topic 16: kinetics II Flashcards
what is the measurement of the change in volume of a gas
this works if there is a change in the number of moles of gas in the reaction. using a gas syringe is a common way of following this
what is the measurement of change of mass
this works if there is a gas produced which is allowed to escape. works better with heavy gases such as CO2
- the mass loss provides measure for reactants
- gas must be sufficiently dense or change in mass is too small to measure
how do you titrate samples of reaction mixture with acid, alkali, sodium thiosulfate etc
small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent
HCOOCH3(aq) + NaOH(aq) —> HCOONa(aq) + CH3OH(aq)
- the NaOH could be titrated with an acid
BrO3–(aq) + 5Br–(aq) + 6H+(aq) —> 3Br2(aq) + 3H2O(l)
- the H+ could be titrated with an alkali
CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + H+(aq) + I–(aq)
- the I- can be titrated with sodium
thiosulfate
what is the rate of reaction
the change in concentration of a substance in unit time
Its usual unit is mol dm-3s-1
what is a rate equation
the rate equation relates mathematically the rate of reaction to the concentration of the reactants
r = k[A]m[B]n
what is order with respect to a substance in a rate equation
orders are usually integers 0,1,2
0 means the reaction is zero order with respect to that reactant
1 means first order
2 means second order
what is the overall order of reaction
the total order for a reaction is worked out by adding all the individual orders together (m+n)
what is the rate constant
rate=k[A][B]
what is the rate - determining step
the slowest step will control the overall rate of reaction
what does the graph for the 1st order with initial rate
a straight diagonal line
what does 1st order mean
the rate of reaction is directly proportional
to the concentration of A r = k[A]1
what does the graph for the 0 order with initial rate
straight line parallel to x axis
what does 0 order mean
the concentration of A has no effect on the
rate of reaction r = k[A]0 = k
what does the graph for the 2nd order with initial rate
exponential graph
what does 2nd order mean
the rate of reaction is proportional to the
concentration of A squared r = k[A]2
how would the initial rate may have been
calculated
from taking gradients from concentration
/time graphs
what happens to the half life for 1st order
independent of the concentration and is constant
what happens to the half life for 0 order
the rate stays constant as the reactant is used up. this means the concentration has no
effect on rate
what happens to the half life for 2nd order
rapidly increase
what is the unit for k for 1st order
s-1
what is the unit for k for 2nd order
mol-1dm3s-1
what is the unit for k for 3rd order
mol-2dm6s-1
what happens to the rate constant if the temperature increases
increases the value of the rate constant k
what is Arrhenius equation
k = Ae-EA/RT
A = Arrhenius constant
Ea = activation energy
R = gas constant
T = temp
how is Arrhenius equation rearranged
ln k = ln A – EA/(RT)
what are the units of the components of Arrhenius equation
Temperature uses the unit K
R = 8.31 J mol-1K-1
activation energy will need to be in J mol-1
to match the units of R
the unit of the Arrhenius constant A will be the same as the unit of the rate constant k
what is k directly proportional to
the rate of reaction
- so ln k = ln (rate)
what is on the x and y axis of the graph
x = 1/T
y = ln rate
what is the gradient of the line on the graph
-Ea/R
what is the molecularity (number of moles of each substance) of the molecules in the slowest
step the same as
the order of reaction for each substance
