ch14

14.1 Factors That Affect Reaction Rates

• 1. Physical state of the reactants

• 2. Reactant concentrations

• 3. Reaction temperature

• 4. The presence of a catalyst

14.2 Reaction Rates

• M/s is units for reaction rate. change in concentration measured in molarity divided by a time interval measured in seconds.

  • Rates are always expressed positively

• Instantaneous rate of reaction is greater when slope is greater. To find instantaneous line draw a tangent line and take its slope (on a molarity versus time graph)

• Rate law:

• For a reaction in which the overall reaction order is 1, k has units of s^-1. For which the overall reaction order is 2, k has units of M^-1s^-1

14.3

• 0th order is when the reaction proceeds at a constant rate regardless of how much reactant is present. Common types of these reactions are when a gas undergoes decomposition on the surface of a solid. 1st order of rate law means that when reactant doubles, rate doubles. 2nd order means that when reactant doubles, rate quadruples. Keep in mind that if chemicals have diff orders in same chem equation, multiply their orders together

• In rate laws, the rate of a reaction depends on concentration but the rate constant does not. the rate constants (and hence the reaction rate) are affected by temperature and by the presence of a catalyst

14.4 The change of concentration with time

• Differential rate law: expresses rate and depends on concentration. This can be transformed into an equation known as the integrated rate law.

• 2nd order 

• Zero order

• When dealing with gases, pressure can be used as concentration 

• A fast reaction has a short half life(time required for concentration of a reactant to reach half its initial value)

• The half-life of a second-order reaction depends on both the rate constant and the initial concentration of A:

• 14.7 sub in ½ for first order. Same for other orders

14.5 Temp and Rate

• The collision model: based on kinetic molecular theory and accounts for effects by both reactant concentrations and temperature at the molecular level

• A molecular collision will occur when: correct molecular orientation, has a certain minimum amount of energy to react (activation energy)

• Top of barrier when considering activation energy is  called the activated complex or transition state.

  • The lower the value of activation energy, the larger the rate constant and the faster the reaction .

  • Reverse reaction is endothermic. Larger barrier to overcome going from right to left than left to right

• R is gas constant (8.314J/mol-K)

• Arrhenius equation. He noticed that most reaction rate data obeyed this equation based on 1. Fraction of molecules possessing activation energy or greater 2. Number of collisions per second 3. The fraction of collisions that have the appropriate orientation. 

• term A is called the frequency factor; it relates to the number of collisions that are favorably oriented for reaction

• Can calculate activation energy be rearranging the formula

14.6 Reaction Mechanisms

• The steps by which a reaction occurs is called the reaction mechanism.

• Both reactions occur in a single event or step and are called elementary reactions.

• The number of molecules that participate as reactants in an elementary reaction defines the molecularity of the reaction.

  • If a single molecule is involved it is unimolecular. Rate law is 1

  • Collision of 2 reactant molecules- bimolecular. Rate law is 2

  • simultaneous collision of three molecules are termolecular. (not likely)

• The net change represented by a balanced chemical equation often occurs by a multistep mechanism consisting of a sequence of elementary reactions. Therefore, The chemical equations for the elementary reactions in a multistep mechanism must always add to give the chemical equation of the overall process.

• When something is neither a reactant nor a product of the reaction—it is formed in one elementary reaction and consumed in the next—it is called an intermediate.

• Because the slow step limits the overall reaction rate, it is called the rate-determining step (or rate-limiting step).

• In general, whenever a fast step precedes a slow one, we can solve for the concentration of an intermediate by assuming that an equilibrium is established in the fast step.

14.7 Catalysis

• A catalyst that is present in the same phase as the reactants in a reaction mixture is called a homogeneous catalyst.

• A heterogeneous catalyst is one that exists in a phase different from the phase of the reactant molecules, usually as a solid in contact with either gaseous reactants or reactants in a liquid solution.

• The initial step in heterogeneous catalysis is usually adsorption of reactants.  Adsorption refers to the binding of molecules to a surface, whereas  absorption refers to the uptake of molecules into the interior of a substance

• A large number of marvelously efficient biological catalysts known as enzymes are necessary for many of these human body reactions to occur at suitable rates

• The reaction any given enzyme catalyzes takes place at a specific location in the enzyme called the active site. The substances that react at this site are called substrates. The lock-and-key model provides a simple explanation for the specificity of an enzyme