1 Intro to Acids and Bases for IB

Fundamental Definitions of Acids and Bases

• There are multiple definitions for acids and bases that are utilized in chemistry, which are typically explored over a period of study spanning two years.

• The Common Sense Definition:

  • An acid is defined as a liquid with a pH value less than $7$.

  • A base is defined as a liquid with a pH value greater than $7$.

  • The International Baccalaureate (IB) curriculum generally does not favor these definitions because they do not involve specific atomic interactions or chemical reactions.

The Arrhenius Definition of Acids

• An Arrhenius acid is a compound that dissociates (breaks apart) into concentration of hydrogen ions ($H^+(aq)$) and a specific anion when it is dissolved in water.

• Understanding this definition is critical for examinations, as IB assessments frequently ask for the specific definition of an Arrhenius acid.

Nature of the Hydrogen Ion ($H^+$)

• A standard hydrogen atom consists of exactly $1$ proton and $1$ electron, with $0$ neutrons.

• An $H^+$ ion is formed when a hydrogen atom loses its single electron.

• Because the electron is removed and no neutron is present, an $H^+$ ion is physically just a single proton.

• Consequently, acids can be described in two equivalent ways:

  • Compounds that break apart into an $H^+$ ion and an anion in water.

  • Compounds that break apart into a proton and an anion in water.

Chemical Equations for Arrhenius Acids

• Hydrochloric acid:     $HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$

• Hydrobromic acid:     $HBr(aq) \rightarrow H^+(aq) + Br^-(aq)$

• Hydrofluoric acid:     $HF(aq) \rightarrow H^+(aq) + F^-(aq)$

• Hydrosulfuric acid:     $H_2S(aq) \rightarrow 2H^+(aq) + S^{2-}(aq)$

• Sulfuric acid:     $H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$

• Phosphoric acid:     $H_3PO_4(aq) \rightarrow 3H^+(aq) + PO_4^{3-}(aq)$

• Nitric acid:     $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$

• Hydroiodic acid:     $HI(aq) \rightarrow H^+(aq) + I^-(aq)$

Identification of Acidic Properties

• If a substance such as even a complex organic molecule like acetic acid ($CH_3COOH$) is identified as an Arrhenius acid, a specific statement must be true regarding its behavior in water:

  • Dissolving it in water will result in the presence of anions and $H^+$ ions.

The Arrhenius Definition of Bases

• An Arrhenius base is a compound that dissociates into hydroxide ions ($OH^-(aq)$) and a specific cation when dissolved in water.

Chemical Equations for Arrhenius Bases

• Sodium hydroxide:     $NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)$

• Calcium hydroxide:     $Ca(OH)_2(aq) \rightarrow Ca^{2+}(aq) + 2OH^-(aq)$

• Aluminium hydroxide:     $Al(OH)_3(aq) \rightarrow Al^{3+}(aq) + 3OH^-(aq)$

• Magnesium hydroxide:     $Mg(OH)_2(aq) \rightarrow Mg^{2+}(aq) + 2OH^-(aq)$

• Lithium hydroxide:     $LiOH(aq) \rightarrow Li^+(aq) + OH^-(aq)$

Distinguishing Bases from Alkalis

• While the terms are often used interchangeably in casual contexts, they have distinct technical meanings in chemistry.

• Base: A chemical substance that has a pH higher than $7$ at room temperature.

• Alkali: A specific type of base that is soluble in water.

• Comparison Examples:

  • $Ca(OH)_2$ (Calcium hydroxide): This substance has a pH above $7$, making it a base. However, it is not soluble in water, therefore it is not an alkali.

  • $NaOH$ (Sodium hydroxide): This substance has a pH above $7$ and is soluble in water, therefore it is classified as an alkali.

Specific Alkalis for Memorization

• To achieve high marks (a 6 or 7) in IB Chemistry, students must memorize the following bases that qualify as alkalis:

  • $LiOH$ (Lithium hydroxide)

  • $NaOH$ (Sodium hydroxide)

  • $KOH$ (Potassium hydroxide)

  • $RbOH$ (Rubidium hydroxide)

  • $CsOH$ (Cesium hydroxide)

  • $NH_4OH$ (Ammonium hydroxide)

  • $Ba(OH)_2$ (Barium hydroxide)

  • $TlOH$ (Thallium hydroxide)

• Memorization is simplified by noting that the first five alkalis listed ($Li, Na, K, Rb, Cs$) involve alkali metals from the periodic table.