Reaction Rates and Measurement

Reaction Rates

Measuring Reaction Rate

  • Reaction rate is measured by observing the change in concentration over time.
    • Specifically, how much reactant is lost and how much product is formed during a time interval (e.g., a second).
  • Molarity is commonly used to express concentration.

Concentration vs. Time Graph

  • The graph plots concentration against time allowing us to visualize reaction rates.
  • Reactants typically start at a high concentration and decrease over time.
  • Products usually start at a low concentration and increase over time.
  • Equilibrium is formed when the concentrations of reactants and products become constant (unchanging).

Reaction Speed and Concentration

  • Reactions usually start fast due to high initial reactant concentrations.
  • As reactants are consumed, the rate of product formation decreases.
  • Example: Reactant A turns into Product B.
    • The concentration of A decreases rapidly at the beginning.
    • Simultaneously, the concentration of B increases.

Stoichiometry From Concentration Changes

  • The slopes of concentration changes reveal stoichiometric ratios.
  • Similar slopes suggest a roughly one-to-one ratio (e.g., A turning into B).
  • Equilibrium arrows indicate a reversible reaction, where A can turn into B, and B can turn back into A: A \rightleftharpoons B
    • Even at equilibrium, both forward and reverse reactions continue; concentrations just don't change.

Interpreting Different Graphs

  • Example of a graph where the slope for the change in concentration of A is steeper than B, indicating a faster reaction rate for A.
  • Equilibrium is reached when the rates of the forward and reverse reactions are equal, and concentrations stabilize.

Product vs. Reactant Favored Reactions

  • Reactions don't always create only products; they can also revert to reactants.
  • Some reactions are product-favored (more products at equilibrium).
  • Others are reactant-favored (more reactants at equilibrium).

Identifying Reactants and Products

  • Consider a reaction with components A (yellow), B (blue), and C (pink).
  • To identify initial materials, look at concentrations at time zero.
  • If a component's concentration is zero at the beginning, it wasn't present initially.

Reaction Direction and Equilibrium

  • If products are present at time zero, the reaction likely starts in reverse.
  • The point where the lines level out indicates where equilibrium begins.
  • At equilibrium, compare reactant vs. product concentrations to determine if the reaction is reactant- or product-favored.

Recap: Reaction Rate and Slope

  • Reaction rate is the change in concentration over time.
  • A steep slope indicates a fast reaction.
  • A shallow slope indicates a slow reaction.