Reactivity with Water – HSC Chemistry
This is part of preliminary HSC Chemistry course under the topic of Periodicity.
HSC Chemistry Syllabus
- Demonstrate, explain, and predict the relationships in the observable trends in the physical and chemical properties of elements in periods and groups in the periodic table, including but not limited to:
Periodicity
Reactivity of Elements with Water: Trends in the Periodic Table
Understanding the reactivity of elements with water is a fundamental aspect of chemistry that reveals much about the nature of elements and their placement in the periodic table. The way an element reacts with water can range from violent to no reaction at all, and this behaviour is deeply influenced by the element's electronic configuration and its position in the periodic table. This article explores these trends and the underlying principles that govern them.
The general trend of elements' reactivity with water is shown; exceptions apply.
Reactivity in Group 1: Alkali Metals
The alkali metals, located in Group 1 of the periodic table, are notorious for their vigorous reactions with water. These elements have a single electron in their outermost shell, which they readily lose to form cations with a stable noble gas electron configuration. The reaction between a metal and water changes the metal atom into a metal cation in the form of metal hydroxide.
The general reaction of alkali metals (`M`) with water can be summarised as follows:
This reaction produces an alkaline solution (due to the hydroxide ions) and hydrogen gas. The reactivity increases down the group from lithium (Li) to caesium (Cs), primarily due to the decreasing ionisation energy, which makes it easier for atoms to lose their outermost electron.
Reactivity in Group 2: Alkaline Earth Metals
The alkaline earth metals in Group 2 also react with water, though generally less vigorously than the alkali metals. This is because their reactions with water involve the removal of two valence electrons instead of only one in alkali metals. This process requires more energy (first and second ionisation energies).
Beryllium (Be) and magnesium (Mg) react very slowly with water, with magnesium requiring hot water to react noticeably. Starting from calcium (Ca), the reaction becomes more noticeable.
The general reaction of alkali Earth metals with water is given by:
$$M(s) + H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)$$
The reactivity increases down the group due to the same reasons as alkali metals, albeit to a lesser extent due to the two outer electrons and the corresponding increase in ionisation energies.
Transition Metals
The reactivity of transition metals with water varies widely. Some, like iron (Fe), react slowly with water, forming rust over time. Others, like gold (Au) and platinum (Pt), do not react with water at all.
Group 13: Boron Family
Aluminium (Al), a member of Group 13, reacts with water only when the protective oxide layer is disturbed. Under normal conditions, it forms an impermeable oxide layer that protects it from further reaction.
Reactivity with Water – Non-metals
Generally, non-metals are less reactive with water compared to Group 1, Group 2 and Group 13 elements. Within non-metal elements, the reactivity with water increases across the period from left to right, and decreases down a group.
- Elements in Group 14, including carbon (C) and silicon (Si), generally do not react with water. However, silicon can react with steam at high temperatures to form silicon dioxide (SiO₂) and hydrogen gas.
- Phosphorus, a member of Group 15, can react with water under certain conditions, but nitrogen (N) and arsenic (As) do not react with water.
- Elements in Group 16 do not react with water under normal conditions.
- The halogens (Group 17) can react with water and also substances that are dissolved in water. For example, chlorine (Cl₂) can react with water to form a mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl). Halogens can also react with metal ions and organic substances dissolved in water. These reactions are different to the reaction between metals and water.
- Noble gases (Group 18) are chemically inert, and do not react with water.
Key Points
The reactivity of elements with water is influenced by several factors:
- Metallic Character: Metals tend to be more reactive with water than nonmetals, with reactivity generally decreasing across a period and increasing down a group.
- Ionisation Energy: Lower ionisation energies facilitate the loss of electrons, making elements more reactive with water. This rule primarily applies to metals. Therefore, reactivity of metals with water decreases across a period form left to right, and increases down a group.
- Non-metal's reactivity with water trend is different to that of metals. It increases across a period from left to right (exception of noble gases), and decreases down a group.
Previous Section: Electronegativity & Ionisation Energy
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