✅16 - Kinetics II Flashcards
What does rate of reaction tell you?
How fast reactants are converted to products
How is rate calculated?
Change in concentration of products/time
How can rate of reaction be determined experimentally?
Gas volume Loss of mass Colour change Change in pH Titration Electrical conductivity
How can gas volume be used to determine rate of reaction?
It can be collected in a syringe and the amount recorded at set intervals, and the ideal gas equation used to work out moles and then molar ratio
How can loss of mas be used to determine rate of reaction?
If a gas is given off, the system will lose mass, measured at regular intervals with a balance
How can colour change be used to determine rate of reaction?
Can be tracked with a colorimeter and a calibration curve plotted
How can change in pH be used to determine rate fo reaction?
If a reaction produces or used up H+, pH can be measured to determine the change in H+ concentration
How can titration be used to determine rate of reaction?
Small samples can be taken at regular intervals and titrated against a standard solution, change in concentration over time
How can electrical conductivity be used to determine rate of reaction?
If the number of ions changes, so will the conductivity
What is the rate constant?
K
What does the value of k change with?
Temperature
What do orders tell you?
How a reactant’s concentration affects the rate
How can orders of reaction be calculated?
From experimental data
What is the overall rider of a reaction?
The sum of all the orders of reaction of the reactants
What does a zero order concentration-time graph look like?
Straight line, top to bottom of Y axis
What does a zero-order rate-concentration graph look like?
Horizontal line
What does a first order concentration-time graph look like?
Curve, top to bottom
What does a first order rate-concentration graph look like?
Directly proportional
What does a second order concentration-time graph look like?
Steep curve, top to bottom
What does a second order rate-concentration time graph look like?
Curve, bottom to top
What is half life?
The time taken for half the reactant to be used up
How can orders of reaction be worked out?
Using the initial rates method
What is an example of the initial rates method?
A clock reaction
How is an initial rates method carried out?
Carrying out separate experiments using different initial concentrations of one reactant, then seeing how the change in Italy concentrations affects the rate
What is done in a clock reaction?
You measure the time taken or a set amount of product to form and how it changes as the concentrations vary
What assumptions are made in clock reactions?
The concentration of arch reactant doesn’t change significantly over th period of the reaction
The temperature stays constant
When the end point is seen, the reaction has not proceeded too far
What happens at the end of a clock reaction?
There’s an easily observable end point as there is a sudden increase in the concentration of a product as the limiting reactant is used up
What is an example of a clock reaction?
The iodine clock reaction
What is the reaction that occurs in the iodine clock?
H2O2 + 2I- + 2H+ ——> 2H2O + I2
What is the indicator used?
Starch
Why is sodium thiosulfate added to the mixture?
To react instantaneously with any iodine that forms
What happens when the thiosulfate is used up?
The iodine becomes obvious with the indicator as it is allowed to form ithout reacting with the thiosulfate
Which reactant’s concentrations are varied?
The iodide or hydrogen peroxide
What is an example of a continuous monitoring method?
Titrations the reaction between iodine and propanone
What is the equation for the reaction between iodine and propanone?
CH3COCH3 + I2 —H+—> CH3COCH2I + H+ + I-
Why is sodium hydrogencarbnate added to samples of iodine-propanone before titrating?
To stop the reaction by neutralising the acid
What do rate equations link?
Rate of reaction and reactant concentrations
What is a rate equation?
Rate = k[A]^m [B]^n
The bigger k is…
…the faster the reaction
What can the rate constant be calculated from?
The orders and rate of reaction
How can k be calculated?
Rate of reaction / concentrations and orders
What is the rate determining step?
The slowest step in a multi-step reaction
What does it indicate if a reactant appears in the rate equation?
That it features in the rate determining step
What is the order of reaction with respect to a reactant?
Shows the number of molecules of that reactant that are involved in or before the rate determining step
What can halogenoalkanes be hydrolysed by?
Hydroxide ions
What do halogenoalkanes undergo?
Nucleophillic substitution
By what mechanism are primary halogenoalkanes hydrolysed?
SN2
By what mechanism are tertiary halogenoalkanes hydrolysed?
SN1
What does an SN2 reaction include?
A transition state
What does an SN1 reaction include?
An intermediate carbocation
By what mechanism do secondary halogenoalkanes react?
SN1 or SN2
What is the Arrhenius equation?
k = Ae^(-Ea/RT)
What is the relationship between activation energy and k?
As Ea gets bigger, k gets smaller
What is the relationship between k and temperature?
As temperature increases, so does k
What is the gradient of an Arrhenius graph?
-Ea/R
How can you find the activation energy from the Arrhenius graph?
By multiplying the gradient by R
What are homogeneous catalysts?
In the same state as the reactant
What are heterogeneous catalysts?
In a different physical state to the reactants
How do solid heterogeneous catalysts work?
They provide a surface for the reaction to take place on, usually as a fine mesh or powder to increase surface area
What is a benefit of heterogeneous catalysts?
They can be easily separated from the products and leftover reactants
How can a heterogeneous catalyst be poisoned?
When a substance clings t the surface more strongly than the reactant does, preventing a catalyst from getting involved in the reaction
