Lecture Eighteen - Acid base equilibrium Flashcards Preview

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Flashcards in Lecture Eighteen - Acid base equilibrium Deck (7)
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1
Q

Define and acid and base.

A

An acid is a proton donor.
It must contain H in its formula.
HA + H2O H3O+ + A-

A base is a proton acceptor.
It must contain a lone pair of electrons to bind the H+ ion to.
B + H2O BH+ + OH-

2
Q

How is pH calculated?

A

The scale used to measure how acidic or how basic a solution is, is known as the pH scale.
The H comes from H+, which is really H3O+ in solution.
If the concentration of H3O+ is known, we can determine the pH by using the equation:
pH = -log10[H3O+]

The pOH scale can also be used.
If the concentration of OH- is known, we can use the equation:
pOH = -log10[OH-]

3
Q

What is the relationship between H3O+ and OH-?

A

The Kw.
In acidic solutions, H3O+ is very high and OH- is very low, and viva versa.
[H3O+][OH-] = 10^-14 = Kw.

4
Q

What is a strong/weak acid/base?

A

A strong acid or base is one which when dissolved in water completely dissociates.
That is, it is safe to assume that the entire reaction has run to completion (not in equilibrium).

Strong acids:
Two main types:
- The hydrophilic acids: HCl, HBr (but not HF).
- The oxoacids in which the number of O atoms exceeds the number of ionisable H atoms by more than two: HNO3, H2SO4, HClO4.

Strong Bases:
Soluble compounds containing O2- or OH- ions.
The cations are usually those of the most active metals.

Weak acids and bases do not completely dissolve in water, and will remain at equilibrium.

Weak acid:
When dissolved in water, and weak acid is one which does not completely dissociate..
Four main types:
- Hydrochloric acid.
- Acids in which H is not bound to O or a halogen, e.g. HCN.
- Oxoacids in which the number of O atoms equals or exceeds by one the number of ionisable H atoms, e.g. HClO, HNO2.
- Organic acids, with the general formula RCOOH.

Weak bases:
Many compounds with an electron rich nitrogen are weak bases.
The common structural feature is an N atom that has a lone electron pair in its Lewis structure.
E.g. ammonium (NH3) and amides.

5
Q

What is the relationship between Ka and pKa?

A

Ka is the acid dissociation constant.
By definition, pKa = -log10Ka.
If there is no pKa value, it means that the reaction has not gone to completion.

6
Q

What are conjugate acids and bases?

A

When an acid loses its proton, it becomes its conjugate base.
When a base gains a proton, it becomes its conjugate acid.

7
Q

How can Kw be found?

A

If Ka and Kb are multiplied together, Kw is found.