topic 6/16- kinetics Flashcards
give two equations for the rate of a reaction
ror= amount of reactant used/time
ror= amount of product formed/time
how could you measure the rate of reaction at a particular point on a rate of reaction graph?
draw a tangent and calculate the gradient
state an example of an experimental method for following ROR and give its equation
Mg (s) + 2HCl -> MgCl2 (aq) + H2 (g)
define rate of reaction
the change in concentration of reactants or products per unit time (=chang in conc/time)
state the 2 most common units for ROR
mol/dm3/s or mol/dm3/min
collision theory states that for a reaction between two particles to occur:
- the particles must collide with each other with sufficient energy (Ea)
- the particles must collide in the correct orientation
define activation energy
the minimum energy needed for a reaction to occur on colliding of particles
state 5 factors affecting the rate of reaction
- concentration
- pressure
- surface area
- temperature
- enzymes
explain how an increase in concentration/pressure affects the rate of reaction
- no of particles per given volume increases
- increased number of frequent (successful collisions)
- rate increases
explain how an increase in surface area affects the rate of reaction
- exposes more reactant particles
- increased number of collisions and successful collisions per second
- increased rate of reaction
the average kinetic energy of particles is proportional to…
their temperature in kelvin
explain the effect of an increase in temperature on the rate of reaction
- increased energy, so KE of particles increases
- a greater proportion of the particles have the required activation energy
- more collisions per second
- increased number of collisions result in a reaction
- ROR increases
draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases.
elsewhere
draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how increasing the temperature affects the ROR
elsewhere
define a catalyst
a substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction
describe the effect of a catalyst on the ROR
a catalyst speeds up the rate of reaction by providing an alternative pathway with a lower activation energy
(= increase frequency of successful collisions)
draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how a catalyst lowers the activation energy
elsewhere
draw an energy profile diagram showing the same reaction with and without a catalyst
elsewhere
give the two major economic advantages of the use of catalysts in industry
- they increase the rate of a chemical reaction meaning that more of the desired product can be made in a given time period
- reactions can take place at lower temperatures resulting in a decrease in the energy costs to the manufacturer
define a heterogenous catalyst
one that is in a different phrase to that of the reactants
investigating the iodine-propanone reaction
- 3 steps
- how can we stop the reaction in the sample?
- what do we titrate the iodine against and how?
- the course of the reaction is followed by taking samples from the iodine-propanone mixture at regular intervals, stopping the reaction and then determining the amount of iodine present by titration.
- by adding an excess of sodium hydrogen carbonate to the mixture, which removes the acid (sulphuric acid) catalysing the reaction
- titrate against sodium thiosulphate- wait for the iodine to go from orange to straw coloured/colourless and then add starch, which is blue/black in the presence of iodine
give 3 ways of monitoring concentration changes in a reaction
- mass (loss of mass as gas released)
- volume (of gas produced)
- colour ( production of solid and cross)
give the equation and units for rate of reaction
change in concentration (mol/dm3)/time (s)
Units: mol/dm3/s
for a reaction A+B-> products, give two scenarios regarding the rate
scenario 1: the rate is proportional to the concentration of A (if we double the concentration of A, the rate also doubles)
scenario 2: the rate is proportional to the square of the concentration of A (if we double the concentration of A, the rate quadruples)
state the rate equation and describe each of the terms
Rate = k[A]^a[B]^b
- k is the rate constant; it is constant for each reaction as long as neither temperature is changed nor catalyst is added
- [A] and [B] are the concentrations in mol/dm3 of reactants A and B respectively
- a and b are the orders of reaction with respect to reactant A and B
zero order with respect to reactant A
- reaction rate not affected by [A]
- if A were doubled, the reaction rate would not change
- rate = k
- zero order reactants do not appear in the equation
first order with respect to reactant A
- reaction rate is proportional to [A]
- if A were multiplied by 2, reaction rate would double, if multiplied by 4, reaction rate would quadruple
- rate = k[A]
second order with respect to reactant A
- reaction rate is proportional to [A]^2
- if [A] were multiplied by 2, reaction rate would quadruple, if multiplied by 4, it would x8
- rate = k[A]^2
how do we work out the units of the rate constant, k, and why?
