Question

Write a general equation for the reaction of an alkali metal with: a) a halogen \(\quad\) b) water

Short answer

a) 2M + X_2 -> 2MX, b) 2M + 2H_2O -> 2MOH + H_2.

Step-by-step solution

Writing Equation with a Halogen

For an alkali metal (M) reacting with a halogen (X_2), the general equation is: 2M + X_2 -> 2MX. Here, X represents a halogen (such as F, Cl, Br, I), and MX will be the formed metal halide.

Writing Equation with Water

When an alkali metal (M) reacts with water (H_2O), the general equation is: 2M + 2H_2O -> 2MOH + H_2. This signifies that the reaction produces a metal hydroxide (MOH) and hydrogen gas (H_2).

Key concepts

Reaction of Alkali Metals with Halogens

Alkali metals are known for their high reactivity, especially when they come into contact with halogens. A halogen refers to elements found in group 17 of the periodic table, which includes fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). When an alkali metal reacts with a halogen, the reaction is highly exothermic and can be vigorous. This is because alkali metals have one loosely-held outer electron, which they readily donate to the halogen's vacant electron shell. The result of such a reaction is the formation of a metal halide, which is a type of salt.

For example, if sodium (Na), which is an alkali metal, were to react with chlorine (Cl), the equation would be: 2Na + Cl_2 -> 2NaCl. In this reaction, each sodium atom donates one electron to a chlorine molecule, forming sodium chloride (NaCl), a common table salt.

Reaction of Alkali Metals with Water

The interaction between alkali metals and water is another area of interest. These metals react vigorously with water to form metal hydroxides and hydrogen gas. The reaction can range from a rapid fizzing with lithium to an explosive reaction in the case of heavier alkali metals like potassium and cesium. The general equation illustrating this reaction is: 2M + 2H_2O -> 2MOH + H_2.

The resulting metal hydroxide dissolves in water to create an alkaline solution, indicating it's a base. Meanwhile, the hydrogen gas produced may catch fire if the reaction generates enough heat. This demonstrates the significance of taking safety precautions when handling alkali metals and water together. Students should be particularly aware that the heat generated by these reactions can be dangerous, and they should be performed under controlled conditions, ideally with a professional present.

Chemical Equations

Understanding chemical equations is fundamental to studying chemical reactions. A chemical equation is a representation of a chemical reaction using symbols and formulas. On the left side are the reactants, the substances that go into the reaction, while the right side shows the products, which are formed as a result of the reaction.

To ensure the equation is balanced, you have to have the same number of each type of atom on both sides of the equation. This respects the law of conservation of mass, which states that matter is not created or destroyed in a chemical reaction. Balancing equations requires knowledge of stoichiometry, which is the calculation of reactants and products in chemical reactions.

Metal Halides Formation

Metal halides form when alkali metals react with halogens, as represented by the general equation 2M + X_2 -> 2MX. The alkali metal gives up its single valence electron to the halogen to form a metal halide compound. In the case of the sodium and chlorine reaction from early, the product sodium chloride is a metal halide.

These compounds are ionic in nature, meaning they consist of metal cations (positively charged ions) and halogen anions (negatively charged ions). The crystal lattice structure and high melting points characteristic of metal halides are indicative of the strong electrostatic forces between the ions.

Metal Hydroxides Formation

Alkali metals react with water to form metal hydroxides, a process that releases hydrogen gas. The general reaction is: 2M + 2H_2O -> 2MOH + H_2. Metal hydroxides, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), are ionic compounds that consist of metal cations and a hydroxide anion (OH-). These compounds are highly soluble in water and are strong bases, meaning they can effectively neutralize acids. The formation of these hydroxides is an indicator of the basic properties of alkali metals and showcases their ability to alter the pH of their environment resulting in an alkaline solution.