3031 Exam 2

• collision theory: molecules are colliding all the time and some fraction‐ but not all‐ of those collisions will lead to transformation of the reactants to the products
• collision theory:

  1. The molecules must come into contact. This is a collision.
  2. They must collide with enough energy to overcome an energy barrier to reaction called the activation energy.
  3. They must collide in an orientation that allows the necessary bond‐breaking and forming needed to transform the reactants to the products.
• Arrhenius equation: Temperature dependence of the Rate Constant, Most reactions speed up as temperature increases
• The magnitude of a first order rate constant is seen to increase exponentially with an increase in temperature.
• The Collision Model: In order for molecules to react they must collide

  • the more molecules present, the greater the probability of collision and the faster the rate
  • reaction rate should increase with an increase in the concentration of reactant molecules.
• As temperature increases, the molecules move faster and the collision frequency increases
• Arrhenius: Molecules must posses a minimum amount of energy to react
• The Activation energy, Ea, is the minimum energy required to initiate a chemical reaction.
• 1. In order to form products, bonds must be broken in the reactants.
  2. Bond breakage requires energy.
  3. Molecules moving too slowly, with too little kinetic energy, don’t react when they collide
• The orientation of a molecule during collision can have a profound effect on whether or not a reaction occurs
• Some collisions do not lead to reaction even if there is sufficient energy
• reaction-rate data obeyed an equation based on three factors:

  1. The number of collisions per unit time.
  2. The fraction of collisions that occur with the correct orientation.
  3. The fraction of the colliding molecules that have an energy greater than or equal to Ea.
• “A” relates to the frequency of collisions and the orientation of a favorable collision probability
• One can determine the activation energy of a reaction by measuring the rate constant at two temperatures
• Thermodynamics: Describes the changes in the form of energy when a reaction occurs, for example, converting chemical energy to heat; feasibility
• Kinetics: Describes how quickly or slowly a reaction occurs.
• reaction rate law expression relates the rate of a reaction to the concentrations of the reactants.
• x and y from rate equation are the reactant orders determined from experiment
• molecularity: the number of ions of atoms or molecules involved in the rate determining or limiting step; whole number only, no zero or fraction
• Reaction Molecularity: Refers to the concentration dependence of a reaction mechanism
• three kinds of chemical events: bond breaking, bond forming, concerted bond breaking and forming
• order of reaction: the sum of the powers of concentration in rate equation
• second order reaction: A plot of 1/[A] vs. time with produce a straight line with the slope equal to k
• For a first-order reaction, the half-life does not depend on concentration
• catalyst: a substance that increases rate of reaction without being consumed by it
• heterogenous catalyst: in phaase different from that of reaction mixture
• homogenous catalyst: present in same phase as reactants
• drug stability: capacity of a drug substance or product to remain within established specification of identity, strength, quality, and purity in a specified period of time
• stability: time lapse during which drug product retains same properties and characteristics that it possessed at time of manufacture
• expiry date: Drug can not be used after this date because the concentration of drug is decreased and become lower than therapeutic concentration. In addition, some products of drug
• After the opening of the drug container, the expiry date will be shorter as a result of the decreased concentration of drug during usage and the effects of external factors
• Eye drops: can be used for one month after opening the droppers
• Syrups and suspension of antibiotics: can be used for one week by storage in room temperature and for two weeks by storage in 4C
• Tablets and capsules remain stable in the package but after removal the expiry date will change
• Ampoules: must be used immediately but the vials (multidose) are stable for 24 h for the presence of preservatives.
• shelf life of a drug: Time taken for a concentration of the drug to be reduced 90% of its value when originally prepared
• chemical decomposition of drugs: Hydrolysis, Oxidation, Isomerization, Photochemical decomposition, Polymerization
• drugs susceptble to hydrolysis: Drugs derivative of carboxylic acid or contain functional group based on this moiety
• prevent hydrolysis: production of insoluble form of drug, modification of chemical structure, presence of complexing agent
• free radical: molecule or molecular fragment that contains one or more unpaired electrons in outermost orbital