kinetics Flashcards
For the given generic reaction:
A + 2B → C
define the rate of consumption of A.
The rate of consumption of A is the disappearance of [A] per unit of time, for example:
rate consumption A = - delta [A]/ delta t
Note the negative sign so that in the end the rate will give a positive value.
For the given generic reaction:
A + 2B → C
define the rate of consumption of B.
The rate of consumption of B is the disappearance of [B] per unit of time, for example:
rate consumption B = - delta [B]/ delta t
Note the negative sign so that in the end the rate will give a positive value.
For the given generic reaction:
A + 2B → C
define the rate of production of C.
The rate of production of C is the production of [C] per unit of time, for example:
rate production C = delta [C]/ delta t
Note that NO negative sign is needed because delta [C] is a positive value.
For the given generic reaction:
A + 2B → C
define the rate of reaction in terms of the reactants and products.
The rate of reaction is the disappearance of [A] or {B] and production of [C] (adjusted to coefficient of 1) per unit of time, for example:
rate reaction = - delta [A]/ delta t = -½ delta [B]/delta t = delta [C]/t
Note that the rate of consumption of B is multiplied by ½ by so to compensate for the coefficient of 2 in the balanced reaction equation.
Define rate law.
The rate law is an equation (specific to a particular reaction) which shows the relationship of reaction rate and concentration of the reactants. This relationship can only determined experimentally.
Define reaction order.
Reaction order describes how changing the concentration of a reactant affects the rate.
For example, for the reaction: A + 2B → C, it is possible that doubling [A} causes the rate to double, but doubling [B] has no effect. We would say the reaction is first order w/r/t A and zero order w/r/t B and the rate law would be:
rate = k [A]1[B]0
Explain the initial rate method.
The intial rate method is an approach to determine the rate law (i.e. the order of the reaction w/r/t each of the reactants).
It involves keeping the setting the concentration of all the reactants and then measuring the initial rate (i.e. the rate before significant amount of any reactant disappears). The concentration of each reactant is then changed seperately (while keeping the concentrations of the other reactants constant) and the initial rate determined. This approach shows how changing the concentration of each reactant affects the rate and therefore the rate law can be determined.
For the given reaction A →B, the units for the rate constant (k) were found to be s-1. What is the order w/r/t A?
The reaction is first order w/r/t A. Consider the first order rate law: rate = k [A]. If we solve for k with units, we get
k = rate / [A] = Ms-1 / M = s-1
Compare and contrast rate law and integrated rate law.
Let us say we have two reactants A and B and the reaction is first order w/r/t to A and second order w/r/t B.
We can write the following rate law: rate = k [A][B]2. This shows the relationship of rate to concentration.
If we keep [B] constant, the disappearance of [A] would be described by the first order integrated rate law:
ln[A] = -kt + ln[A]0
This shows the relationship of concentration to time.
If we keep [A] constant, the disappearance of [B] would be described by the second order integrated rate law:
1/[B] = kt + 1[B]0
Which again shows the relationship of concentration to time.
For the reaction A→B, assume the rate is first order w/r/t A. Describe several approaches to determine the rate constant (k) at a particular temperature.
Depending on what data is available, the rate constant (k) can be determined from:
- The rate law ⇒ k = rate / [A]
- The integrated rate law and a plot of ln [A] vs t. The slope of this plot = -k
- The half-life ⇒ k = ln 2/ t½
Explain collision theory in relation to rates of reaction.
Collision theory posits that a chemical reaction requires molecules to collide with the correct orientation and sufficient energy. Sufficient energy means that the colliding molecules have a kinetic energy equal or greater than the activation energy for the reaction.
Define activation energy.
Activation energy is the minimum of energy required by a molecule for a successful collision and a reaction to occur.
Explain the effect of temperature on the rate the of reaction.
As temperature increases the rate of reaction increases.
As the temperature increases, molecules move more quickly and therefore will collide more frequently. But most importantly, as the temperature increases, more molecules will have sufficient energy to react when they collide.
Explain the effect of a catalyst on the rate of reaction.
A catalyst provides an alternative pathway with a lower activation energy. Therefore, although molecules do not have a different kinetic energy (if the temperature is constant), more molecules will have sufficient energy to react.
What important parameter can be determined from the slope of a plot of ln k vs 1/T?
The slope of a plot of ln k vs 1/T is equal to - Ea/R. Therefore this slope can be used to determine activation energy. Ea = - slope x R.
