# Trends in Chemical Reactivity with Oxygen, Water and Dilute Acids ## Introduction Group 2 elements, also known as alkaline earth metals, display characteristic trends in their chemical reactivity, particularly with oxygen, water, and dilute acids. This section explores these trends, detailing the reactions, properties of resulting compounds, and variations down the group. --- ## (a) Reaction with Oxygen / Formation of Basic Oxides The Group 2 metals readily react with oxygen (burning more rapidly in pure oxygen than in air) to form white solid oxides. **General Reaction:** $$2M_{(s)} + O_{2(g)} \rightarrow 2MO_{(s)}$$ *Example:* Magnesium ribbon burns with a bright white flame when ignited: $$2Mg_{(s)} + O_{2(g)} \rightarrow 2MgO_{(s)}$$ **Formation of Peroxides:** Strontium and Barium can also form peroxides ($MO_2$) in addition to normal oxides. In the case of Alkali metals (Group 1), Lithium forms a normal oxide, while others form peroxides or superoxides. **Solubility of Oxides:** Beryllium oxide ($BeO$) and magnesium oxide ($MgO$) are insoluble in water. The solubility of the remaining oxides ($CaO$, $SrO$, $BaO$) *increases* down the group, as they form increasingly soluble hydroxides. **Acidic/Basic Character of Oxides:** All oxides of Group 2 elements are basic in character, except for Beryllium oxide ($BeO$). $BeO$ is amphoteric, meaning it reacts with both acids and bases. The basic character increases down the group. **Reactions of Amphoteric Beryllium Oxide ($BeO$):** With acid: $$BeO_{(s)} + 2HCl_{(aq)} \rightarrow BeCl_{2(aq)} + H_2O_{(l)}$$ With base: $$BeO_{(s)} + 2NaOH_{(aq)} \rightarrow Na_2BeO_{2(aq)} + H_2O_{(l)}$$ (Here, $Na_2BeO_2$ is sodium beryllate) **Reactivity Trend with Oxygen:** The Group 2 metals become more reactive with oxygen going down the group. This is because larger atoms ($Ba$ vs $Mg$) lose their outer two electrons more readily due to decreased ionization energy. *Example:* Barium metal is highly reactive and must be stored under oil to prevent contact with air. **Covalent Character of Beryllium Oxide ($BeO$):** Unlike other Group 2 oxides which are ionic, $BeO$ exhibits significant covalent character. This is attributed to the small size and high charge density of the $Be^{2+}$ ion. The high polarizing power of $Be^{2+}$ distorts the electron cloud of the oxide ion ($O^{2-}$), leading to electron sharing and a covalent bond. --- ## Flame Test Many Group 2 metal ions produce characteristic colours when heated in a Bunsen flame, which is used for their qualitative identification. **Procedure:** A clean nichrome wire (cleaned with concentrated hydrochloric acid) is dipped into a sample of the salt and then heated in a non-luminous Bunsen flame. **Characteristic Colours:** - Calcium compounds: Brick-red - Strontium compounds: Scarlet/Red - Barium compounds: Apple-green --- ## (b) Reaction with Water The reactivity of Group 2 metals with water shows a clear trend down the group. **Beryllium ($Be$):** Does not react with water due to a stable, protective layer of beryllium oxide ($BeO$) on its surface. **Magnesium ($Mg$):** Does not react with cold water. Reacts vigorously with steam to form magnesium oxide and hydrogen gas: $$Mg_{(s)} + H_2O_{(g)} \rightarrow MgO_{(s)} + H_{2(g)}$$ **Calcium ($Ca$):** Reacts more readily than magnesium with cold water. The reaction forms a cloudy white suspension of slightly soluble calcium hydroxide and hydrogen gas: $$Ca_{(s)} + 2H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + H_{2(g)}$$ The dissolved calcium hydroxide makes the solution weakly alkaline. Hydrogen gas is released at a steady rate. **Trend Down the Group:** Reactivity of Group 2 elements with water increases down the group. Hydrogen gas is released more and more rapidly as you go from Ca to Ba. **General Reaction (for Group 2 metals except Be and Mg with water):** $$M_{(s)} + 2H_2O_{(l)} \rightarrow M(OH)_{2(s)} + H_{2(g)}$$ *(Note: $M(OH)_2$ can be sparingly soluble to soluble depending on the metal)* **Reaction of Peroxides with Water:** Sodium peroxide reacts with ice-cold water to produce