General Chemistry
Rate orders describe the amount that a reaction rate changes when a reactant's concentration is changed and are represented by x and y in the rate law. The overall rate order for a reaction is found by adding x and y. Zero-order reactions are unaffected by changes in concentration, so the reaction rate equals the rate constant k. First-order reactions have rates that are directly proportional to one reactant's concentration. Second-order reactions have rates that are directly proportional to two reactants or that are proportional to the square of one reactant. Mixed-order reactions have reaction rates that change throughout the reaction, and broken-order reactions have rate orders that are fractions.
Lesson Outline
<ul> <li>Rate law and rate orders (x and y)</li> <li>Overall rate order (x + y)</li> <li>Zero-order reactions <ul> <li>Concentration of reactants doesn't matter</li> <li>Reaction rate equals the rate constant k</li> <li>Rate changes with catalyst or temperature changes</li> </ul> </li> <li>First-order reactions <ul> <li>Rate proportional to one reactant's concentration</li> </ul> </li> <li>Second-order reactions <ul> <li>Rate proportional to the product of two reactants' concentrations</li> <li>Rate can also rely on the square of one reactant's concentration (when two molecules of one reactant are interacting with each other)</li> </ul> </li> <li>Mixed-order reactions <ul> <li>Reaction rates change throughout the reaction</li> </ul> </li> <li>Broken-order reactions <ul> <li>Rate orders are fractions</li> </ul> </li> </ul>
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FAQs
A rate order is the relationship between the rate of a reaction and the concentration of its reactants. It is an integer (or sometimes fractional) value that is used to express how the reaction rate is influenced by the concentration of a reactant in the rate law. The rate law is a mathematical expression that establishes the connection between the reaction rate and the concentration of the reactants, which is typically expressed as: rate = k[A]^x[B]^y, where k is the rate constant, A and B are the reactants, and x and y are the order of A and B, respectively.
Zero-order, first-order, and second-order reactions refer to the order of reactions with respect to their reactants, which determine how the reaction's rate depends on the concentration of reactants. Zero-order: The reaction rate does not change with a change in the concentration of the reactant. The rate law expression is of the form: rate = k (rate constant). First-order: The reaction rate is directly proportional to the concentration of the reactant. The rate law expression is of the form: rate = k[A], where A is the reactant. Second-order: The reaction rate is proportional to the square of the concentration of the reactant. The rate law expression is either rate = k[A]^2 for a single reactant or rate = k[A][B] for two reactants.
To determine the rate order experimentally, researchers typically conduct experiments under controlled conditions, measuring the changes in reactant concentrations and reaction rates over time. The method of initial rates is commonly used in this process, where the initial rates of a reaction at various concentrations of reactants are monitored, and the reaction order is deduced by comparing the effects of changing concentrations on the reaction rate. A plot of the reaction rate versus the reactant concentration will yield a straight line for zero-order, first-order, and second-order reactions, with different slopes and plot types (linear, semi-log, or reciprocal plot) for each order.
Mixed-order reactions are those where the reaction rate depends on the concentration of one or more reactants but does not follow simple zero-order, first-order, or second-order behavior. In these reactions, the order with respect to one or more reactants may be a non-integer or may change during the reaction itself. Broken-order reactions are characterized by non-integer orders with respect to one or more reactants. It indicates that the reaction rate's dependence on the reactant concentration deviates from simple whole-number reaction orders, typically due to complex mechanistic factors that influence the overall reaction rate.
