Kinetic Flashcards Preview

Physical Chemistry University > Kinetic > Flashcards

Flashcards in Kinetic Deck (30)
Loading flashcards...

What is the rate of an equation?

How quickly the concentrations of reactants and products are changing
mathematically described as the rate of change of [A] with respect to time: d[A]/dt


How to determine rate of reaction at time t?

Gradient of tangent to the graph at time t
d[A]/dt = -kt (negative so reactant used)
d[B]/dt = kt (positive so product formed)


What is instantaneous rate?

Rate at a specific moment in time


Units of rate?

Rate of change of concentration of a reaction or product with time


Rate law?

Rate = k[A]a[B]b


What is an elementary reaction?

A single step process that involves one or two molecules


Two important features of elementary reactions?

Equation describe what is actually going on at the molecular level, for a complex reaction which is made up of elementary reactions the equation only gives the stoichiometry
For an elementary reactions it is possible to use the chemical equation to form the rate equation but this is not the case for complex reactions


Rate equation form elementary reaction?

An elementary reaction is a reaction that takes place at the molecular level ie what is really happening. The stoichiometric equation represents the overall reactions in terms of initial reactants and final products but does not show how a reaction actually happens, the molecularity of an elementary reaction is the number of molecules that come together to react



1 molecular involves eg isomerisation ot radioactive decay



2 molecules react


What is a reaction mechanism?

The sequence of elementary molecular events that lead from reactant to products, what happens at the molecular level during a chemical reaction from experimental determined kinetic data are used to postulate the mechanism for a reaction


Using the rate law to test a proposed mechanism?

Rate law is experimentally determined, we want to propose a reaction mechanism that accounts for the observed rate law, to do this we must be able to formulate the rate law that corresponds to the proposed mechanism and compare this with the experimentally observed rate law


What is the rate law?

Relates the concentration of reactants and the overall rate of reaction, empirically the rate of reactions is often but not always found to be related to the concentration of that reactant raised to a simple power


Rate constant k?

Rate constant k relates concentrations of reactants to the overall rate of reaction


Reaction order?

Power to which the concentration or pressure of a reactant is raised in the rate law, the order overall is the sum of order of reactant, until you have carried out experiments you cannot know that this is true, in general the rate law cannot be found from a chemical equation it must be found experimentally, order in an experimental observation depends on the mechanism of the reaction


How does the rate constant affect reaction rate?

The larger the value of k the faster the reaction is


Rate constant units?

Units for the rate constants are important and depends on the type of reaction and the reaction order
1st order rate law k = s-1
2nd order rate law k = dm3mol-1s-1
3rd order rate law k = dm6mol-2s-1
zero order rate law k = moldm-3s-1


Integrated rate law?

By going from the rate law to the integrated rate law the time information is available which can allow us to determine how much of a species has reacted at a particular time


First and second order decay proportionality dependence?

First order decays proportionally to exp(kt)
Second order decays proportionally to 1/t


Pseudo first order reactions?

Take one of the reactants out of the equation by making its concentration much greater than the other one, therefore its changes in concentration are negligible called the isolation technique


What is half life?

The time taken for the concentration of a reagent to fall to 1/2 of its initial value


Methods to determine order of reaction?

Tangents to the concentration vs time curve
Initial rates
Integrated rate equations
Half lives


Tangents to the concentration vs time curve ?

Plot rates against [A] if its a straight lie tenth results are 1st order, if there is a curve its not first order, test for second order by plotting rate vs [A]2 if linear the reaction is second order. Can be complicated and take a long time so use log log plots, equation is always a straight line, if gradient is 0 then 0th order reaction, if gradient is 1 then 1st order reaction etc


What is a reaction theory?

Based on a model of how the reaction occurs at a molecular levels - these may be used to help design appropriate experiments to measure rate constants


What is collision theory?

Simple model in which reactant modules are assumed to behave like hard spheres, collisions between spheres may result in reaction and a calculated collision frequency is used to estimate A


What is transition state theory?

A more sophisticated theory in which one assumes that reactions form a transition state which is a short lived complex at the maximum on the energy profile, the A factor is calculated base on an assumed structure for the transition state


What is molecularity?

Number of molecules in the transition state - applies only to elementary reactions



Transition state involves one molecules of reactant



Transition state involves two reactant molecules wither coming together to form a large molecule as the product or bond break and new bonds form and two new molecules are formed (products)


What is a catalyst?

A catalyst increases the rate of a chemical reaction without being consumed in the reaction - catalysts provide an alternative pathway in which the rate determining step has a lower Gibbs energy of activation than uncatalysed reactions thus speeding up the reaction