R3.1 - proton transfer reactions Flashcards
What is a bronsted lowry acid?
A proton doner.
What is a bronsted lowry base?
A proton acceptor.
What is a proton?
In aqueous solution, it can be represented by H+ (aq) or H3O+ (aq).
What were some key discoveries about acids and bases?
1887 - Arhenius said that acids form H+ ions and alkalis form OH- ions.
1823 - Bronsted Lowry said that acids and bases donate and accept acids.
What is the difference between a base and an alkali?
Base - substances which accept H+ ions.
Alkalis - substances that dissolve in water to form OH- ions.
What are conjugate pairs?
Acid and base pairs which differ by H+ (1 proton).
How do you work out the conjugate acid?
Add H+.
How do you work out the conjugate base?
Remove H+.
What is amphiprotic?
Substances that can act as both Bronsted Lowry acids and bases; have a double identity.
Why is water amphiprotic?
It can act as a proton acceptor and a proton donator.
- Must possess both a lone pair of electrons and H that can be released as H+.
What is the equation for water acting as an acid with NH3?
NH3 (B) + H2O (A) <–> NH4+ (CA) + OH- (CB)
What is the equation for water acting as a base with ethanoic acid?
CH3COOH (B) + H2O (A) <–> CH3COO- (CA) + H3O+ (CB)
What allows species to have a double identity?
- To act as a Bronsted-Lowry acid, they must be able to dissociate and release H+.
- To act as a Bronsted-Lowry base, they must be able to accept H+, which means they must have a lone pair of electrons.
What does amphoteric mean?
It can behave as an acid or base by reacting with the other substance.
E.g) Ammonium oxide in group 3.
What are the equations for Al2O3?
Al2O3 (s) + 3H2SO4 (aq) –> Al2(SO4)3 (aq) + 3H2O (l)
Al2O3 (s) + 3H2O (l) + 2OH- (aq) –> 2Al(OH)4- (aq)
Why can Al2O3 not be amphiprotic?
It has no proton (H+) to donate and reacts with acids to form salt and water.
How does the acid and base in a conjugate pair differ?
Differs by H+.
What is acid rain?
Solution with a pH less than 5.6 that is formed when non-metal oxides react with water.
- Weak acids become stronger as they react.
What is the equation for sulfur dioxide and water?
1) SO2 (g) + H2O (l) <–> H2SO3 (aq)
2) H2SO3 (aq) <–> 2H+ (aq) + SO32- (aq)
What is the equation for nitrogen oxide and water?
2NO2 (g) + H2O (l) –> HNO2 (aq) + HNO3 (aq)
What is the pH scale?
Logarithmic scale (uses powers of 10) used to measure the concentration of H+ ions.
- Power of Hydrogen scale.
What are the key equations?
pH = -log [H+]
[H+] = 10^-pH
How can we measure pH?
Using a pH probe or universal indicator.
What are the key properties of pH?
- pH doesn’t have any units.
- pH is inversely proportional to [H+].
- For each increase of 10x in [H+], pH decreases by 1 unit.
- At pH > 7, we still use [H+] to determine pH.
What happens to pH as [H+] ions increases?
As [H+] increases by 10x, the pH decreases by 1 unit.
How will pH decrease for each increase of 10x in [H+]?
pH will decrease by 1 unit.
What are the characteristics of pH < 7?
- Acidic solutions
- [H+] > [OH-]
What are the characteristics of pH > 7?
- Alkaline solutions.
- [OH-] > [H+]
What are the characteristics of pH = 7?
- Neutral solutions.
- [H+] = [OH-]
What is the ion product constant of water, Kw?
Kw = 1.0 x 10^-14
Kw = [H+] [OH-]
pH = 7 (water is neutral)
What is the process of ionization of water?
It is an endothermic process.
H2O (l) <–> H+ (aq) + OH- (aq)
K = ([OH-][H+]) / [H2O]
K[H2O] = Kw = [H+][OH-]
- [H2O] is a constant as the concentration of water is so large.
What is Kw dependent on?
Temperature:
Increase temperature = Kw shifts to the right, so Kw, [H+] and [OH-] increases, but pH falls.
Decrease temperature = Kw shifts to the left, so Kw, [H+] and [OH-] decreases, but pH increases.
What type of reaction is the disassociation of water?
The dissociation of water into [H+] and [OH-] is endothermic.
- When temperature increases, the equilibrium shifts to the right, in favour of the forward reaction.
How can we determine the type of solution using concentration?
Acidic: [OH-] < [H+]
Alkaline: [H+] < [OH-]
Neutral: [H+] = [OH-]
How can we work out Kw and pH of pure water?
1) Pure water = [OH-] [H+]
2) Kw = [OH-] [H+]
3) Kw = [H+]^2
4) [H+]^2 = 1.0 x 10^-14
5) [H+] = 1.0 x 10^-7
6) pH = -log (1.0 x 10^-7) = pH 7
What is the relationship between [H+] and [OH-]?
[OH-] and [H+] are inversely proportional in aqueous solutions.
What does strength of an acid depend on?
Degree of ionisation
What is strength?
The extent to which an acid or alkali ionizes.
What is concentration?
The measure of the amount of [H+] ions.
What is a strong acid?
An acid that completely dissociates in water.
- A strong acid has a weak conjugate base.
