Reaction Kinetics Flashcards
Explain the term “rate equation”
The experimental relationship between reaction rate and concentration of reactants.
E.g. k[A]^m[B]^n
Explain the term “order of reaction”
The power of the reactant’s concentration term in the experimentally-determined rate equation is the order of reaction with respect to that reactant. (m/n)
Overall order= m+n
Explain the term “rate constant”
Proportionality constant in the experimental determined rate equation.
k increased when temperature increases and Ea decreases (use of catalyst)
k = Ae^-EaIRT
Explain the term “half life of a reaction”
t1/2 represents the time taken for the concentration of a reactant to be halved. Half life is constant for first order reactions.
t1/2= ln2/k
Note that half life of first order reaction is independent of concentration
Explain the term “rate determining step” (RDS)
Slowest step in the reaction. Rate of reaction = rate of RDS
note that rate equation does not involve intermediates.
Explain the term “activation energy”
Minimum amount of energy that reactant particles must possess before a reaction could occur.
Explain the term “rate of reaction”
The change in concentration of reactant or product per unit time.
Rate of reaction = -d[reactant]/dt = d[product]/dt. Rate of reaction differs from rate of change of concentration of a product/reactant by a constant factor (reciprocal of its stoichiometric coefficient)
What are the continuous methods (physical) of determining rate of reaction?
- Volume measurement - reactions that produce gaseous products, measuring volume of gas collected in syringe at regular time intervals
- Mass measurement - reactions that produce gaseous products, measuring the mass of reactants at regular time intervals
- Gas pressure measurement - reactions where stoichiometric amount of gaseous product is different from stoichiometric amount of gaseous reactants, measuring total pressure at regular time intervals
- Colorimetric method - reactions with coloured reactants/products, measuring amount of light transmitted through solution at regular time intervals
- Conductometric method - reactions with change in number of ions, measuring change in electrical conductivity of solution with time
- Rotation of plane of polarisation of plane-polarised light - reactions with change of optical activity
What are the steps to continuous method (sampling)
- Reactions carried out at constant temp
- Known conc and vol of reactants mixed and start stop watch
- Withdraw samples at suitable time intervals
- Quench sample to stop/slow reaction
- Conc of reactant/product determined by titration
What is the discontinuous method (physical method) of determining rate of reaction?
Sulfur clock reaction, iodine clock reaction. Measurement of time taken to reach an easily seen stage after reactants are mixed. The faster the reaction, the sooner the easily seen stage is reached.
Factors affecting rate of reaction?
- Concentration/pressure of reactants
- Surface area of solid reactants
- Temperature
- Catalyst
Explain qualitatively, in terms of collisions, the effect of concentration changes on the rate of a reaction
The rate of reaction increases with concentration/pressure of one or more of the reactant species. As concentration/pressure of the reactant is increases, the number of reactant particles per unit volume increases -> increase in frequency of collision between reactant particles, increases frequency of effective collisions.
Explain, in terms of collision, effect of surface area of solid reactants on rate of reaction
Rate of reaction increases when solids are in a finely divided state, due to larger surface area of contact between the reactant particles and hence higher frequency of effective collisions between reactant particles.
Explain, in terms of collision frequency and distribution curve, the effect of temperature change on a rate constant (and hence rate) of a reaction
When temperature increases, the average kinetic energy of reactant particles increases, frequency of collisions between reactant particles increases, rate of reaction increases. Temperature rise of 10*C will roughly double reaction.
At higher temperature, there is a larger number of reactant particles with energy greater than or equal to activation energy, resulting in increase in freq of effective collisions between reactant particles, increase rate of reaction.
Note: distribution curve in other flashcard deck
Definition of catalyst
Substance that increases rate of a reaction but remains chemically unchanged at the end of reaction. Provides an alternative reaction pathway with lower activation energy.
