Glossary
Activation Energy
The minimum amount of energy required for reactant molecules to collide effectively and transform into products.
Example:
A match needs a certain amount of friction to generate enough heat to overcome the activation energy and ignite the chemicals on its tip.
Average Rate
The change in concentration of a reactant or product over a specific, measurable time interval, calculated as the slope between two points on a concentration vs. time graph.
Example:
To find the average rate at which a reactant was consumed during the first 10 minutes of a reaction, you would calculate the change in its concentration over that 10-minute period.
Catalysts
Substances that increase the rate of a chemical reaction without being consumed, by providing an alternative reaction pathway with a lower activation energy.
Example:
The platinum in a car's catalytic converter acts as a catalyst, speeding up the conversion of harmful exhaust gases into less toxic substances.
Concentration
The amount of a substance present in a given volume of solution, often expressed in molarity (moles per liter).
Example:
Increasing the concentration of hydrogen peroxide in a solution will generally make it decompose faster, as there are more molecules available to react.
Equilibrium
A dynamic state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.
Example:
When a bottle of soda is sealed, the carbon dioxide dissolving into the liquid reaches equilibrium with the carbon dioxide escaping into the gas phase above the liquid.
Instantaneous Rate
The rate of a reaction at a particular moment in time, determined by the slope of the tangent line to the concentration vs. time curve at that specific point.
Example:
If you want to know the exact speed of a reaction precisely 5 seconds after it begins, you would determine its instantaneous rate at that moment.
Kinetics
The branch of chemistry that studies the rates of chemical reactions and the factors that influence their speed.
Example:
Understanding the kinetics of how quickly a drug breaks down in the body is crucial for determining its dosage and shelf life.
Pressure (Effect on Rate)
For gaseous reactions, increasing the pressure increases the concentration of gas molecules, leading to more frequent collisions and a faster reaction rate.
Example:
In industrial synthesis of ammonia (Haber-Bosch process), high pressure is used to increase the reaction rate between nitrogen and hydrogen gases.
Rate Law
An experimentally determined equation that expresses the relationship between the rate of a reaction and the concentrations of its reactants, often including a rate constant.
Example:
For the reaction A + B → C, a common rate law might be Rate = k[A]²[B], indicating how the initial concentrations affect the reaction speed.
Rate of Reaction
A measure of how quickly reactants are consumed or products are formed over a specific period, typically expressed as change in concentration per unit time.
Example:
If a chemical process produces 0.2 moles of product per liter every second, its rate of reaction is 0.2 M/s.
Stoichiometry (and Reaction Rates)
The quantitative relationship between reactants and products in a balanced chemical equation, used to relate the rates of consumption and production of different chemical species.
Example:
In the reaction 2H₂ + O₂ → 2H₂O, stoichiometry dictates that hydrogen gas is consumed twice as fast as oxygen gas.
Surface Area (Effect on Rate)
The total exposed area of a solid reactant; increasing it provides more contact points for collisions, thereby increasing the reaction rate.
Example:
A finely powdered sugar cube dissolves much faster in water than a whole sugar cube because of its significantly larger surface area.
Temperature (Effect on Rate)
A measure of the average kinetic energy of molecules; increasing it generally increases reaction rates by leading to more frequent and energetic collisions.
Example:
Baking a cake at a higher temperature than recommended can cause it to cook too quickly or burn, as the chemical reactions proceed at an accelerated rate.