Trends in Reactivity with Water
In this discussion, we will explore the reactions of Group 1 elements—lithium, sodium, potassium, rubidium, and cesium—with water. By examining these reactions, we aim to uncover the trend in reactivity within Group 1.
Except for lithium, alkali metals are extremely soft and easily fused. They are highly malleable (can be pressed into sheets) and ductile (can be drawn into wires). When freshly cut, these metals exhibit a bright luster that quickly tarnishes upon exposure to air.
Summary of the Trend in Reactivity
The reactivity of Group 1 metals with water increases as you move down the group. This means that lithium reacts the least vigorously, while cesium reacts the most vigorously.
Explaining the Trend in Reactivity
Examining the Enthalpy Changes for the Reactions
The enthalpy change (ΔH) for a reaction provides insight into the energy changes occurring during the reaction. The table below provides estimates of the enthalpy change for each element when reacting with water:
| Element | Enthalpy Change (kJ/mol) |
|---|---|
| Li | -222 |
| Na | -184 |
| K | -196 |
| Rb | -195 |
| Cs | -203 |
As observed, there is no consistent pattern in these values. They are quite similar, and surprisingly, lithium releases the most heat during the reaction.
Chemical Reactions of Alkali Metals with Water
Let’s look at the chemical equations and thermodynamic values for the reactions of each alkali metal with water:
Lithium and Water
The reaction of lithium with water is relatively gentle compared to other alkali metals:
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g) (ΔH): -222 kJ/mol
Sodium and Water
Sodium reacts more vigorously with water, producing sodium hydroxide and hydrogen gas:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) (ΔH): -184 kJ/mol
Potassium and Water
Potassium’s reaction with water is even more vigorous, often producing enough heat to ignite the hydrogen gas produced:
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) (ΔH): -196 kJ/mol
Rubidium and Water
Rubidium reacts very violently with water, and the reaction can be explosive:
2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2(g) (ΔH): -195 kJ/mol
Cesium and Water
Cesium has the most violent reaction with water among the alkali metals, often resulting in explosions:
2Cs(s) + 2H2O(l) → 2CsOH(aq) + H2(g) (ΔH): -203 kJ/mol
Understanding the Reactions
To understand these reactions, we need to consider what happens to the metal atoms. In each case, the process starts with metal atoms in a solid state and ends with metal ions in solution. The overall reaction for the metal is:
X(s) → X+(aq) + e–
The reactivity trend can be explained by the ionization energy of the metals. As you move down the group, the ionization energy decreases, making it easier for the metals to lose their outer electron and react with water. This explains the increasing reactivity from lithium to cesium.
By understanding these trends and reactions, we can better predict and safely handle these highly reactive elements in various chemical applications.
