CHEM 121 Module 6

CHEM 121 Foundations of General Chemistry

rate

a measure of how a property varies with time

speed

a familiar rate that expresses the distance traveled by an object in a given amount of time

chemical reaction

a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time

rate of reaction

the change in the amount of a reactant or product per unit of time.

determined by measuring the time dependence of a property that can be related to reactant or product amounts.

rate expression

the mathematical representation of the change in species concentration over time during a chemical reaction

brackets

indicate molar concentrations

symbol delta (Δ)

indicates “change in”

average rate

describes the changes in concentration of the species in the reaction over a specific time interval using the concentrations at the beginning and end of the time period

instantaneous rate (t_n)

the rate at which a reaction is proceeding at a specific time

initial rate (t₀)

the instantaneous rate of a reaction at “time zero,” when the reaction begins

instantaneous rate of a reaction may be determined

experimental conditions permit the measurement of concentration changes over very short time intervals.

then average rates computed as described earlier provide reasonably good approximations of instantaneous rates.

five factors typically affecting the rates of chemical reactions

the chemical nature of the reacting substances.

the state of subdivision of the reactants.

the temperature of the reactants.

the concentration of the reactants.

the presence of a catalyst.

chemical nature of the reacting substances

the rate of a reaction depends on the nature of the participating substances

physical states of reactants

a chemical reaction between two or more substances requires close contact between the reactants

heterogeneous reaction

between a solid phase and either a liquid or gaseous phase

temperature of reactants

chemical reactions typically occur faster at higher temperatures

concentration of reactants

the rates of many reactions depend on the concentrations of the reactants

rates usually increase

when the concentration of one or more of the reactants increases

catalyst

a substance that speeds up a chemical reaction by lowering the activation energy (E_a) required to complete the reaction

rate laws, or rate equations

mathematical expressions that describe the relationship between the rate of a chemical reaction and the concentration of its reactants

rate constant

k.

specific for a particular reaction at a particular temperature.

reaction orders

exponents m and n.

are typically integers.

overall reaction order

the sum of orders for each reactant

method of initial rates

a common experimental approach to the determination of rate laws

 integrated rate laws

determine a second form of each rate law that relates the concentrations of reactants and time

half-life of a reaction (t_1/2)

the time required for one-half of a given amount of reactant to be consumed

collision theory

The rate of a reaction is proportional to the rate of reactant collisions.

The proportional symbol (∝) is used to indicate that there is a direct relationship between reaction rate and the number of collisions over time, where both are increasing or decreasing by the same factor. 

The reacting species must collide in an orientation that allows contact between the atoms that will become bonded together in the product.

The collision must occur with adequate energy to permit mutual penetration of the reacting species’ valence shells so that the electrons can rearrange and form new bonds (and new chemical species).

activated complex or a transition state

reactant species collide with both proper orientation and adequate energy, they combine to form an unstable species

activation energy (E_a)

the minimum energy necessary to form a product during a collision between reactants

reaction diagrams

widely used in chemical kinetics to illustrate various properties of the reaction of interest

enthalpy

amount of heat energy in a thermodynamic system

equilibrium

a dynamic phenomenon in which opposing forces are balanced

chemical equilibrium

occurs when the reaction converting reactants to products is happening at the same rate as the reaction converting products to reactants.

when the concentration of reactants and products in a reversible reaction remain constant over time and the rates of the forward reaction and reverse reaction are equal to one another.

reaction at equilibrium

reactants converting to products and products to reactants at the same rate

reaction quotient (Q)

measures the relative amounts of products and reactants present during a reaction at a particular point in time

equilibrium constant, K

the constant value of Q exhibited by a system at equilibrium

law of mass action

at a given temperature, the reaction quotient for a system at equilibrium is constant

magnitude of an equilibrium constant

explicitly reflects the composition of a reaction mixture at equilibrium, and it may be interpreted with regard to the extent of the forward reaction

 heterogeneous equilibrium

involves reactants and products in two or more different phases

coupled equilibrium

reactions, those which have one or more reactant or product species in common

basic manipulations

changing the direction of a chemical equation essentially swaps the identities of “reactants” and “products”, and so the equilibrium constant for the reversed equation is simply the reciprocal of that for the forward equation.

changing the stoichiometric coefficients in an equation by some factor x results in an exponential change in the equilibrium constant by that same factor.

adding two or more equilibrium equations together yields an overall equation whose equilibrium constant is the mathematical product of the individual reaction’s K values.

 Le Châtelier’s principle

if an equilibrium system is stressed, the system will experience a shift in response to the stress that reestablishes equilibrium

reactant is added

(increasing the denominator of the reaction quotient) or product is removed (decreasing the numerator), then Q_c < K_c and the equilibrium will shift right

if Q<K

the reaction will proceed in the forward direction to achieve equilibrium

if Q=K

the reaction is at equilibrium

if Q>K

the reaction will proceed in the reverse direction to achieve equilibrium

temperature increase endothermic reactions  (ΔH>0)

forward shift

K increases

temperature increase exothermic reactions  (ΔH<0) 

reverse shift

K decreases

temperature decrease endothermic reactions  (ΔH>0)

reverse shift

K decreases

temperature decrease exothermic reactions  (ΔH<0) 

forward shift

K increases