Kinetics (rates of reactions) Flashcards
describe the features of the Maxwell-Boltzmann distribution
- number of molecules on y-axis
- energy on x-axis
- graph begins at (0,0) as all molecules have some energy
- roughly bell shaped but skewed to the left
- x-axis is an asymptote as some molecules have very high energy
- peak is the most probable energy (Emp)
- mean energy is found just to the right of Emp
- Ea is found on the right
explain how temperature affects the rate of reaction with reference to the Maxwell-Boltzmann distribution and collision theory
- the higher the temperature, the higher the Ek of the molecules = molecules move faster = more frequent successfull collisions in a given time
- when the temperature is increased, the Maxwell-Boltxmann distribution graph shifts to the right. this means the area to the right of Ea is increased
- therefore a greater proportion of molecules have an energy greater than or equal to Ea = more frequent successful collisions in a given time
what happens to the Maxwell-Boltzmann distribution if a catalyst is added?
- shape of graph stays the same
- only Ea moves to a lower value
what happens to the Maxwell-Boltzmann distibution if the concentration increases?
graph increases in height but shape stays the same
define rate of reaction
the change in concentration of reactants or products per unit of time
why are initial rates of reactions used in calculation and not the rate during the reaction?
as the reaction progresses, the reactants are used up making it more difficult to relate the conc of the reactants to the RoR
describe the effects of changes in concentration on the rate of reaction using the rate equation
- if the order of reaction with respect to the species is zero, rate is unchanged by changes in the concentration
- produces horizontal linear graph of time against conc
- if the order of reaction with respect to the species is one, rate is directly proportional to the concentration
- produces linear graph of time against conce
- e.g if the conc doubles, rate doubles
- produces linear graph of time against conce
- if the order of reaction with respect to the species is two, rate is directly proportional to the concentration squared
- e.g if the conc double, rate quadruples
- produces very steep exponential graph of time against conc
give the two forms of the Arrhenius equation and the units of each components
k = rate constant ( units vary based on overall order of reaction in rate equation )
A = arrhenius constant (same units as k)
Ea = activation energy (J mol-1)
R = gas constant 8.31 J mol-1 K-1
T = temperature in K
how do you convert from degrees celsius to Kelvin?
+ 273
give the form of the Arrhenius equation used to plot a graph and explain how it would be used to find Ea and A
- plotting the graph of ln of k and 1/T from experimental data gives you a straight line graph
- the gradient is -Ea/R
- Ea = -R(gradient)
- the y-intercept is lnA
- A = elnA
define the rate determining step and explain how it relates to the rate equation
- the slowest step in the mechanism for a reaction
- the species present in the rate equation are the reactants involved in reactions that occur before the RDS
state the key points needed in explaining the rate determining step
- state the order of reaction with respect to each reactant included in the rate equation
- identify the step(s) in the mechanisms that has these reactants in the same ration as given in the rate equation
- step …. is the RDS because up to and including the RDS, … particles of … and … react
- step … shows that … react in a … ratio
- if a reactant has zero order you must state : … is not involved in the RDS. step … does not involve …
how can concentration-time graphs be used to determine the order of reaction with respect to a reagent?
- in a zero order reaction, straight line graph
- rate=k so the gradient of the line gives the rate constant (but remember to change the sign as you will be given -k)
- in a first order reaction, exponential graph
- begins high then approaches zero
- in a second order reaction, very steep exponential graph
describe how you would determine the order of reaction using the initial rate method
- pick the reactant whose order of reaction you want to determine
- carry out several experiments at different initial concentrations and measure the time of the reaction (e.g measuring the gas volume produced at different inital concentrations)
- for each initial conc, determine the rate at t=0
- either calculate initial rate directly from data (1/time)
- plot a graph of quantity against time for each separate concentration and draw a tangent at t=0
- plot a graph of initial rate against conc (which must go through origin)
- the shape of the graph would tell you the order of reaction
- to confirm the order of reaction for curve plot log rate = n log [A] + log k . the gradient of this line gives you the order of reaction
describe how you would determine the order of reaction with respect to a reactant using the continuous rate method
- involves following a reaction from start to finish e.g by taking samples at intervals and doing titrations to determine how the conc of a reactant is changing OR by measuring the volume of a gas produced
- samples taken should be cooled rapidly or have another chemical added to stop the reaction
- usually used if the conc of the reactant can be easily determined directly
- plot a graph of conc against time
