Acids, Bases and Buffers Flashcards
What is a Bronsted-Lowry acid?
A proton donor.
What is a strong acid?
An acid that completely dissociates ie it fully ionises when dissolved in water.
What happens when a strong acid reacts with water? What are the products?
H+ is transferred to a water molecule to produce a hydroxonium ion and a negative ion depending on what acid you use from the start. HA + H2O H3O+ + A-
Why is it that we assume the dissociation of a strong acid (when it reacts with water) is one wayed?
It is a reversible reaction but the forward reaction is must more successful than the backward one i.e. the acid is soo good at giving away H+.
What is the reaction of HCl and H2O? Why is it one wayed?
H2O(l) + HCl(g) –> H3O+(aq) + Cl-(aq) So little of the reverse reaction happens hence why we write it one wayed. Virtually 100% of H+ have reacted to form hydroxonium ion so it is 100% ionised/dissociated.
What is pH?
The measure of the concentration of H+ ions in a solution. The lower the pH, the higher the concentration of H+.
What is the equation for calculating pH?
pH = -log[H+(aq]
How do you calculate [H+] from the pH of a strong acid?
[H+] = 10^-pH
What is weak acid?
An acid that partially dissociates i.e. it doesn’t fully ionise in water.
When writing the ionisation etc of a weak acid e.g. CH3COOH + H2O H3O+ + Ch3COO-, why do you write an equilibrium sign?
The back reaction is more successful than the forward reaction. The further the position is to the left, the weaker the acid is.
What is the equation of the acid dissociate constant, Ka?
Ka = [H+][A-] ———— [HA]
What is the acid dissociate?
The measure of position of equilibrium.
What is the equation for acid dissociation?
HA H+ + A-
How do you calculate pKa?
pKa = -log Ka
How do you calculate Ka from pKa?
Ka = 10 ^ pKa
What does the pKa tell you? e.g. the higher the pKa…
The HIGHER the pKa, the WEAKER the acid. The LOWER the pKa, the STRONGER the acid.
Do Ka and pKa have units?
Ka does but pKa doesn’t.
What is a base?
A proton acceptor.
What is a strong base?
A base that completely ionises/dissociates when dissolved in water.
What are strong bases e.g. KOH or NaOH? I’m referring to ionic stuff.
They’re fully ionic e.g. you can think of them being completely split into metal and hydroxide ions.
CaOH is insoluable in water. Is it still a base?
Yes as it ionises 100%.
How do you work out the pH for a strong base?
pH = 14 - pOH
How do you calculate pOH?
pOH = -log [OH-]
How do you calculate [OH-] from pOH?
[OH-] = 10^ -pOH
What is a weak base?
A base that partially dissociates?
What is Kw?
The ionic product of water. Kw = [H+] [OH-]
What changes Kw?
Temperature
What is the value of Kw at room temperature?
1x10^14 mol2 dm-6
What does the titration curve for strong acid v. strong base look like?
What does the titration curve for strong acid v. weak base look like?
What does the titration curve for weak acid v. weak base look like?
What does the titration curve for weak acid v. strong base look like?
What is a buffer solution?
A buffer solution is one that resists changes in pH when small amounts of acid or alkali are added.
What do buffer solutions consist of?
A weak acid and it's salt e.g. ethananoid acid (CH3COOH) and sodium ethanoate (CH3COO-Na+).
What are acidic buffers? How do they work?
They are buffer solutions with a pH of under 7.
- The acid is a weak acid so partially dissociates and the salt fully dissociates.
- This means the solution contains: lots of ionised ethanoic acid, lots of ethanoate ions and enought H+ to make the solution acidic.
How does a acidic ethanoic acid and sodium ethanoate acid buffer solution work?
- Ethanoic acid partially dissociates: CH3COOH ⇔CH3COO- + H+ pos. of equilibrium left
- Sodium ethanoate partially dissociates: CH3COONa → CH3COO- + Na+ pos. of equil. further pushed left
- Plenty of ethanoate ions and undissociated ethanoic acid with enough H+ ions =
CH3COOH(aq) ⇔ CH3COO-(aq) + H+(aq)
What happens if you add an alkali to an acidic buffer system?
The addiction of OH- raises the concentration of OH-. The buffer solution removes these by reacting the added OH- with H+ ions so OH-(aq) + H+(aq) → H2O
The equilibrium shifts to the right to replace to replace the H+ ions.
What happens if you add an acid to an acidic buffer system?
The addiction of an acid raises the concentration of H+ ions. The H+ ions react with A- e.g. CH3COO- in the CH3COOH/CH3COO- buffer system. So CH3COO- + H+ <——-===> CH3COOH
The equilibrium shifts to the left, reducing the concentration of H+ ions.
BUFFER SOLUTIONS AND BIOLOGICAL ENVIRONMENTS: Why do cells need a constant pH? How is this achieved?
- To allow biochemical reactions to take place. This pH is based on the equilibrium between dihydrogen phosphate ions and hydrogen phosphate ions:
- H2PO4- <===> H+ + HPO42-
BUFFER SOLUTIONS AND BIOLOGICAL ENVIRONMENTS: What pH must blood be kept at? How is this achieved?
- Blood needs to be at constant pH of 7.4.
- It is buffered using carbonic acid. The levels of H2CO3 are controlled by respiration. By breathing out CO2, levels of H2CO3 is reduced as it moves equilibrium to the right: H2CO3 <==> H2O + CO2
- The levels of HCO3- are controlled by the kidneys with excess being secreted in the urine: H2CO3 <==> H+ + HCO3-
