Question

Write a general equation for the reaction of an alkali metal with: a. a halogen b. water

Short answer

a. 2M + X2 -> 2MX, b. 2M + 2H2O -> 2MOH + H2, where M is an alkali metal, X is a halogen, MX is an alkali metal halide, and MOH is a metal hydroxide.

Step-by-step solution

General Equation with a Halogen

When an alkali metal (M) reacts with a halogen (X), they form an alkali metal halide (MX). The general equation for this reaction is: 2M + X2 -> 2MX, where M is the alkali metal and X is the halogen.

General Equation with Water

The general equation for the reaction between an alkali metal (M) and water (H2O) is: 2M + 2H2O -> 2MOH + H2. In this reaction, M represents the alkali metal, MOH is the resultant metal hydroxide, and H2 is the evolved hydrogen gas.

Key concepts

Alkali Metal and Halogen Reaction

Alkali metals are known for their high reactivity, especially with halogens, which are elements like fluorine, chlorine, bromine, and iodine. When they react, they tend to do so violently, often producing a bright flame. This is because alkali metals have one electron in their outer shell that they are eager to donate, while halogens have seven electrons in their outer shell and are just as eager to receive.

Consider sodium (Na), an alkali metal, reacting with chlorine (Cl), a halogen. The equation for this reaction, as mentioned in the exercise, is: 2Na + Cl_2 -> 2NaCl.This results in sodium chloride, also known as table salt. It's the transfer of electrons between the reactants that forms the ionic bond in the salt. All alkali metal and halogen reactions will follow this general pattern, forming an alkali metal halide.

Alkali Metal and Water Reaction

When alkali metals come into contact with water, they react to form metal hydroxides and hydrogen gas. This type of reaction is highly exothermic, meaning it releases a great deal of energy. For example, if you were to drop a piece of potassium into water, you would observe a vigorous reaction that releases enough heat to ignite the produced hydrogen gas.

The general equation for an alkali metal reacting with water is perfectly illustrated in the solution: 2M + 2H_2O -> 2MOH + H_2.The 'M' represents any alkali metal and 'MOH' is the metal hydroxide formed. For instance, if 'M' were lithium (Li), the equation would be: 2Li + 2H_2O -> 2LiOH + H_2,showing the formation of lithium hydroxide. Safety precautions must be taken when performing this type of reaction due to the intense reactivity.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They display the starting substances, known as reactants, and the ending substances, called products. For example, the general equation for the reaction between an alkali metal and a halogen is expressed as: 2M + X_2 -> 2MX.This tells us that two atoms of an alkali metal (M) react with one molecule of a diatomic halogen (X_2) to produce two units of alkali metal halide (MX).

These equations follow the law of conservation of mass which states that matter is neither created nor destroyed in a chemical reaction. Thus, the number of atoms for each element must be the same on both sides of the equation. Balancing chemical equations is crucial for accurately representing a chemical reaction. In education, understanding how to read and balance chemical equations is fundamental.

Metal Hydroxide

Metal hydroxides form when alkali metals react with water. These compounds consist of metal cations and the hydroxide anion (OH-). In the context of alkali metals, the hydroxides produced are strong bases due to their high solubility in water and their ease of dissociation to release hydroxide ions.

For instance, when sodium reacts with water, it forms sodium hydroxide: Na + H_2O -> NaOH + H_2.Sodium hydroxide (NaOH) is used in many applications, such as soap making and as a cleaning agent. Each alkali metal forms its own unique hydroxide, all of which share the general formula MOH, where 'M' represents an alkali metal. These hydroxides are often studied in chemistry due to their strong reactive properties and their implications in various industries.