Chemical Reactions: Definition, Equation, and Factors Affecting Reaction Rate
Chemical Reaction
A chemical reaction, or chemical change, is a process in which two or more substances (called reactants) transform into other substances called products due to an energy factor.
Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction.
Classification of Chemical Reactions
By How They Perform:
- Synthesis: Elements react to form a compound.
- Decomposition: A compound breaks down into two or more simpler substances.
- Replacement: An element reacts with a compound, displacing an element in the compound to form a new one.
- Double Replacement: Two compounds exchange elements, forming two new compounds.
Because of Their Speed:
- Instantaneous: Reactions that occur very quickly.
- Non-instantaneous: Reactions that take time to occur.
For Their Energy:
- Endothermic: Reactions that absorb energy.
- Exothermic: Reactions that release energy.
Activation Energy
The activation energy (
) in chemistry and biology is the minimum energy required for a system to initiate a particular process, such as a chemical reaction.
Reaction Mechanism
The reaction mechanism is the sequence of steps or stages that make up a chemical reaction. Reaction mechanisms are related to chemical kinetics and dynamics.
The reaction rate is the amount of substance reacting per unit of time. For example, the oxidation of iron under atmospheric conditions is a slow reaction that can take many years, while the combustion of butane in a fire is a reaction that happens in a fraction of a second.
Factors Affecting Reaction Rate
- Nature of the Reaction: Some reactions are inherently faster than others. Factors like the number of reacting species, their physical state (solids react slower than gases or solutions), and the complexity of the reaction can influence the rate.
- Concentration: Reaction rate increases with concentration, as described by the rate law and explained by collision theory. Higher reactant concentration leads to more frequent collisions.
- Pressure: The rate of gas reactions increases significantly with pressure, which is equivalent to increasing gas concentration. For reactions in condensed phases, pressure dependence is weak unless it’s very high.
- Order: The reaction order determines how concentration (or pressure) affects the reaction rate.
- Temperature: Higher temperature generally increases reaction rate by providing more energy to the system, leading to more collisions and a greater number of particles with sufficient activation energy. The Arrhenius equation describes the influence of temperature. A rule of thumb is that reaction rates double for every 10°C temperature increase, but this can vary significantly.
- Solvent: For reactions in solution, solvent properties affect the reaction rate. Ionic strength also plays a role.
- Electromagnetic Radiation and Light Intensity: Electromagnetic radiation can speed up or initiate reactions by providing energy to reactant particles. This energy can break bonds, excite molecules, and create reactive intermediate species. Higher light intensity increases the energy absorbed and thus the reaction rate.
- Catalyst: A catalyst increases the reaction rate (both forward and reverse) by providing an alternative pathway with lower activation energy.
- Isotopes: The kinetic isotope effect refers to different reaction rates for molecules with different isotopes (e.g., hydrogen vs. deuterium) due to mass differences.
- Contact Surface: In surface reactions (e.g., heterogeneous catalysis), the rate increases with increasing contact surface area because more solid particles are exposed to reactant molecules.
- Mixing: Mixing can significantly affect the reaction rate in both homogeneous and heterogeneous reactions.
The reaction rate can be independent of temperature (non-Arrhenius) or decrease with increasing temperature (anti-Arrhenius). Reactions without an activation barrier (e.g., some radical reactions) tend to have an anti-Arrhenius temperature dependence.