Video Transcript
The reaction of hydrogen and iodine to produce hydrogen iodide is shown below: H2 gas plus I2 gas react to form 2HI gas. What is the meaning of the arrow in this chemical reaction equation? (A) Heat is needed for the reaction to work. (B) There is a change in the state of matter. (C) The reaction is reversible. (D) The reaction is irreversible. (E) Ultraviolet light is needed for the reaction to work.
Every chemical equation includes an arrow called a reaction arrow. A reaction arrow separates the reactants from the products in a chemical equation. There are two reactants in the given equation: hydrogen gas and gaseous iodine. When hydrogen and iodine react, they form the product hydrogen iodide, which is also a gas. There are two types of reaction arrows used in chemical equations. The forward reaction arrow is a single arrow that points from left to right. The equilibrium arrow is a double arrow with the top arrow pointing from left to right and the bottom arrow pointing from right to left.
The forward arrow is used for irreversible reactions. One example of an irreversible reaction is combustion. During the combustion of methane gas, carbon dioxide and water are produced and heat and light are released. Because this reaction is irreversible, the carbon dioxide and water molecules cannot react with each other to be converted back into the reactants.
In contrast, the equilibrium arrow is used for reversible reactions. The reaction provided in the question is an example of a reversible reaction. The forward reaction is represented by the top arrow and proceeds from left to right. During the forward reaction, the reactants H2 and I2 react to form the product HI. In a reversible reaction, the reverse reaction, which proceeds from right to left, happens simultaneously. During the reverse reaction, the product molecules react with each other and are converted back into the reactant molecules.
The double arrow in the provided chemical equation means that the reaction is reversible. The correct answer is answer choice (C).
