MCAT® ExamChemical and Physical Foundations of Biological SystemsEasy
A student is designing an experiment to measure the rate of a chemical reaction. They are investigating the decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen gas (O₂), catalyzed by iodide ions (I⁻). The reaction is represented as: 2 H₂O₂(aq) → 2 H₂O(l) + O₂(g). To accurately determine the initial rate of reaction, which of the following experimental parameters is MOST critical to control and measure precisely?
- AThe initial concentrations of hydrogen peroxide and iodide ions.
- BThe atmospheric pressure in the laboratory.
- CThe final volume of oxygen gas produced.
- DThe total volume of the reaction vessel.
Show answer & explanationAnswer & explanation
Correct answer: A. The initial concentrations of hydrogen peroxide and iodide ions.
To determine the initial rate of a reaction, the initial concentrations of reactants and catalysts are paramount. These values directly influence how quickly the reaction begins, and changes in them can significantly alter the observed rate. Other factors like temperature also play a role, but among the given options, initial concentrations are most critical for defining the starting conditions.
Why the other options are wrong
- B. Atmospheric pressure has a negligible effect on the rate of reactions occurring in solution, especially compared to reactant concentrations.
- C. The final volume of oxygen produced relates to the extent of the reaction, not the initial rate at which it begins.
- D. While reaction volume is important for calculating concentrations, the total volume of the vessel itself is less critical than the initial concentrations for determining the initial rate.
Reaction Rate Factors
Chemical reaction rates are influenced by several factors, including reactant concentrations, temperature, surface area, and the presence of catalysts.
- Higher reactant concentration generally leads to a faster reaction rate due to more frequent collisions.
- Increased temperature typically increases reaction rate by providing more kinetic energy to molecules.
- Catalysts speed up reactions by lowering the activation energy without being consumed.
- Surface area is important for heterogeneous reactions, as more surface area allows for more contact between reactants.
Memory trick: Concentration, Temperature, Catalyst, Surface Area: The CTCS of reaction speed!