Chemical Reactions: Rates, Equilibrium, and Entropy
Chemistry
Vocabulary
Rate
A ratio between two related quantities expressed in different units.
Reaction Rate
The rate at which the reactants of a chemical reaction form the products.
The calculated average rate can be negative for a reactant or positive for a product.
Collision Theory
States that in order for a collision to be effective, reactant particles must possess a sufficient amount of kinetic energy to break the necessary bonds in the reactants.
There are two things needed for a reaction to occur: enough energy and correct orientation.
Incorrect Orientation
The molecules are not oriented in a way that allows the required new bonds to form.
Too Little Energy
Many collisions with the molecules oriented correctly are not energetic enough to break the bonds in the reactant molecules.
There are four things that affect reaction rate: increasing temperature, increasing concentration, reducing particle size, and using a catalyst.
Effect of Concentration on Reaction Rate
A higher concentration means more reactant molecules in a given volume, which results in more collisions and provides more chances for a reaction to occur.
Effect of Temperature on Reaction Rate
A higher temperature results in more kinetic energy, which results in a higher chance of collisions breaking bonds and forming new ones.
Effect of Reduced Particle Size on Reaction Rate
Reduced particle size results in more surface area, which provides more area where collisions could occur.
Effect of a Catalyst on Reaction Rate
A catalyst is a substance that increases reaction rates by providing a lower energy path for the reaction without being used up during the reaction.
Activation Energy
The minimum energy requirement for colliding particles to react.
Activated Complex
A transition state that exists for a short period in a chemical reaction as the bonds in the reactants are breaking and the bonds in the products are forming.
Reaction Intermediate
A product of one step in a multistep reaction and a reactant in a following step.
Enzymes
Proteins or other molecules that fold into specific shapes to form indentations called active sites.
Reversible Reactions
Product molecules, under ordinary conditions, can react to form the original reactant molecules.
Chemical Equilibrium
A state of balance in which the rates of the forward and reverse reactions are equal.
Equilibrium Position
Marked by the relative concentrations of the reactants and products at equilibrium.
Le Châtelier’s Principle
States that if a chemical system at equilibrium experiences a disturbance, it changes in a way that counteracts the change as it returns to equilibrium.
There are three main factors that affect equilibrium:
- Concentration: If the concentration increases, the reaction shifts to the right, while if the concentration decreases, the reaction shifts to the left.
- Temperature: If the reaction is exothermic, raising the temperature causes the reaction to shift to the left, and decreasing the temperature causes it to shift to the right, while if the reaction is endothermic, the opposite occurs.
- Pressure: If the pressure increases, the reaction shifts to the right, while if it decreases, it shifts to the left.
Free Energy
The overall energy change for a process.
Thermodynamically Favorable Process
A process that produces free energy.
Thermodynamically Unfavorable Process
It is not spontaneous and does not produce free energy.
Entropy
S, is a measure of the disorder of a system—the opposite of the order of the system.
The four factors that affect entropy are:
- Changing State: Since entropy is the measure of the disorder of a system, states of matter that allow for particles to move freely have a higher entropy than states of matter that do not, so gases > liquid > solids.
- Dividing into Parts: Entropy increases when a substance is divided into parts, so a reaction that has more moles in the products has a higher entropy than a reaction that has more moles in the reactants.
- Increasing the Number of Molecules: When the total number of product molecules is higher than the total number of reactant molecules, entropy increases.
- Increasing the Temperature: Entropy increases when temperature increases due to particles moving faster.