IB Chemistry: Strong and weak acids and bases Strong and weak acids and bases 1. Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity. In general, when any acid is added to water, the following reaction happens: HA + H2 O ↔ H 3O+ + A– A reversible reaction occurs where the acid dissociates and forms H3O+. However, some of the H3O+ may also lose an H+ ion, reacting to the A– ions floating about and reforming the acid and water. The degree to which an acid dissociates and the strength of the forwards and backwards reaction determine the strength of an acid. Strong acids: Acids that completely dissociate in a solution. For example, when hydrochloric acid is added to water, the forward reaction where the acid ionizes and forms H3O+ is much stronger than the reverse reaction. hence, we can write thereaction as: HCl + H2 O → H 3O+ + Cl– Under the Bronsted-Lowry definition of an acid, virtually 100% of the H+ ions in the hydrochloric acid has dissociated and formed H3O+, making HCl a strong acid. To put it simply, strong acids completely ionize in the presence of water, splitting into their component ions: HA → H + + A– Since strong acids completely dissociate into their respective ions, they have the ability to vastly improve the electrical conductivity of an aqueous solution, as the more ions there are in a solution, the greater its electrical conductivity. Weak acids: Acids that partially dissociate in a solution. For example, when CH3COOH is added to water, not all of the acid dissociates to form its conjugate base pair and H3O+: CH3COOH + H2 O ↔ H 3O+ + CH3COO– In this case, the backwards reaction of reforming the acid is more successful than the forward reaction of forming the ions. Hence, not all of the acid molecules will ionize to form H3O+ + CH3COO–, meaning that CH3COOH is a weak acid since it partially dissociates.
Since weak acids only partially dissociate into their respective ions, their ability to conduct electricity is much weaker than strong acids, since there are less ions floating in the solution. Strong Bases: Bases which completely dissociate in a solution Much like strong acids, when strong bases are added to water, they dissolve and completelydissociate into their metal ions and hydroxide ions: NaOH(s) → Na +(aq) + OH–(aq) NaOH is a strong base since it completely converts to OH– ions when aqueous. Since they completely dissociate into their component ions, strong bases have good electrical conductivity properties. Weak Bases: Bases that partially dissociate in a solution. Like weak acids, when a weak base is added to water, only a portion of the base ionizes to form its conjugate acid pair and OH– ions. Take ammonia for example: NH3 + H2 O ↔ NH 4+ + OH– Only a percentage of the ammonia turns into hydroxide ions. Ammonia molecules still remain in the solution as some of ions reform to produce the base and water. Hence, ammonia is a weak base since it doesn’t fully convert to OH– ions. Their ability to conduct electricity is like weak acids. There are much less ions present in thesolution in comparison to strong acids and bases, hence they have very little conductivity. 2. State whether a given acid or base is strong or weak. Examples of strong acids: ● Hydrochloric acid – HCl ● Sulphuric Acid – H2SO4 ● Nitric Acid – HNO3 Example of weak acids: ● Carboxylic acids (-COOH group) ○ Ethanoic acid – CH3COOH ● Carbonic acid (aqueous carbon dioxide) – H2CO3 Example of strong bases:
● Group 1 hydroxides ○ Potassium hydroxide – KOH ○ sodium hydroxide – NaOH ○ Calcium hydroxide – CaOH ● Barium hydroxide – Ba(OH)2 Examples of weak bases: ● Ammonia – NH3 ● Amines (Chemical group that has a nitrogen atom with a lone pair) 3. Distinguish between strong and weak acids and bases and determine the relative strengths of acids and bases, using experimental data. Dissociation constant – Ka The dissociation constant is something we can use to determine the strength of an acid: HA ↔ H + + A– Ka = ([H+] [A–])/ [HA] Like the equilibrium constant KC, Ka is the ratio of dissociated ions to undissociated acid. If the Ka value is higher, it means that the acid is stronger since there is a greater amount of dissociated ions in comparison to the acid. The higher the Ka, the stronger the acid. Ka is used only to distinguish between the strength of weak acids. Since strong acids dissociate completely, the value of [HA] approaches 0 as it reacts with the water. Mathematically speaking, all strong acids have a Ka of infinte. pKa is also another term we can use to describe the strength of an acid. pKa = – log10 Ka Just remember the lower the pka, the stronger the acid. ___________________________________ Likewise, bases have the exact same thing to distinguish their strength, except this time, it’s called Kb: B + H2 O ↔ BH + OH – Kb = ([BH][OH–])/[B]
We ignore the concentration of the water because it’s simply too large in comparison to the other numbers. Kb simply tells the ratio of the concentration of dissociated ions to undissociated base. As it dissociates, concentration of the base decreases as the concentration of the ions increase. A base is stronger if it produces more OH– ions whilst aqueous, so the higher the value of Kb, the stronger base. pKb is also another term we can use to describe the strength of an base. pkb = – log10 Kb Once again, the lower the value of pKb the stronger the base.