Book: UTC: 16 Flashcards
"Understand these concepts"
What variables does reaction rate depend on? How does it depend on these variables?
- Concentration, physical state, and temperature.
- Concentration: molecules must collide in order to react. Reaction rate is proportional to the number of collisions, which depends on the concentration of reactants. Physical state: molecules must mix to collide. The more finely divided a solid or liquid reactant, the greater its surface area, the more contact it makes with the other reactant, and the faster the reaction occurs. Temperature: molecules must collide with sufficient energy. At a higher temperature, reactant particles move faster and collisions occur more frequently; furthermore, more sufficiently energetic collisions occur. Both of these lead factors lead to more reactions occurring.
What is the meaning of reaction rate in terms of changing concentrations over time?
Reaction rate depends on the concentration of reactants. As the reaction progresses, concentration decreases. This means that the rate of reaction decreases throughout the course of the reaction.
How can the rate be expressed in terms of reactant or product concentrations?
For aA + bB -> cC + dD, where a, b, c, d are the coefficients of the balanced equation, we have r = -1/a ∆[A]/∆t = -1/b ∆[B]/∆t = 1/c ∆[C]/∆t = 1/d ∆[D]/∆t
Explain the distinction between average and instantaneous rate and why the instantaneous rate changes during the reaction.
Average rate computes the rate of reaction over a time interval. Instantaneous rate does the same but as the time interval shrinks to zero. Instantaneous rate changes during the reaction because the concentration of reactants changes.
Explain the interpretation of a reaction rate in terms of reactant and product concentrations.
Per r = -1/a ∆[A]/∆t = -1/b ∆[B]/∆t = …, products appear at the same rate at which reactants disappear.
Explain the experimental basis for the rate law and the information needed to determine it—initial rate data, reaction order, and rate constant.
The rate law has exponent values called reaction orders. These orders may be determined by experiment. If order can be determined, the instantaneous rate of reaction can be found at any given set of concentrations and rate data.
What is the importance of reaction order in determining the reaction rate?
It is a variable in the equation for the rate law which can be determined experimentally.
How is reaction order determined from initial rates at different concentrations?
If you have reactants A and B whose initial values are known along with initial rates, fix [B], and set up the equation r_1/r_2 = ([A]_1/[A]_2)^m, and solve for m.
How do integrated rate laws show the dependence of concentration on time?
Time is a variable in all of the equations.
What does reaction half-life mean and why is it constant for a first-order reaction?
It is the time it takes for half the concentration of reactants to react. When [A] and 0.5[A] are plugged into the integrated rate equation, a constant values pops up for half life.
Why are concentrations multiplied in the rate law?
Rate depends on all of them in the concentrations in a generic way.
Explain activation energy and the effect of temperature on the rate constant (Arrhenius equation).
In collision theory, particles must collide with sufficient energy for a reaction to occur. The rate constant must depend on temperature as the rate of reaction depends on temperature. As temperature affects how many particles have sufficient energy for an effective collision to occur, rate constant depends on temperature as shown: k = Ae^(-E_a/RT)
How does temperature affect reaction rate?
It speeds up reaction rate by increasing the frequency of collisions and the number of collisions with sufficient energy to exceed activation energy.
Why do molecular orientation and complexity influence the number of effective collisions and the rate?
The atoms that become bonded in the product must make contact, according to collision theory. The larger the molecules, the smaller chance there is that this will happen during a collision. Thus, more complex molecules and larger molecules generally lower reaction rates as less collisions are effective.
Explain the transition state.
An unstable species with partial bonds forms as two molecules approach each other. This is a transition state in which the species is not quite reactant nor is it quite product.
