Problem 13 - Chemistry The Molecular Science

Question

For each process, write a chemical equation and classify the process as reactant-favored or product-favored. (a) Water decomposes to its elements, hydrogen and oxygen. (b) Gasoline spilled on the ground evaporates (use octane, \(\mathrm{C}_{8} \mathrm{H}_{18},\) to represent gasoline). (c) Sugar dissolves in water at room temperature.

Step-by-Step Solution

Answer

(a) Reactant-favored, (b) Product-favored, (c) Product-favored.

Step 1: Write the Equation for Water Decomposition

The decomposition of water into hydrogen and oxygen can be represented by the chemical equation:\[2 \text{H}_2\text{O} (l) \rightarrow 2 \text{H}_2 (g) + \text{O}_2 (g)\]This process involves breaking down water molecules (H2O) into hydrogen gas (H2) and oxygen gas (O2).

Step 2: Classify Water Decomposition

Since the decomposition of water into hydrogen and oxygen requires energy (endothermic reaction), it is reactant-favored under standard conditions because it does not occur spontaneously without an input of energy.

Step 3: Write the Equation for Gasoline Evaporation

The evaporation of octane, representing gasoline, can be written as:\[\text{C}_8\text{H}_{18} (l) \rightarrow \text{C}_8\text{H}_{18} (g)\]This equation indicates the phase change from liquid octane to gaseous octane.

Step 4: Classify Gasoline Evaporation

Evaporation is a physical process driven by volatility and temperature. At the right conditions, such as room temperature, evaporation is typically product-favored because it proceeds spontaneously.

Step 5: Write the Equation for Sugar Dissolving

When sugar (sucrose) dissolves in water, the equation can be written as:\[\text{C}_{12}\text{H}_{22}\text{O}_{11} (s) + \text{H}_2\text{O} (l) \rightarrow \text{C}_{12}\text{H}_{22}\text{O}_{11} (aq)\]This equation represents solid sugar dissolving into an aqueous solution form.

Step 6: Classify Sugar Dissolving

The dissolution of sugar in water at room temperature is typically product-favored because the solubility of sugar in water at these conditions allows the process to proceed spontaneously until the solution reaches saturation.

Key Concepts

Understanding Reactant-Favored Processes

In chemical reactions, reactant-favored processes are those in which the reactants are more stable, or more energetically favorable, than the products. This means the reaction tends to "favor" the reactants, often requiring the input of energy to proceed.

For example, in the decomposition of water into hydrogen and oxygen, the chemical equation is:\[2 \text{H}_2\text{O} (l) \rightarrow 2 \text{H}_2 (g) + \text{O}_2 (g)\]In this reaction, energy must be supplied for it to proceed, classifying it as an endothermic process. Under standard conditions, such as room temperature and pressure, the water does not decompose spontaneously.

• High energy input is required to overcome the stability of the water molecules.
• The reaction is not spontaneous at normal conditions.
• That means it's reactant-favored because, left on its own, the process prefers to remain as reactants.

Understanding Product-Favored Processes

Product-favored processes are those where the products of the reaction are more stable or more energetically favorable than the reactants. This implies that the reaction tends to proceed spontaneously, favoring the formation of products.

An example of this is the evaporation of gasoline, represented by the equation:\[\text{C}_8\text{H}_{18} (l) \rightarrow \text{C}_8\text{H}_{18} (g)\]In this scenario, at room temperature, octane often evaporates naturally.

• The molecules transition from a liquid to a gaseous state without needing extra energy input.
• The increase in temperature or decrease in pressure facilitates the spontaneous phase change.
• Therefore, it is termed product-favored under the right conditions, typical of physical changes like evaporation.

Grasping Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They show the transformation from reactants to products using symbols and formulas.

Consider when sugar dissolves in water, represented as:\[\text{C}_{12}\text{H}_{22}\text{O}_{11} (s) + \text{H}_2\text{O} (l) \rightarrow \text{C}_{12}\text{H}_{22}\text{O}_{11} (aq)\]This equation formulates:

• Sugar transitioning from solid to dissolved (aqueous) form in water.
• The arrow indicates the direction of the reaction, from reactants to products.
• The state symbols (s, l, g, aq) correspond to solid, liquid, gas, and aqueous, clarifying the physical states involved.

Understanding these equations is crucial as they not only communicate the substances involved but also help classify the process based on its energy dynamics and spontaneity.