Rubidium metal reacts extremely rapidly and violently with water, producing rubidium hydroxide and hydrogen gas, releasing a significant amount of heat.
Rubidium (Rb), an alkali metal, exhibits a highly vigorous reaction when it comes into contact with water. This reaction is one of the most exothermic and visually dramatic among the alkali metals, typically occurring instantly upon contact.
The Reaction Process
When rubidium metal is introduced to water, it immediately reacts to break down the water molecules. The reaction can be summarized as follows:
- Speed: The reaction is incredibly rapid, often instantaneous.
- Products: It forms a colourless solution of rubidium hydroxide (RbOH), a strong base, and hydrogen gas (H₂).
- Exothermicity: The reaction is highly exothermic, meaning it releases a substantial amount of heat. This heat is often sufficient to ignite the hydrogen gas produced, leading to a lilac-colored flame, characteristic of rubidium.
- Basicity: The resulting solution is strongly basic due to the presence of dissolved rubidium hydroxide, which dissociates into rubidium ions (Rb⁺) and hydroxide ions (OH⁻).
Chemical Equation
The balanced chemical equation for the reaction of rubidium with water is:
2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g)
Where:
Rb(s)is solid rubidium metalH₂O(l)is liquid waterRbOH(aq)is aqueous rubidium hydroxideH₂(g)is hydrogen gas
Key Characteristics and Observations
The reaction of rubidium with water is characterized by several distinct features:
- Explosive Nature: Due to its extreme reactivity and the rapid production of hydrogen gas, the reaction can be explosive.
- Lilac Flame: The intense heat generated during the reaction often ignites the hydrogen gas, which then burns with a characteristic lilac (or purplish-red) flame. This flame color is a unique property of rubidium compounds when heated.
- Dissolution: The rubidium metal quickly disappears as it reacts and forms the soluble rubidium hydroxide.
- Sound: A distinct "hissing" sound may be heard as the hydrogen gas is rapidly released.
Reactivity Trend
Rubidium's position in Group 1 (alkali metals) of the periodic table places it below sodium and potassium. As you move down Group 1, the reactivity with water increases due to the increasing atomic radius and decreasing ionization energy. This makes rubidium's reaction with water more violent than that of lithium, sodium, or potassium. Cesium, below rubidium, reacts even more explosively.
| Alkali Metal | Reaction with Water Observations |
|---|---|
| Lithium | Fizzing, dissolves slowly, hydrogen gas. |
| Sodium | Melts into a sphere, fizzes vigorously, hydrogen ignites (orange flame). |
| Potassium | Immediately melts, fizzes very vigorously, hydrogen ignites (lilac flame). |
| Rubidium | Explodes on contact, hydrogen ignites (lilac flame). |
| Cesium | Extremely violent explosion, shatters glass. |
Table: Reactivity of Alkali Metals with Water
Safety Precautions
Given the highly vigorous and exothermic nature of this reaction, handling rubidium metal, especially near water, requires extreme caution. It must be stored under an inert atmosphere (like argon) or in mineral oil to prevent contact with moisture and air. Any experiments involving rubidium and water should only be conducted by trained professionals in a controlled environment with appropriate safety equipment.
For more detailed information on rubidium and its properties, you can refer to resources from organizations like the Royal Society of Chemistry or Wikipedia's entry on Rubidium.
