Problem 17 - Introductory Chemistry Atoms First

Question

Tap water contains dissolved oxygen gas, \(\mathrm{O}_{2}(a q)\). Adding heat to tap water causes the dissolved oxygen to leave. Which of the two equations below represents a chemical reaction, and which does not? Defend your answer. (a) \(2 \mathrm{H}_{2} \mathrm{O}_{2}(l) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\) (b) \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(a q)\) Heat \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\)

Step-by-Step Solution

Answer

Equation (a) represents a chemical reaction, as it involves the decomposition of hydrogen peroxide into water and oxygen gas. Equation (b) does not represent a chemical reaction, as it only shows the change in the physical state of dissolved oxygen gas when heat is added to the water.

Step 1: Analyze the first equation

We have the first equation as: \(2 \mathrm{H}_{2} \mathrm{O}_{2}(l) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(l) + \mathrm{O}_{2}(g)\) Here, hydrogen peroxide (H\(_2\)O\(_2\)) decomposes into water (H\(_2\)O) and oxygen gas (O\(_2\)). Since new products are formed from the reactants, this is a chemical reaction.

Step 2: Analyze the second equation

The second equation is: \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(a q)\) Heat \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\) In this case, we're starting with water (H\(_2\)O) and dissolved oxygen gas (O\(_2\)(aq)) and adding heat. As a result of adding heat, the dissolved oxygen leaves the water and becomes oxygen gas (O\(_2\)(g)). Notice that no new substances are formed; it's simply a change in the physical state of the dissolved oxygen gas. Therefore, this is not a chemical reaction.

Step 3: Conclusion

Equation (a) represents a chemical reaction because it involves the decomposition of hydrogen peroxide into water and oxygen gas. Equation (b) does not represent a chemical reaction, as it only shows the change in the physical state of dissolved oxygen gas when heat is added to the water.

Key Concepts

Decomposition Reaction

Decomposition reactions are a type of chemical reaction where one compound breaks down into two or more simpler substances. These reactions are characterized by a single reactant breaking apart.
It is essential to note that decomposition reactions often require energy input, such as heat, light, or electricity, to proceed. For example, the decomposition of hydrogen peroxide (H\(_2\)O\(_2\)) into water (H\(_2\)O) and oxygen gas (O\(_2\)) is a classic example.
In this scenario, hydrogen peroxide acts as the single reactant that decomposes to form two distinct substances, demonstrating a change at the molecular level.

Physical Change

A physical change involves a change in the physical properties of a substance without altering its chemical composition. These changes are often reversible. For instance, when you melt ice, it changes from a solid to a liquid, but it's still water.
One of the common indicators of a physical change is a state change, such as solid to liquid, liquid to gas, and so on. In the provided exercise, heating dissolved oxygen in water to produce gaseous oxygen is an example of a physical change.
The key here is that no new substances are formed, and only the physical state of oxygen changes from aqueous (dissolved) to gaseous.

Hydrogen Peroxide

Hydrogen peroxide (H\(_2\)O\(_2\)) is a chemical compound that is often used as a disinfectant or bleaching agent. However, it's not stable and can easily decompose, especially in the presence of light, heat, or catalysts.
This decomposition is typically what students encounter in chemistry when discussing chemical reactions. During decomposition, hydrogen peroxide breaks down into water and oxygen gas, making it a useful demonstration of a decomposition reaction.
The interesting aspect of hydrogen peroxide is its ability to store a higher energy state, which releases oxygen when it breaks down.

Dissolved Oxygen

Dissolved oxygen refers to the oxygen gas that is mixed within a liquid, particularly water. It's crucial for the survival of aquatic life, as organisms such as fish and plants rely on it.
The amount of oxygen that can dissolve in water varies based on temperature and pressure. When water is heated, its ability to hold oxygen decreases, causing oxygen to escape as a gas.
In the example from the exercise, heating tap water liberates oxygen as gas, which shows the necessity to understand how temperature affects dissolved gases within liquids.