3.5 Chemical kinetics Flashcards
Different ways of measuring rate
1) Measuring gas volume at constant pressure by using a gas syringe
2) Mass pan balance
3)Colorimetry
4) measuring gas pressure at constant volume
5) sampling + quenching
Gas syringe?
labelling
delivery tube
conical flask
HCl
Mg
wire
change in volume at constant pressure
Mg+2Hcl—- Mgcl2+H2
Mass pan balance?
Cotton wool
conical flask
mass pan balance
change in mass
cotton wool prevents entry of gases
Colorimetry
colour change
turbidity (colourness)
Ns2S2O3(aq) +2HCl(aq)
—- S2(g) + 2 NaCl (aq) + H2O(l) + S(s)
Sodium thiosulfate
Smell of rotten egg
Cloudy
Manometer?
measuring gas pressure at constant volume
machine used for measuring pressure reactants + products
N2 (g) + 3 H2(g) ——– 2 NH3(g)
Sampling + quenching?
used when no other method is suitable
involves removing small samples of reaction mixture at regular time intervals
the sample is placed in ice cold water - to lower the concentration of the reactants
als to stop the reaction continuing
the sample is tested using titration
Equation for the rate of reaction?
change in concentration / time
draw a tangent to the curve close to the line as possible
Rate definition?
rate of change of concentration of a reactant or product
Rate constant?
the constant in the rate equation which is not affected by changing the concentration of the reactants at a specific temperature
Order of reaction?
the power to which the concentration is raised in the rate equation
0 order?
the concentration of the reactant does not affect the rate of reactant + will not appear in the rate equation
1st order?
if the concentration doubles, the rate also doubles
if the concentration triples, the rate also triples
What is the unit for 0 order?
Rate = mol 1 dm -3 s-1
concentration = mol dm-3
K = mol 1 dm -3 s -1
what is the unit for 1st order?
Rate = mol 1 dm -3 s -1
concentration = mol 1 dm -3
K = S -1
what is the unit for second order?
Rate = mol 1 dm -3 s -1
concentration = mol 1 dm3
k = mol -1 dm -3 s -1
Rate determining step?
the slowest step in a reaction mechanism
each step in the mechanism occurs at a different rate with its own rate equation
slowest step which prevents the overall reaction from taking place
Mechanism?
a series of steps which occur in a chemical reaction
Collision theory?
for a reaction to occur, the reacting particles must collide
the particles must have sufficient energy for the reaction to proceed
In a 2nd order reaction, 2 particles must collide
In a third order reaction, 3 particles must collide
In a first order reaction, only 1 particle in a rate determining step
Arrhenius equation?
Increasing temp of a reaction increases the rate explained by collision theory
Increasing temp means there are more collisions with sufficient energy to react.
This is known as activation energy
Boltzmann’s distribution curve?
higher temp, lower peak
Equation?
K = Ae ^ - Ea/RT
K = rate constant
A = frequency factor
e = Button on calc
Ea = activation energy
R = molar gas
T = temp
A?
frequency factor - frequency of the collisions between the particles
( only a constant over a limited temp range)
- Ea / RT?
shows the fraction of collisions that possess energy above the activation energy
Effects of catalysts on rate?
Catalysts increase the rate of a chemical reaction by providing an alternative route with a lower activation energy
This does not affect the concentrations n rate equation
So it is the rate constant that changes
Reducing activation energy will increase the value of
-Ea/RT and increase the rate constant
