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CHEM 471 Midterm
| Question | Answer |
|---|---|
| What is a complex reaction? | The transformation of reactants to products occurs in more than one elementary step, the reaction is defined as a complex reaction |
| If the reaction mechanism only consists of a single step, this reaction is known as | a simple reaction |
| The condition for the existence of a pre-equilibrium | The rate of decay of the intermediate into the reactants is much faster than the rate at which it forms the products |
| If the rate of I going back to the reactants is much faster than the rate of formation of I into P then | a pre equilibrium exists between I and A,B |
| Unimolecular reactions are those | that have the form A->P and follow the first order kinetics (R=k[A]) |
| Unimolecular reactions are the | overall reactions |
| Two limiting conditions of the Lindemamm-Hinshelwood mechanism | 1) the deactivation rate of A* is much greater than the consumption rate of A* into P 2) the deactivation rate of A* is much lower than the consumption rate of A* into P |
| Apparent rate constant of Lindemann mechanism | (kuni). |
| The apparent rate constant of Lindemann mechanism depends on | [A] or the pressure of A if it is a gas phase reaction. |
| At low pressures Kuni showed | a linear relationship with pressure: Kuni=k1[A] |
| At high pressures Kuni | is independent of pressure, Kuni=(k1k2)//k-1 |
| A plot of 1/Kuni vs 1/[A] gives | a line |
| Catalyst | reduce Ea, not consumed, speeds up reaction rate. A substance that can change the rate of reaction but undergoes no net chemical change itself |
| How can you increase reaction rate | Increase the reactant concentration, raise the rate constant by increasing temperature, raise the rate constant by decreasing activation energy |
| A catalyst increases the reaction rate by | providing an alternative mechanism that avoids the slow, rate determined step of the uncatalyzed reaction to lower Ea |
| Enzyme | Binds to substrate. Its a catalyst existing in the human body. Homogenous biological catalysts |
| Enzyme has special proteins | was a size between 10nm and 100nm |
| Enzyme is | extremely specific to the reaction it catalyzes |
| The reactants in an enzyme are usually called | the substrate |
| Enzyme DOESN'T | act as a reactant |
| At a constant temperature for a given initial concentration of substrate [S]0 | the initial rate of product formation is proportional to the total concentration of enzyme [E]0 |
| At a constant temperature for a given [E]0 at low [S] | the rate of product formation is proportional to S |
| At a constant temperature for a given [E]0 at high [S] | the rate of product formation becomes independent of [S] reaching a maximum value, Rmax |
| The Michaelis constant is | the dissociation constant for ES. (Km) it measures how well E binds to S. In concentration units |
| Non-radiative decay processes | the excess energy of the excited state is transferred into the vibration, rotation, and translation of the surrounding |
| non radiative decay processes examples | vibrational relaxation, internal conversion, intersystem crossing, dissociation |
| Radiative processes | The excess energy is discarded as a photon, and the molecule passes into a lower energy state. |
| Radiative processes examples | Fluorescence and phosphorescence |
| Photochemical reaction | A chemical reaction that is initiated by the absorption of a photon by at least one component of a reaction mixture |
| Primary photochemical processes | When the products are formed directly from the excited states of the reactant molecule |
| Secondary photochemical processes | Where the products are formed from the intermediates originated from the excited states of a reactant |
| Quantum yield | To characterize the efficiency of a photochemical process |
| Quantum yield is defined as | the number of reactant molecules consumed in photochemical processes per photon absorbed |
| Primary quantum yield (quantum yield of primary photochemical processes) | It is the number of events divided by number of photons absorbed. OR the rate of primary process divided by rate of photon absorption=R/Iabs |
| Light intensity (Iabs) | is the absorbed energy per unit time per unit volume of reactant |
| The first law of photochemistry (The Grotthus-Draper Law) | Light must be absorbed by a chemical substance in order for a photochemical reaction to take place |
| The second law of photochemistry (The Stark-Einstein law)-photo equivalence law | In a primary process, for each photon of light absorbed by a chemical system, only one molecule can be activated |
| The sum of primary quantum yields for all photophysical and photochemcial processes should be | 1 |
| Overall quantum yield | MAY EXCEED 1. The number of reactants that react/number of photons absorbed |
| The reaction rate of photochemical reactions depends on | the light intensity |
| Diffusion | a material transport by molecular or atomic motion. It is a process of spatial drift of molecules or atoms due to their kinetic motion |
| The driving force of diffusion is | a spatial gradient in material concentration |
| Diffusion Flux | a quantity of matter transferred through a given area in a given time interval. The flux always acts in opposition to the gradient |
| Definition of the Diffusion coefficient D | experimental observations show that the flux is linearly proportional to the spatial gradient in the transport property |
| Fick's First Law of Diffusion | The flux of matter diffusing in the x axis of a container is proportional to the concentration gradient |
| Definition of the flux plane | the plane located at x=0 |
| Diffusion is | always in one plane |
| Fick's second law of diffusion shows | that the time evolution of the number density is proportional to the second derivative (or curvature) of the spatial distribution of the number density |
| The Stokes-Einstein equation can be applied to | both neutral molecules and charged ions |
| The condition for applying Stokes-Einstein equation to diffusion in solution is | that the radius of the solute particles is significantly greater than the radius of the solvent particles |
| Collision theory is only applicable to | the reactions between simple molecules in the gas phase |
| Effective collisions | the collisions that can lead to the formation of products |
| Effective collisions require | a minimum kinetic energy (activation energy) for the collisions to be effective. It needs to be oriented properly with each other. |
| The steric requirement | the disagreement between the experimental and theoretical values of the pre-exponential can be accomodated using a steric factor |
| Eyring equation assumption 1 | The activated complex and the reactants are in equilibrium |
| Eyring equation assumption 2 | The transition state breaks up to form products through one of its vibrations and displacement along this vibration mode leads to products |
| Activated Complex theory is also called | Absolute rate theory and transition rate theory |
| The steric factor in the collision theory is the result of | an increase in order, and consequently a decrease in entropy, when the activated complex is formed |
| The saddle point | This point represents the highest potential energy with respect to the reactants and products. This point also represents the lowest potential energy regarding the configuration of the activated complex |
| Spectroscopy is the study of | the interaction between EM radiation and matter |
| Spectroscopy deals with | the absorption, emission or scattering of electromagnetic radiation by atoms or molecules |
| Atomic spectra | electronic transitions sharp because atoms don't have freedom of rotation |
| In molecular spectroscopy | asides from electronic transitions, rotational and vibrational transitions can also be observed. |
| A spectrum is | a plot of the change in light intensity when passing through the molecules caused by absorption, emission, or scattering vs the wavelength or frequency |
| An electromagnetic traveling wave may be characterized by | its wavelength or frequency or wavenumber |
| The intensities of spectra lines are associated with | a transition between different energy levels |
| Transition probabilities and selection rules | Not all the transitions between different energy levels in a molecule are allowed |
| Under normal condition, which energy states are more populated | Lower energy states |
| Transition probability is determined by | the square of the transition dipole moment |
| If the transition dipole moment vanishes | the transition probability is 0 |
| Selection rules | The rules that govern the non-vanishing of the transition dipole moment. Are statements about whether a transition is allowed or forbidden |
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