2.3 Kinetics (from CH1401: definitions, rate laws) Flashcards
Thermodynamics vs kinetics: what do they tell us?
Kinetic stability vs thermodynamic stability
ex.: H2O2 decomposition to produce O2 (elephant toothpaste)
Explain what is kinetics and what it deals with
What are the possible progressions / mechanisms of reactions
- One step
- Two step
- Unimolecular (one molecule involved)
- Bimolecular (two molecules involved)
In the overall reaction 3 molecules involved ut needs to be split up into steps to determine molecularity
What are the potential energy contour plots, explain them
How are the rates of rreactions measured experimentally?
What is the definition of rate of a reaction?
Rate - the slope at a particular time / concentration
How to calculate the rate of a recation from a data plot?
Rate - the slope at a particular time / concentration
How can the reaction rates of different secies in the same reaction be determined from a reaction equation?
According to the reaction equation ratios - relative differences in rates of different species in the same reaction
What is the Rate Law? Explain the concept
- Rate Law can only be dteermined experimentally
- The Rate Law: expresses the rate of reaction in terms of the molar concentrations of the species in the overall reaction
- Rate constant k: independent of the concentration of species in the reaction but depends on temperature
What does the reaction order explain about the reaction?
How is the rate of a reaction determined and how can it be expressed?
What is the zeroth order plots for conc vs time and rate vs conc
What is the first order plots for conc vs time and rate vs conc
As conc decreases, so does the rate
What is the second order plots for conc vs time and rate vs conc
As the conc drops, the rate drops even quicker
What is the integrated rate law for a zeroth order reaction?
What is the integarted rate law for first order reaction?
What is the integarted rate law for second order reaction?
What is the difference between the first order differential rate law and integrated rate law?
Differential rate law gives rate in terms of conc
Integrated rate law gives rate as a function of time
What is the order of radioactive decay and why
1st order because the rate of decay will be dependent only on the number of species that are in the block + each nucleus of the species has an equal chance of decay → number of decays per unit time depends on the number of nuclei
How does carbon dating works? How to estimate length of time since death?
What is the differential rate law for first order A+B→products? Why
Because the collision frequency only depends how often A and B collide and react, so the higher the conc - the more collisions
How does the integrated rate law look for first order A+B→products reaction?
Complex but can be simplified when said that on of reactants is in large excess
How do differential rate laws combine in equilibrium?
How can thermodynamics and kinetics be related at equilibrium?
What are the two ways to determine rate law from experimental data?
Rate laws can only be datermined experimentally → calculated in two ways
How is the initial rate law determined from experimental data?
Measuring the tangent at the very start of the curve - linear relationship for some time
Explain how to determine reactions order via initial rate method
x
Explain how to determine reactions order via method of integration
Key points from rate law
When does rate constant k change?
When temperature changes (when increases - increase in k - rate increases)
rate decreases when T raised above certain point - catalysed by enzymes (denature)
Which equation describes the relationship between k and T?
Arrhenius equation (molecules have to have a threshold of E to be able to react)
What is a reaction profile?
How to determine Ea and A for a particular reaction?
A - measure of frequencies of collisions in the correct orientation to react
Explain catalysts (define, working mechanism, types)
both forward and reverse reactions are catalysed at the same time
What is the detailed mechanism of metal catalysts?
- catalyst provides lower energy pathway
- reactants form weak bonds with the metal → after some time start making bonds with the surface → make bonds between each other
- Example: liquid oil to margerine using Ni catalyst
How do catalyst alter the activation energy curve?
- very high E needed to split gaseous H2 - with Ni not needed
- free E change same with or without of catalysts
Summary of all equations in kinetics
