To increase the rate of a reaction in the liquid phase, one should increase the molarity of the reactants. This is achieved by adding more solute to the same volume of solvent or by evaporating some of the solvent to reduce the total volume.
To increase the rate of a gaseous reaction, one can either add more gas to a fixed-volume container or decrease the volume of the container (compression). Both actions increase the partial pressure of the reactants.
When analyzing experimental data, the rate is often determined by measuring the change in concentration over time (). On a concentration-time graph, the gradient of the tangent at any point represents the instantaneous rate.
| Factor | Primary Effect | Resulting Change |
|---|---|---|
| Concentration | More particles per unit volume | Increased collision frequency |
| Pressure | Particles forced closer together | Increased collision frequency |
| Temperature | Increased kinetic energy | Increased frequency AND higher proportion of successful collisions |
Use Precise Terminology: When explaining why rate increases, always use the phrase "per unit volume" or "per unit time." Simply saying "there are more collisions" is often insufficient; you must state there are "more collisions per second" or "more particles per ."
Identify the State: Check if the reactants are aqueous or gaseous. Use the term "concentration" for solutions and "pressure" for gases to demonstrate technical accuracy.
Graph Interpretation: In exams, you may be asked to compare two curves on a rate graph. A steeper initial gradient indicates a higher concentration or pressure, but the curve will level off sooner if the total amount of limiting reactant is the same.
The Energy Fallacy: A common mistake is claiming that increasing concentration gives particles more energy. This is incorrect; only temperature changes the average kinetic energy of the particles.
Volume vs. Concentration: Students often confuse the total amount of a substance with its concentration. Doubling the volume of a solution while keeping the moles of solute the same actually decreases the concentration and slows the reaction rate.
Activation Energy: Increasing concentration or pressure has no effect on the activation energy () of a reaction. is a fixed property of the reaction pathway and is only "lowered" by the addition of a catalyst.
