Chapter 6 - Rates of Chemical Reactions

Reaction rate: change in the number of reactants or products over time * Rate of reaction = ∆Quantity / ∆t * Moles per second * In a gaseous or solution units are in concentration A (moles/Liters or mol/L) over seconds * Rate of reaction = ∆[A] (in mol/L) / ∆t (in seconds) * Reaction rates are always positive
Average rate: average change in the concentration of a reactant or product per unit time over a given time interval * Only give an idea of how the reaction is progressing * Finding slope by secant line of the graph
Instantaneous rate: rate of the reaction at a particular time at a tangent line
You can measure reaction rates through: * Mass, pH, and Conductivity * Pressure * Color: through a spectrophotometer * Volume

Rate law equation: Rate ∝ k[A]m[B]n * Shows relationship between the concentration of the reactants and the rate of their reaction * **Rate constant: the letter k (represents a proportionality constant) Depends on temperature and is constantSpeed of reaction In s−1 * [A] and [B] are rates of reactants * Rate law exponents m and n do not change with temperature and have to be found experimentally * Exponents of 1: first order * Exponents of 2: second-order
Overall reaction order: sum of exponents (m and n)
First-order reaction: overall reaction order is equal to 1
Second-order reaction: overall reaction order is equal to 2
Other times reaction rates can lead it to equal to k[A]0, so the rate of reaction is constant
Initial rates method: comparing the initial rate of each reaction
Half-life, t1/2: reaction is the time that is needed for the reactant mass or concentration to decrease by one half of its initial value * In seconds * t1/2 = 0.693/k

Collision theory: for a reaction to occur, reacting particles (atoms, molecules, or ions) must collide with one another.Increasing surface area more collisions occurs Not all collisions occur in reactions, for a collision to be effective:Must be correct orientation and sufficient collision energy
Activation energy, Ea: minimum collision energy that is required for a successful reaction
Maxwell-Boltzmann distribution: fraction of collisions vs energy graph at a constant temperature * The area under the graph represents the distribution of kinetic collision
Transition state theory: explain what happens when molecules collide in a reaction. It examines the transition, or change, from reactants to products * The kinetic energy of the reactants is transferred to potential energy as the reactants collide, due to the law of conservation of energy
Potential energy diagram: a diagram that charts the potential energy of a reaction against the progress of the reaction * axis represents potential energy * *x-*axis, labeled “Reaction progress”, represent the progression of reaction through time
There is no way to predict the activation energy of a reaction from its enthalpy change
Transition state: top of the activation energy barrier
Activated complex: chemical species that exist at the transition state neither product nor reactant
Reactions rate increases at higher temperatures

Reaction mechanism: a series of steps that make up an overall reaction
Elementary reaction: involves a single molecular event, such as a simple collision between atoms, molecules, or ions
Reaction intermediates: molecules/atoms/ions that are formed in an elementary reaction and consumed in a subsequent elementary reaction
Molecularity: refers to the number of reactant particles (molecules, atoms, or ions) that are involved in an elementary reaction
Bimolecular: when two particles collide and react
Unimolecular: when one molecule or ion reacts
For an elementary reaction, the exponents in the rate law equation are the same as the stoichiometric coefficients for each reactant in the chemical equation
Rate-determining step: slowest elementary reaction slower which becomes the overall rate-determining step
A catalyst is a substance that increases the rate of a chemical reaction without being consumed by the reaction * Lowers activation energy so reactants have sufficient energy to react * Homogeneous catalyst: exists in the same phase as the reactants * Heterogeneous catalyst: exists in a phase that is different from the phase of the reaction it catalyzes
Enzymes: biological catalysts enormous protein active site: a small part of enzyme part of the catalyst reaction Substrate: reactant molecule which binds to the active site to models:Lock and key model: the enzyme is like a lock Induced fit model: changes shape to fit the substrate