rates Flashcards
What is rate?
how quickly the concentrations of reactants and products are changing.
How is rate mathematically defined?
the rate of change of [A] with respect to time.
How can you work out the rate of a reaction from a conc. against time graph?
rate of reaction at time, t, is the gradient (slope) of the tangent to the graph at time t.
What is the instantaneous rate?
rate at a specific moment in time
units of rate:
mol dm^-3 s^-1
If something is being used is the rate positive or negative?
If something is being USED its rate is NEGATIVE
If something is being formed is the rate positive or negative?
If something is being FORMED its rate is POSITIVE
Is the gradient on a concentration against time graph positive or negative and why?
Negative because the reactant is being used up.
What is an Elementary reaction?
a reaction in which the order of the reaction is governed by the stoichiometry of the reaction.
An elementary reaction is a single step process that involves one or two molecules.
What is the rate law equation?
rate = k[A]^a [B]^b
Factors affecting the rate of reaction:
the concentrations of the reactants;
the temperature of the reaction;
the intensity of radiation (if the reaction involves radiation such as light);
the particle size of a solid (related to the surface area of the solid particles);
the polarity of a solvent (for reactions in solution involving ions);
the presence of a catalyst
Two important features of elementary reactions:
The equations are actually what is going on at the molecular level
For an elementary reaction it is possible to use the chemical equation to form the rate equation - not the case for complex reactions
What does a stoichiometric (chemical) equation represent?
represents the overall reaction in terms of initial reactants and final products, but does not show how a reaction actually happens.
What is molecularity?
the molecularity of an elementary reaction is the number of molecules which come together to react
for a zero order reaction, what is the relationship between the concentration and rate?
zero order: change of concentration has no effect on rate.
for a first order reaction, what is the relationship between the concentration and rate?
First order: you double the concentration, it doubles the rate.
for a second order reaction, what is the relationship between the concentration and rate?
Second order: double the concentration, it quadruples the rate.
What is the Overall order?
the sum of the orders of reactants.
Relationship between rate and k:
The larger the value of k, the faster the reaction is.
What is a differential rate equation?
A rate equation where the rate of consumption of a reactant, or the rate of formation of a product, is written as a differential
What is the integrated rate equation for a elementary first order equation?
ln [A]t = ln [A]0 − kt
On a graph of ln[A] against time, what does the y-intercept and gradient tell you?
- y-intercept = ln[A]0
- gradient = -k
What is the integrated rate equation for a elementary second order equation?
1/[A]t = 1/[A]0 + kt
On a graph of 1/[A] against time, what does the y-intercept and gradient tell you?
- y-intercept = 1/[A]0
- gradient = k
Definition of half-life?
The time for the concentration of a species to fall to half its initial value
Relationship between half-life and first order reactions:
in a first order process as all the half-lives are the same.
Relationship between half-life and the initial concentration of a first order reaction:
The half life of a 1st order reaction is independent of the initial concentration of reactants. - would take the same time to go from 1 to ½ than it would to go from 20 to 10.
Half life equation:
half life = ln(2) / k
Relationship between half-life and the initial concentration of a second order reaction:
For a 2nd order process, our half-life does depend upon our initial concentration - so our half-life is inversely proportional to our initial concentration
What is the steady state approximation?
states that the rate of change of concentration of a reactive species, X, in a sequence of consecutive reactions is approximately zero. X gets used up as fast as it is formed, so its concentration stays constant—it reaches a steady state.
Can we apply the SSA is we have a fast step then a slow step?
no
Can we apply the SSA is we have a slow step then a fast step?
yes
When does a series of reactions reach a steady state?
when the rate of formation of an intermediate is the same as its rate of removal—but these two processes are not the reverse of one another and they don’t come to equilibrium.
What does the Arrhenius equation summarise?
The rate constant, k, changes with temperature and determines the way in which the rate of reaction varies with temperature.
What is “A” in the Arrhenius equation?
“A” is the constant of proportionality between the concentration of the reactants and the rate at which they collide.
Essentially, it is a frequency factor relating the number of collisions and reactivity.
In an Arrhenius plot/graph what is the gradient and y-intercept equivalent to?
gradient = -Ea/R y-intercept = lnA
What does the activation energy value tell us?
The value of the activation energy, Ea, determines how sensitive the reaction is to changes of temperature.
What does a high activation energy mean?
A high value of Ea corresponds to a reaction that is very sensitive to changes of temperature
What does a low activation energy mean?
a low value of Ea corresponds to a reaction that is less sensitive to changes of temperature
What is the highest point on an energy profile?
a transition state
What are the 2 main theories of reactions?
- collision theory
- transition state theory
What is the collision theory?
based on a simple model in which the reactant molecules behave like hard spheres. Collisions between the spheres may result in reaction. A calculated collision frequency is used to estimate the A factor in the Arrhenius equation for the reaction.
What is the transition state theory?
a more sophisticated and detailed theory that assumes the reactants form a transition state—a short-lived complex at the maximum on the energy profile. The A factor is then calculated, based on an assumed structure for the transition state.
In the collision theory, what must occur?
molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation
Activation energy: basic definition
minimum amount of energy required for reaction
What is a catalyst?
a substance that increases the rate of a chemical reaction without being consumed in the reaction.
What is heterogeneous catalysis?
the catalyst is in a different physical state from that of the reactants. Many industrial processes involve this type of catalysis, in which a mixture of gases or liquids is passed over a solid catalyst, such as a d-block metal or one of its compounds. An advantage of heterogeneous catalysis is that the products (and any unreacted starting materials) are easily separated from the catalyst.
What is homogeneous catalysis?
the catalyst is in the same physical state as the reactants. Many organic acid–base catalysed reactions involve this type of catalysis. Enzyme reactions in cells take place in aqueous solution and involve homogeneous catalysis.
