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Ch.14 - Chemical Kinetics
All textbooksMcMurry 8th EditionCh.14 - Chemical KineticsProblem 124b
Chapter 14, Problem 124b

Consider the reaction H2(g) + I2(g) → 2 HI(g). The reaction of a fixed amount of H2 and I2 is studied in a cylinder fitted with a movable piston. Indicate the effect of each of the following changes on the rate of the reaction. (b) An increase in volume at constant temperature

Verified step by step guidance
1
Understand that the rate of a reaction is influenced by the concentration of the reactants. According to the rate law, the rate is proportional to the concentration of the reactants raised to a power, which is determined experimentally.
Recognize that increasing the volume of the container at constant temperature will decrease the concentration of the gaseous reactants, H2 and I2, because concentration is defined as moles per unit volume.
Recall that a decrease in concentration of reactants generally leads to a decrease in the rate of reaction, as there are fewer molecules per unit volume to collide and react.
Consider the reaction mechanism and the collision theory, which states that reaction rate depends on the frequency of effective collisions between reactant molecules. A lower concentration results in fewer collisions per unit time.
Conclude that increasing the volume at constant temperature will decrease the rate of the reaction because it reduces the concentration of the reactants, leading to fewer effective collisions.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Reaction Rate

The reaction rate refers to the speed at which reactants are converted into products in a chemical reaction. It is influenced by several factors, including concentration, temperature, and the presence of catalysts. In gas-phase reactions, the concentration of reactants can be affected by changes in volume, which in turn impacts the frequency of collisions between molecules.
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Ideal Gas Law

The Ideal Gas Law (PV = nRT) describes the relationship between pressure (P), volume (V), temperature (T), and the number of moles (n) of a gas. When the volume of a gas increases at constant temperature, the pressure decreases, which can lead to a change in the concentration of the gaseous reactants. Understanding this law is crucial for predicting how changes in volume affect reaction rates.
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Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. In the context of the reaction H2(g) + I2(g) → 2 HI(g), increasing the volume decreases the concentration of the reactants, which can shift the equilibrium position and potentially affect the rate of the forward reaction.
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Related Practice
Textbook Question

Consider the reaction H2(g) + I2(g) → 2 HI(g). The reaction of a fixed amount of H2 and I2 is studied in a cylinder fitted with a movable piston. Indicate the effect of each of the following changes on the rate of the reaction. (a) An increase in temperature at constant volume

Textbook Question

Consider the reaction H2(g) + I2(g) → 2 HI(g). The reaction of a fixed amount of H2 and I2 is studied in a cylinder fitted with a movable piston. Indicate the effect of each of the following changes on the rate of the reaction. (c) The addition of a catalyst

Textbook Question
Concentration–time data for the conversion of A and B to D are listed in the following table. (c) What is the rate law?