sodium hydroxide and hydrogen peroxide. --- ## (c) Reaction with Dilute Acids Group 2 metals undergo redox reactions with dilute acids, producing a salt and hydrogen gas. **Reactivity Trend:** The reactions become more vigorous and exothermic down the group. **General Reaction (with dilute HCl):** $$M_{(s)} + 2HCl_{(aq)} \rightarrow MCl_{2(aq)} + H_{2(g)}$$ --- ## (d) Reaction of Group 2 Oxides with Acid Group 2 oxides, being basic (except $BeO$), react with acids to form salts and water. These reactions are important for preparing useful compounds. **Examples:** Magnesium oxide with sulfuric acid: $$MgO_{(s)} + H_2SO_{4(aq)} \rightarrow MgSO_{4(aq)} + H_2O_{(l)}$$ Calcium oxide with hydrochloric acid: $$CaO_{(s)} + 2HCl_{(aq)} \rightarrow CaCl_{2(aq)} + H_2O_{(l)}$$ **Applications of Products:** - **Magnesium Sulfate ($MgSO_4$):** Used for short-term relief of constipation (Epsom salts) and as a soaking solution for minor sprains, bruises, muscle aches, or joint stiffness. - **Calcium Chloride ($CaCl_2$):** Used as a solidifying agent in paint production and as a coagulant in the manufacture of rubber. --- ## Possible Questions/Answers Based on "Concept Assessment Exercise 5.1": 1. **Q:** What is the general trend in the melting points going down Group 2? **A:** Generally, melting points decrease down Group 2, though there can be some irregularities (e.g., $Mg$ has a lower melting point than $Ca$). This is because the metallic bonding becomes weaker as the atomic radius increases, leading to weaker attraction between the positive ions and the delocalized electrons. 2. **Q:** Explain why the atoms in Group 2, as in any other group, get larger with increasing atomic number. **A:** Atoms get larger down a group due to the addition of new electron shells. Each subsequent element down the group has electrons occupying an extra main energy level (shell), which increases the atomic radius. The shielding effect of inner electrons also increases, reducing the effective nuclear charge felt by the outermost electrons, allowing them to be further from the nucleus. 3. **Q:** Write a balanced chemical equation, including state symbols, for the reaction of: i. strontium with oxygen ii. strontium oxide with hydrochloric acid. **A:** i. $$2Sr_{(s)} + O_{2(g)} \rightarrow 2SrO_{(s)}$$ ii. $$SrO_{(s)} + 2HCl_{(aq)} \rightarrow SrCl_{2(aq)} + H_2O_{(l)}$$ 4. **Q:** Barium reacts vigorously when added to water. i. Write a balanced chemical equation, including state symbols, for the reaction of barium with water. ii. State two observations that could be made during this reaction. iii. Suggest the approximate pH of the resulting solution. iv. Will the reaction be more or less vigorous than the reaction of barium with water? Explain your answer. **A:** i. $$Ba_{(s)} + 2H_2O_{(l)} \rightarrow Ba(OH)_{2(aq)} + H_{2(g)}$$ ii. Observations: - Rapid effervescence/production of gas ($H_2$) - Barium metal disappears/dissolves quickly - Heat is produced (exothermic reaction) - A clear solution or a slightly cloudy solution (if $Ba(OH)_2$ is formed in high concentration) is formed iii. Approximate pH: The resulting solution will be strongly alkaline, so a pH of **12-14** would be expected. iv. *Assuming comparison with calcium:* The reaction of **calcium** with water will be **less vigorous** than that of barium. This is because reactivity increases down Group 2. Barium atoms are larger and lose their valence electrons more easily than calcium atoms, resulting in a more vigorous reaction. 5. **Q:** Describe what you would see when magnesium reacts with cold water and steam. Also write an equation for the reaction with steam. **A:** - **With cold water:** Magnesium shows no visible reaction with cold water, or perhaps extremely slow effervescence after a very long time, as it is protected by a thin oxide layer. - **With steam:** Magnesium burns with a bright white flame in steam, forming a white solid (magnesium oxide) and producing hydrogen gas (effervescence/hissing of gas). - **Equation for reaction with steam:** $$Mg_{(s)} + H_2O_{(g)} \rightarrow MgO_{(s)} + H_{2(g)}$$