Show Arrhenius bases produce hydroxide ions in aqueous solution: An Arrhenius acid reacts with an Arrhenius base (OH-(aq)) to produce a salt. An amphoteric substance can therefore react with either an Arrhenius acid (H+(aq)), or, with an Arrhenius base (OH-(aq)) to produce a salt.
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This is part of the HSC Chemistry course under the topic Using Brønsted-Lowry Theory.
This video discusses the difference between strong and weak acids/bases. The video also outlines the difference between strength and concentration.
Brønsted-Lowry Acids and Bases
$$HF(aq) + H_2O(l) \leftrightharpoons H_3O^+(aq) + F^-(aq)$$ $$NH_3(aq) + H_2O(l) \leftrightharpoons NH_4^-(aq) + OH^-(aq)$$
Brønsted-Lowry Theory – Conjugate Acid and Base PairsThe Brønsted-Lowry theory:
The acid and base of a conjugate acid-base pair differ from each other in structure by only 1 proton. For example, the reaction between acetic acid and water: $$CH_3COOH(aq) + H_2O(l) \leftrightharpoons CH_3COO^-(aq) + H_3O^+(aq)$$ Acidic Hydrogen
Strength of Acids and Bases
In the diagram on the left, the hydrochloric completely dissociates into H+ and Cl-. This means that HCl is a strong acid. In the diagram on the right, the acetic acid only partially dissociates into H+ and CH3COO-.This means that CH3COOH is a weak acid. Chemical Equation for Strong Acids Since the dissociation/ionisation of a strong acid is complete, its chemical equation is written with a single-direction arrow. For example, the dissociation of HCl in water: $$HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$$ This is also true for the ionisation of water: $$HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)$$ Chemical Equation for Weak Acids Weak acids partially dissociate/ionise because their conjugate bases are able to re-gain protons to reform the weak acids. In other words, the ionisation of a weak acid is reversible. The same concept applies to weak bases and their conjugate acids. Since the ionisation of weak acids and bases in water are reversible, they can reach a dynamic equilibrium where the rate of forward and reverse reactions are equal. The dissociation/ionisation of a weak acid is incomplete (partial), its chemical equation is written with a reversible arrow. For example, the dissociation of acetic acid in water: $$CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)$$ This is also true for the ionisation in water: $$CH_3COOH(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + CH_3COO^-(aq)$$ Strength vs Concentration
In the diagram on the left, there are more molecules of HCl than on the right. This means the solution on the left can be described as being more concentrated while the solution on the right can be described as being more dilute.
Base StrengthThe conventions used to describe acid dissociation are also applied to bases. This means
Chemical Equation for Strong Bases Most strong bases are inorganic bases. Since the dissociation of a strong base is complete, its chemical equation is written with a single-direction arrow. For example the dissociation of sodium hydroxide ion water: $$NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)$$ Chemical Equation for Weak Bases Most weak bases are organic bases which are Brønsted-Lowry bases (proton acceptor). Since the ionisation of a weak base is incomplete (partial), its chemical equation is written with a reversible arrow. Similar to weak acid ionisation, weak base ionisation can also reach a dynamic equilibium. For example, the ionisation of ammonium in water: $$NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$$ Sparingly Soluble Bases Although Group 1 and 2 metal hydroxides are mostly strong bases, Group 2 hydroxides e.g. Ca(OH)2 are less soluble in water. As a result, some undissociated solid Ca(OH)2 will be found undissolved in the water or lying at the bottom of the solution as precipitate. Thus, sometimes the dissociation of a base can be limited by its solubility in water. In contrast, Group 1 hydroxides e.g. NaOH are very soluble in water which means only hydrated metal cations (Na+) and hydroxide ions will be found in the solution. Use of Acid and Base Dissociation Constants (Ka, Kb )The value of acid and base dissociation constants can be used to compare the strength of acids and bases.
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