chem1102 #2 – semester 2microsoft word - chem1102 #2 – semester 2.docx created date: 12/14/2015...
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CHEM1102 #2 – Semester 2 List common acids and bases ���
Use pH and KW
Instead of using large numbers , we use a logarithmic scale (pH scale):
• pH = -‐log10[H+]
• pOH = -‐log10[OH-‐]
• pH + pOH = 14
• pKW = – log10 KW = 14.00 at 25 °C
•
Define acids & bases
Arrhenius Definition
ACID: H+ producer in aqueous solution e.g. HCl
BASE: OH-‐ producer e.g. NaOH
Bronsted Lowry Definition
ACID: proton donor (H+) e.g. HCl
BASE: proton acceptor e.g. NH3
Autoionisation of Water ⇒
Temperature Dependence of pH
• Reaction is endothermic: it is more favourable at higher temperature
•
2 H2O(l) à H3O+ (aq) + OH-‐ (aq)
At 25°C: KW = [H3O+][OH-‐] = 1.0 × 10-‐14
Complete acid / base calculations for strong and weak acids and ���bases ���
Use pH, pKw, pKa and pKb ���
Define strong and weak acids & bases ��� Strong acids and bases:
• completely ionise in water (100% dissociation)
• have equilibrium lying completely to the right, Ka ≈ ∞
Strong acids: • H2SO4, HCl, HBr, HI, HNO3, HClO4,
• Lose protons easily
• More polarized the H-‐X bond, the weaker the bond, the stronger the acid
Strong bases: • All hydroxides of Groups 1 & 2 (except Be): NaOH, Ca(OH)2,
• Hold onto protons tightly
Weak acids and bases:
• do not completely ionise in water (<100% dissociation)
• are in equilibrium
Understand acid and base equilibria ���& Identify conjugate acid/base pairs ���
Perform calculations with strong acids/bases ���
The larger the value of Ka, the stronger the acid and the lower the value of pKa
An ACID –BASE equilibria will ALWAYS lie in the direction of the weaker acid and weaker base
• increasing pKa = decreasing Ka
: Ka1
> Ka2
> Ka3
• REASON: harder to remove +ve charge (i.e. protons) against increasing -‐ve charge
• large difference in pKa values means that we only need to consider one equilibrium at a time
Use the buffer concept and construct buffers ���
A solution containing both a weak acid and its salt that withstands pH changes when acid or base (limited amounts) are added.
Condition Procedure
Buffer Preparation Required pH of buffer = pKa of acid available use equimolar of acids and its conjugate base
If pH required by buffer ≠ pKa of acid available use the Henderson-‐Hasselbalch equation
Buffer Capacity • the amount of strong acid or base that can be added without causing significant pH change
• depends on amount of acid & conjugate base in solution
• Highest when [HA] ≈ [A-‐] OR when [HA] ≈ [A-‐] are large
• Most effective buffers have acid/base ratio < 10 and > 0.1 ⇒ pH range is ±1
Buffers in biological systems include:
• blood, contain buffers: pH control essential because biochemical reactions are very sensitive to pH. Death occurs at pH <6.8 and pH >7.8
• “Extracellular” buffer (outside cell)
Apply the Henderson-‐Hasselbalch equation ���
* Applies ONLY TO WEAK ACIDS AND THEIR CONJUGATED BASES (not acids or bases alone)