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MCAT Chem
Ch. 5 Kinetics and equalibrium
| Question | Answer |
|---|---|
| when can you use the stoichiometric coefficients as the orders of reaction | 1. when the rxn mech. is a single step and the balanced "overall" reaction is reflective of the entire chemical process. 2.when the complete rxn mech. is given and rate determining step is indicated |
| zero-order reaction | formation of product C is independent of concentrations of any reactants A and B. rate=k[A]^0[B]^0=k, rate constant itself is dependent on temp. |
| first order reaction | rate that is directly proportional to only one reactant. rate=k[A]^0 or k[B]^0 |
| second order reaction | has a rate that is proportional either to the product of the concentrations of two reactants or to the square of the concentrations of a single reactant. rate=k[A]^1[B]^1 or k[A]^0[B]^2=k[B]^2 = k[A]^2[B]^0 = k[A]^2 |
| collision theory of chemical kinetics | the rate of a reaction is proportional to the number of collisions per second between the reacting molecules. An effective collision occurs only if the molecules collide with each other in correct orientation and sufficient energy |
| activation energy | the minimum energy of collision necessary of a reaction to take place |
| transition state | has greater energy than either the reactants or the products. Once an activation complex is formed, it can either dissociate into products or revert to reactants w/o any additional energy |
| enthalpy change of the reaction (DH) | the difference between the potential energy of the products and the potential energy of the reactants. -enthalpy=exothermic, +enthalpy=endothermic |
| factors affecting reaction rate | reactant concentrations, temperature, medium, catalysts |
| temperature | the reaction rate will increase as the temperature increases. As temp increases so does the average kinetic energy of the molecules |
| medium | polar solvents preferred because their molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, which permits reaction to occur faster |
| catalysts | substances that increase rxn rate without being consumed. homogeneous-same phase as reactants. heterogeneous-distinct phase. change only rate, will not affect spontaneity |
| Qc<Keq | then the reaction has not reached equilibrium |
| Qc>Keq | then the reaction has exceeded equilibrium |
| Qc=Keq | then the reaction is in dynamic equilibrium |
| properties of the law of mass action 1 | the concentrations of pure solids and pure liquids do not appear in the equilibrium constant. do not change course of reaction. |
| properties of the law of mass action 2 | Keq is characteristic of a particular reaction at a given temperature: the equilibrium constant is temperature dependent |
| properties of the law of mass action 3 | generally, the larger the value of Keq, the farther to the right we'll find the equilibrium and the more complete the reaction |
| properties of the law of mass action 4 | if the equilibrium constant for a reaction written in one direction is keq, the equilibrium constant for the reaction written in reverse is 1/Keq |
| Le Chatelier's Principle | a system to which a "stress" is applied tends to shift so as to relieve the applied stress. No matter what the particular form the stress takes(change in concentration, change in pressure, change in temp) |
| how will a system always react with Le Chatelier's Principle | the system will always react in the direction away from the added species or toward the removed species |
| Changes in pressure (by changing volume) | the system will move toward whichever side has the lower number of moles of gas. |
| A(aq)+2B(g) <-> C(g) + heat will shift to the right if | A or B added, C removed, pressure increased or volume reduced, temperature reduced. |
| A(aq)+2B(g) <-> C(g) + heat will shift to the left if | C added, A or B removed, volume increased or pressure reduced, temperature increased. |
| when reaction occurs by more than one step, the equilibrium constant for the overall reaction is found by.. | multiplying together the equilibrium constants for each step of the reacion |
| if the ration of Q/Keq is less than 1 (Q<Keq) then | the natural log will be negative and the free energy change will be negative, so the reaction will proceed forward until equilibrium is reached |
| if the ration of Q/Keq is greater than 1 (Q>Keq) then | the natural log will be positive, the the free energy change will be positive. The reaction will spontaneously move in the reverse direction until equilibrium is reached. |
| If the ration of Q/Keq is 1 then | the reaction quotient is equal to the equilibrium constant; the reaction is at equilibrium, and by definition, the free energy change is zero |
| A compound with a Ka greater than 10^-7 contains? | more H^+ cations than HA^- anions, which makes it a weak acid. |
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