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Some rate laws depend on the concentration of more than one species. For instance, oxidation of iodide ions by persulfate ions has the rate law: Rate of consumption of I- = k[S2O82-][I-]. In such cases explain, in detail, how we can simplify analysis.
The idea is to ensure that one of the species remains effectively at the same concentration throughout the reaction, which allows us to treat it as a constant. We can achieve this by starting with such a high concentration of persulfate ions that their concentration barely changes in the course of the reaction.
If we started with an extremely high concentration persulfate in a reaction involving the oxidation of iodide ions by persulfate, what would be the role of persulfate? What would be the new rate law? What is K'? Finally, if the reaction is no longer a second order reaction, what is it and why?
- persulfate becomes a constant
- Rate of consumption of I- = k'[I-]
- k' = k[S2O82-] (another constant)
- We now have a pseudo-first order reaction, because our rate law depends on the concentration of only one substance [I-]
How would you deduce the overall rate constant in the reaction described in the previous card?
knowing the concentration of persulfate ions, we can use the expression: k = k'/[S2O82-]
The half life (t1/2), of a substance is the time needed for its concentration to fall to ____-____ of its _____ value
The _______ the value of k, the more rapid the consumption of the reactant. It would only follow that we should be able to write an equation relating the _____ rate constants to ______ half-lives. What is that equation? What condition must be present for this to be used?
- t1/2 = ln2/k
- it must be a first order reaction
The half-life of a reactant that decays in a first order reaction is _______ ______ to the rate constant of the reaction.
State two possible equations for the integrated rate law for second order reactions with the rate law: Rate of consumption of A = k[A]2
- 1/[A]t - 1/[A]0 = kt
- [A]t = [A]0/1+kt[A]0
Going from zero to second order, state the rate law, integrated rate law, slope of the line plotted, and the half life.
The half-life of a second order reaction is inversely proportion to the _______ of the ______
An elementary reaction describes a distinct event, often a _______ of particles. To describe how reactions take place, chemists propose a reaction mechanism, which is a ______ of elementary reactions or steps that they believe take place as ______ are transformed into ______.
- sequence or series
Elementary reactions are classified by their_______, which is the number of _______ molecules, atoms, or ions taking part in a specified elementary reaction.
Kinetic information can only ______ a proposed mechanism; it can't prove that a mechanism is ______.
To construct an overall rate law from a mechanism, we write the rate law for each of the ______ _______ that have been proposed, then combine them into an overall rate law. State a procedure that helps with this.
- elementary mechanisms
- Steady-state approximation, which assumes that any intermediate formed remains at a constant, low concentration
When constructing an overall rate law, we sum all the rate laws for the elementary reactions. First, we identify any elementary reaction that results in the _______ or __________ of the overall product and write the equation for its net rate of formation. Define net rate of formation.
- the sum of the rates of all elementary reactions resulting in formation minus the rates of any elementary reactions that lead to its consumption
The equilibrium constant for an elementary reaction is equal to the ratio of the _______ and _______ rate constants of the reaction or, for multi-step reactions, the ratio of the product of the ______ rate constants to the product of the ______ rate constants.
The rates of chemical reactions depends on ________. The qualitative observation is that most reactions _______ in speed as temperature is raised. An increase of 10°C from room temp typically ______ the rate of reaction of organic species in solution. This is why we cook foods, heating accelerates reactions that lead to the breakdown of _____ _____ and the decomposition of ______. We refrigerate foods to slow down the natural chemical reactions that lead to their ________.
- cell wall
Svante Arrhenius found that the plot of the logarithm of the rate constant (ln k) against the inverse of the absolute ______ (__) is a straight line. State what he established in equation form, then state the proper empirical Arrhenius equation.
- absolute temperature (1/T)
- ln k = intercept + slope *1/T
- ln k = lnA - (Ea/RT)
State an alternative of the Arrhenius equation using antilogarithms
k = Ae-Ea/RT
The two constants, A and Ea, are known as ______ ______ for the reaction and are found from experiment; A is the ___-______ factor, and Ea is the ________ energy. Both A and Ea are nearly independent of ________ but have values that depend on the reaction being studied.
- Arrhenius parameters
- pre-exponential factor
- activation energy
Activation energies for ORGANIC reactions are typically between ___ and _____ kJ * mol-1
The derivation of the Arrhenius equation that we use to predict thte rate constant at one temperature from its value at a different temperature is: ln(k2/k1) = Ea/R (1/T1 - 1/T2). When T2>T1, the right hand side is ______, so ln(k2/K1 is ______, which means that k2>K1. That is, the rate constant _______ with temperature. For fixed values of T1 and T2, ln(k2/k1) is large when Ea is ______; so the increase in rate constant is large when the activation energy is ______.