Question

Sulfur dioxide is a reducing agent. When it is bubbled through an aqueous solution containing \(\mathrm{Br}_{2}\), a red-colored solution, it reduces the bromine to colorless bromide ions and forms sulfuric acid. Write a balanced equation for this reaction and identify the oxidizing and reducing agent.

Short answer

Balanced equation: SO2 + Br2 + 2H2O -> H2SO4 + 2Br-. Sulfur dioxide (SO2) is the reducing agent and bromine (Br2) is the oxidizing agent.

Step-by-step solution

Write the unbalanced chemical equation

Start by writing the skeleton equation with reactants and products. Sulfur dioxide (SO2) reacts with bromine (Br2) to give bromide ions (Br-) and sulfuric acid (H2SO4). The unbalanced equation is: SO2 + Br2 -> H2SO4 + Br-.

Balance the chemical equation

Balance the atoms other than oxygen and hydrogen first. There are 2 bromine atoms on the reactant side, so place a coefficient of 2 in front of Br- on the product side: SO2 + Br2 -> H2SO4 + 2Br-. Now, balance the sulfur and oxygen atoms. There is 1 sulfur atom on both sides, so sulfur is balanced. There are 4 oxygen atoms in H2SO4 and only 2 in SO2, thus to balance the oxygen atoms, place a coefficient of 2 in front of H2O on the product side: SO2 + Br2 + H2O -> H2SO4 + 2Br-. Lastly, balance the hydrogen atoms by placing a coefficient of 2 in front of H2O on the reactant side: SO2 + Br2 + 2H2O -> H2SO4 + 2Br-.

Verify that the equation is balanced

Count the number of atoms of each element on both sides of the equation to confirm they are equal. Reactants: 1S, 2O (from SO2) + 2Br + 4H and 2O (from H2O); Products: 1S, 4O (from H2SO4) + 2Br + 4H. The equation is balanced.

Identify the Oxidizing and Reducing Agents

Recognize that the oxidation state of sulfur increases from +4 in SO2 to +6 in H2SO4, so SO2 is oxidized and is the reducing agent. Conversely, the oxidation state of bromine decreases from 0 in Br2 to -1 in Br-, so Br2 is reduced and is the oxidizing agent.

Key concepts

Chemical Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is satisfied in a chemical reaction. When substances react, the same number of atoms should be present before and after the reaction.

For instance, in the reaction where sulfur dioxide (SO2) becomes sulfuric acid (H2SO4) while reducing bromine (Br2) to bromide ions (Br-), the initial step is to list unbalanced reactants and products. Afterward, a step-by-step approach follows, starting with balancing atoms that are neither oxygen nor hydrogen.

This systematic approach requires adjusting coefficients in front of compounds and elements to ensure that the number of atoms of each element is equal on both sides of the equation. It may also be helpful to consider adding products like water (H2O) to balance hydrogen and oxygen, which are often last to be balanced.

Once the equation appears balanced, it's important to verify by counting each type of atom. If the numbers match on both sides, the equation is correctly balanced. This process ensures that the reaction obeys the physical laws and provides a better understanding of the reaction stoichiometry.

Oxidation States

Understanding oxidation states is crucial for analyzing redox (reduction-oxidation) reactions. The oxidation state, often referred to as oxidation number, is a measure of the degree of oxidation of an atom in a substance. It can be thought of as the hypothetical charge an atom would have if all bonds to atoms of different elements were completely ionic.

Oxidation states can change during chemical reactions as electrons are transferred between atoms. An increase in oxidation state corresponds to oxidation, while a decrease corresponds to reduction.

During the reaction between sulfur dioxide and bromine, the sulfur’s oxidation state increases from +4 in SO2 to +6 in H2SO4, indicating oxidation. Conversely, bromine’s oxidation state decreases from 0 in Br2 to -1 in Br-, showing that it has been reduced. By determining the changes in oxidation states, we can understand how electrons are transferred and identify the roles substances play in the reaction.

Redox Agents Identification

In redox reactions, identification of the reducing agent and the oxidizing agent reveals which substances donate and accept electrons, respectively. The reducing agent is the substance that loses electrons, thus being oxidized, while the oxidizing agent is the substance that gains electrons, thereby being reduced.

Looking at the reaction between sulfur dioxide and bromine, the increase in oxidation state of sulfur signifies that SO2 is donating electrons. Therefore, SO2 is the reducing agent. On the other hand, the decrease in oxidation state of bromine means that Br2 is accepting electrons, making Br2 the oxidizing agent.

Identifying the redox agents not only helps in understanding the flow of electrons during the reaction but also helps in predicting the direction of chemical reactions and understanding the relative strengths of oxidizing and reducing agents involved in the process.