What are some examples, with equations of strong acids?
- HCl, HBr, HI, HNO3, H2SO4
HCl (aq) + H2O (l) –> H3O+ (aq) + Cl- (aq)
What is a weak acid?
An acid that partially dissociates in water.
- Weak acids have a strong conjugate base.
What are some examples, with equations of weak acids?
- CH3COOH, H2CO3, H3PO4, DNA
CH3COOH (aq) + H2O (l) <–> H3O+ (aq) + CH3COO- (aq)
What is a strong base?
A base that completely dissociates in water.
- Strong bases have weak acid conjugates.
- OH ions show Bronsted-Lowry base behaviour by accepting protons.
What are some examples, with equations of a strong base?
- NaOH, LiOH, KOH
NaOH (aq) –> Na+ (aq) + OH- (aq)
What is a weak base?
A base that partially dissociates in water.
- A weak base has a strong acid conjugate.
- There is a low concentration of ions and equilibrium lies to the left.
What are some examples, with equations of a weak base?
- NH3, C2H5, NH2
NH3 (aq) + H2O (l) <–> NH4+ (aq) + OH- (aq)
Where does acid-base equilibria lie?
Lies in the direction of the weaker conjugate.
What base is a better proton acceptor and why?
Strong bases are better proton acceptors as they readily accept protons to form conjugates with non-acidic properties, whereas weak acids make acidic conjugates.
OH- (aq) + H+ (aq) –> H2O (l)
What is the most common substance?
Weak acids - any molecule with a carboxyl group (COOH)
What is the relationship between bond length, bond strength and acid strength?
Increasing bond length and decreasing bond strength increases acid strength.
What is the strongest acid?
HI - has the longest bond length and weakest bond strength.
What is the weakest acid?
HF - has the shortest bond length and strongest bond strength.
How are strong and weak acids and bases different in terms of electrical conductivity?
Conductivity increases with greater concentration of ions, and smaller size of ions.
- Strong acids and bases will shows a greater electrical conductivity.
How are strong and weak acids and bases different in terms of pH?
Strong acid = higher [H+] and lower pH
Weak acid = lower [H+] and higher pH
Strong base = higher pH
Weak base = lower pH
How are strong and weak acids and bases different in terms of rate of reaction?
Rates of reactions are faster with strong acids in comparison to weak acids.
What is neutralisation?
Exothermic reaction in which an acid and base react to form a salt and water.
What are alkalis?
Soluble bases that produce OH- ions when dissolved in water.
What is the reaction between K2O and water?
K2O (s) + H2O (l) –> 2K+ (aq) + 2OH- (aq)
What are salts?
Ionic compounds where the H in an acid is replaced by a metal or other positive ions.
What are reactions like with hydroxides and oxides?
HNO3 (aq) + NH4OH (aq) –> NH4NO3 (aq) + H2O (l)
- NO3 and NH4- are spectator ions
Ionic equation: H+ (aq) + OH- (aq) –> H2O (l)
What is a spectator ion?
Ions that don’t change state or structure during the reaction, so can be cancelled out.
- Not included in the ionic or half-equation.
What are reactions like with carbonates?
2HCl (aq) + CaCO3 (s) –> CaCl2 (aq) + h2O (l) + CO2 (g)
Ionic equation: 2H+ (aq) + CO3 2- (s) –> CO2 (g) + H2O (l)
What are reactions like with metals?
Mg (s) + 2HCl (aq) –> MgCl2(aq) + H2 (g)
Oxidation: Mg (s) –> Mg2+ (aq) + 2e-
Reduction: 2H+ (aq) + 2e- –> H2 (g)
What solutions are used to determine pH curves?
Strong acids and bases
What is the point of inflection?
Point where there is a large jump in the pH of a solution.
What is the point of equivalence?
Point where neutralization occurs.
How can the pH curve be explained?
1) pH is initially low.
2) pH gradually changes.
3) Very large jump between pH 3 and 11.
4) Curve flattens out at high values.
5) Point of equivalence is pH 7.
What is a monoprotic species?
When neutralization occurs with equal volumes of acids and bases.
What is effervescence?
When a gas visibly produces bubbles when being released - usually refers to CO2.
How do you carry out an investigation of the pH curve?
1) Add 23cm3 of NaOH to a conical flask using a pipette.
2) Add 50cm3 of HCl to a burette and let 1cm3 drop each time into the conical flask, gently stirring.
3) Record pH for every 1cm3 of HCl added 50 times using a pH probe and plot a graph.
What is the pOH scale?
It describes the [OH-] ions in a solution.
What are the equations involving the pOH scale?
pOH = -log(10) [OH-]
[OH-] = 10 ^-pOH
- Subtract from 14 if the solution is an alkali
What are equations to show relationship between pOH and Kw?
pH + pOH = 14
pH + pOH = pKw
What is the link between pKw and Kw?
As Kw gets larger, pKw decreases.
- Inversely proportional relationship
What are equations to show the relationship between pKw and Kw?
pKw = -log(10) Kw
Kw = 10 ^-pKw
How are pH and pOH derived?
They are derived from the concentrations of H+ and OH- at equilibrium.
How are the strengths of weak acids and bases described?
Why can we not determine the concentration of ions in a solution of a weak acid?
As the ions only partially ionise in water
