SO3(g)
Chemical equation for the dissolution of sulfur oxides in water
- H2O(l) + SO2(g) => H2SO3(aq)
- H2O(l) + SO3(g) => H2SO4(aq)
What is the source of nitrogen oxide?
Internal combustion engines
Chemical equation for the formation of nitrogen oxides from combustion reactions
- N2(g) + O2(g) => 2NO(g)
- 2NO(g) + O2(g) => 2NO2(g)
Chemical equation for the dissolution of nitrogen oxides in water
H2O(l) + 2NO2(g) => HNO2(aq) + HNO3(aq)
Effects of acid deposition
- Impact on materials
= Expansion and stress in stonework
= Erosion of structures
= Corrosion reaction (Rusting) - Impact on the environment / human health
Expansion and stress in stonework
a) Chemical equation for the reaction of SO2(g) and CaCO3(s)
2CaCO3(s) + 2SO2(g) + O2(g) => 2CaSO4(aq) + 2CO2(g)
Erosion of structures
a) Chemical equation for the reaction of CaCO3 and H2SO4 / HNO3
- CaCO3(s) + H2SO4(aq) => CaSO4(aq) + CO2(g) + H2O(l)
- CaCO3(s) + 2HNO3(aq) => Ca(NO3)2 (aq) + C02(g) + H2O(l)
Corrosion reaction
a) Chemical equation for the reaction of Fe and SO2
- Fe(s) + SO2(g) + O2(g) => FeSO4(s)
Corrosion reaction
a) Chemical equation for the reaction of Fe and H2SO4
b) Chemical equation for the reaction of Al2O3 and 6HNO3
a) Fe(s) + H2SO4(aq) => FeSO4(aq) + H2(g)
b) Al2O3(s) + 6HNO3(aq) => 2Al(NO3)3 + 3H2O(l)
Impacts of acid deposition on environment
- Displaces metal ions from soil and prevents the growth / development of plants
- Elevates acid levels in lakes and rivers, affect pH sensitive ecosystems
Impacts of acid deposition on human health
- Causes the poisoning of fish, eventually resulting in damage to human health
- Irritates mucous membrane causing respiratory illness (asthma)
Methods used to reduce SO2 emissions
- Pre-combustion
- Post-combustion
Precombustion methods used to reduce SO2 emissions
Hydrodesulfurization (HDS) is a catalytic process that removes sulfur from refined petroleum products by reacting it with hydrogen to form H2S, which is used to manufacture H2SO4
Postcombustion methods used to reduce SO2 emissions
Flue-gas desulfurization can remove up to 90% of SO2 from flue gas in the smokestacks of coal-fired power stations before it is released into the atmosphere