the units of k will depend on the form of the rate equation, so need to be derived separately from each reaction equation
effect of an increased concentration of a reactant on
- rate of reaction
- value of rate constant
- increases unless the order is 0
- no change
effect of an increased temperature of a reactant on
- rate of reaction
- value of rate constant
- increases
- increases
effect of the addition of a catalyst on
- rate of reaction
- value of rate constant
- increases
- increases
give two strategies that can be employed to find reaction orders from experiments
- continuous monitoring
- using initial rates
define continuous monitoring
the quantity (ie concentration) of a reactant is measured at intervals (eg every 30 seconds) during the course of a reaction
describe 3 ways in which continuous monitoring can be used
- concentration-time graphs
- finding half-lives
- rate-concentration graphs
- concentration-time graphs
concentration on y axis, time on x axis, where the shape indicates the order of reactant with respect to that reactant
draw the graph shapes for zero order, first order, and second order concentration-time
- finding half-lives
the half-life of a reactant is the time taken for its concentration to reduce by half
- first order- half life is constant
- second order- half life not constant
- rate-concentration graphs
- to determine how the rate changes with concentration it is necessary to take tangents of the concentration-time graph
- these tangents give the rate of the reaction at that particular time
- concentration on x axis, rate on y axis
draw the graph for zero, first, and second order rate-concentration graphs
in a ‘clock method’, rate =
1/t
strategy B; using initial rates
- several experiments undertaken, changing concentration of only one reactant at a time and keeping all others the same
how is initial rate determined?
- finding the gradient of a reactant concentration against time graph at time=0
- timing how long it takes for a certain amount of product to be made and calculating 1/time as initial rate
why is the reaction between iodine and propane in acid solution known as an auto-catalysis reaction?
because the catalyst (H+) is made as a product
describe the reaction order of an SN1 reaction mechanism for halogenoalkanes
overall order=1 check booklet for more info
describe the reaction order of an SN2 reaction mechanism for halogenoalkanes
overall order=2 check booklet for more info
the effect of temperature on the rate is determined by the
rate constant
altering the temperature or adding a catalyst will change the
rate constantt
Arrhenius equation
k=Aexp(-Ea/RT)
A- Arrhenius constant/pre-exponential factor/frequency factor/steric factor - incorporates effect of collision frequency and orientation
Ea- minimum energy for reaction to occur (J/mol)
R- gas constant (8.31 J/K/mol)
absolute temperature (K)
taking the natural logarithm of the Arrhenius equation converts it to a more useful form
ln(k) = ln(A) - (Ea/RT)
- this equation is in the form y=mx + c
- ln(k) is y axis
- ln(A) is c
- (-Ea/R) is gradient, m
- 1/T is x axis
describe how you would use the Arrhenius equation to find activation energy
- measure the rate of reaction at a range of different temperatures
- calculate the nature logarithm of each rate to give ln(rate)
- convert each ‘C temp to Kelvin by adding 273 and find 1/T (units K^-1)
- plot a graph of ln(rate) on the y axis against 1/T/K^-1 and draw a line of best fit
- find the gradient by using m=y2-y1/x2-x1
- activation energy (j/mol) = -R x Gradient
- convert answer to units kJ/mol
remember
1/T values will often be of the order !0^-3 so don’t forget to write x 10^-3 on your temperature scale
reactions may occur by more than one step; what determines the rate of reaction?
the slowest step (rate determining step)
look at how to form the rate equation from reactions with more than one step
the molecularity of an elementary step is
the number of reactant particles taking part in that step
the order of a reaction can either be —– or —— in nature, and can describe
integer, fractional; the number of a specific reactant’s particles taking part in the rate-determining step.
rate equations can only be determined
experimentally
how do catalysts alter a reaction mechanism?
they introduce a step with lower activation energy
which step of the reaction will have a higher Ea
the rate-determining step
