For reaction :
X + Y + Z = products
rate equation determined experimentally as rate = k[X]2[Y]
where k is the rate constant
- 2nd order with respect to X
- 1st order with respect to Y
- Zero order with respect to Z
On a concentration-time graph, 1st order has a constant half life.
Strong and Weak Acids
In accordance with the Bronstead-Lowry theory:
- An acid is a proton donor
- A base is a proton acceptor
A strong acid is one which will completely dissociate into its ions:
- HCl ------> H+ + Cl-
A weak acid is one which only partially dissociates into its ions:
- HCOOH <-----> H+ + HCOO-
Extent of dissociation is the 'strength' of an acid, and is measure by the acid dissociation constant, Ka.
Ka = ( [H+] x [HCOO-] ) / [HCOOH], pKa = -log(Ka)
The stronger the acid, the greater the value of Ka, and the smaller the value of pKa.
Acid base pairs differ by H+
- HA + H2O <-----> H3O+ + A-
In the above:
- HA is acid 1.
- H2O is base 2.
- H3O+ is acid 2.
- A- is base 1.
Acid 2 and Base 2 can also be known as conjugate, as they are accepting or donating H+.
Equilibrium Constant, Kc
- N2(g) + 3H2(g) <-----> 2NH3(g)
Kc = [NH3]2 / ( [NH2] x [H2]3 )
products over reactants
Kc only changes with temperature
Depends on sign of deltaH for forward reaction:
deltaH +ve (endo), Kc increases with an increase in temperature
deltaH -ve (exo), Kc decreases with an increase in temperature
STRONG ACID, [H+] = [HA]
WEAK ACID, [H+] = square root of (Ka x [HA])
STRONG BASE, [H+] = (Kw) / [OH-]
BUFFER, [H+] = Ka x ( [HA] / [A-] )
- Ka is the acid dissociation constant
- Kw is the ionic product of water, and is equal to 1.00 x 10-14
ALL RELATE TO:
- pH = -log[H+(aq)]
A buffer is a solution which is able to resist changes to its pH when small amounts of acid and alkali impurities are added to it.
It is usually comprised of a weak acid, HA, and its conjugate base, A-. For example,
CH3COOH and CH3COO-NA+.
The mixture forms two equations:
- CH3COOH <----> H+ + CH3COO-
- CH3COO-Na+ ----> CH3COO- + Na+
When impurites are added:
- Acid added: H+ reacts with CH3COO- to form CH3COOH, causing equilibrium to shift to the left.
- Alkali added: OH- reacts with H+ to form water, causing equilibrium to shift to right, as H+ ions need to be replaced.