Rate Of Reaction Flashcards
Rate of reaction def
Change in concentration over time
Order def
The power to which a concentration is raised
Overall order def
The sum of powers in the rate equation
Rate constant def
Proportionality constant
Half-life def
Time taken for concentration of substance to reduce by half
Rate-determining step def
Slowest step in a reaction mechanism
How is rate measured
Amount of product produced / time:
- Moles per decimeters(cubed) / seconds(time)
- mol dm3 s-1
What is the rate equation?
Rate = k[A]^a(x)[B]^b
- rate = mol dm-3 s-1
- a = order of reaction with respect to a
- b = order of reaction with respect to b
- k = rate constant
- A and B = concentrations in mol dm-3
Zero order meaning
The concentration of that reagent does not affect the rate
First order meaning
The concentration of that reagent is proportional to the rate
Second order meaning
The rate of that reaction is proportional to the square of the concentration of A
What change in the reaction conditions would cause the rate constant to change?
Change in temperature
- Increase the temperature = increase the rate of reaction
- Decrease the temperature : decrease rate of reaction
Equation to work out half life
Kt1/2 = ln(2)
How to work out k in rate of reaction from half life
K = ln2/half life
Half life - t1/2
Reaction Mechanism def
It is the order in which bonds are made and broken for a reaction to happen
This results in a series of steps
What is an intermediate in a reaction mechanism?
It is a substance that is made in the rate-determining step(slowest step) and is then used up in the fast reaction
What does the rate equation only include?
Only includes reacting species in the rate-determining step - the slowest step
Explain why it is unlikely that a reaction will occur in one step
- Unlikely that all the reactants will form exactly all the products in one reaction
- Number of particles in the rate-determining step don’t match those in the reaction
What is the Arrhenius equation?
How do you rearrange this to find Ea on a graph
K = Ae^(-Ea/RT)
K: rate constant A: frequency factor or pre-exponential factor e: mathematical quantity, e Ea: Activation energy R: the gas constant (8.31 J K-1) T: Temperature (Kelvin)
Rearrangement:
lnK = -(Ea/R) x ((1/T) + lnA
Y = Mx + c
How do you find the initial rate of reaction using a graph of data?
- Draw a tangent at t = 0
- Calculate the gradient of the tangent
- The gradient of the tangent is the initial rate of reaction
Explain the purpose of the sodium thiosulfate solution in reaction between sodium thiosulfate and iodine
- Reacts rapidly with iodine as it is formed and reduces it
- Prevents iodine starch solution/blue black colour forming until a fixed amount of iodine has reacted
- Prevents iodine starch blue/black colour forming until all the thiosulfate has reacted
Outline a method, not involving sampling the mixture, which would be suitable for measuring the iodine concentrations in this experiment between sodium thiosulfate and potassium iodide.
(3 Marks)
- Use a colorimeter to measure absorbance/wavelength of light in iodine solution
- Measure absorbance of light/colour at set intervals/ periods, e.g. every 10 seconds
- Use a calibration curve to convert measured absorbance into concentration
What affects the value of Kc, the equilibrium constant?
Only temperature affects the equilibrium constant, Kc
What affects the value of the rate constant, k?
Rate of reaction affects the rate constant, k
- increase rate of reaction increases the value of rate constant
- decreasing rate of reaction decreases value of the rate constant
Outline a titremetric method that could be used to measure the change in concentration over time of a compound A in the reaction:
A + B (+ catalyst) —> C + D
(3 Marks)
- Take samples of the reaction mixture at various times
- Remove the catalyst by quenching with the ice
- Titrate the mixture with a standard solution of acid/ of known concentration.
In a reaction mixture, how could you show that equilibrium had been established?
Information on exo/endothermic is not given.
(2 Marks)
Use a colorimeter to absorb wavelengths of light of mixture
Colour/wavelengths of light absorbed will not change.
in an experiment to determine rate, explain why an excess of a substance is used in a reaction.
(1 Mark)
- to keep the concentration of the substance (effectively) constant
- to keep the order of reaction for the substance as zero
- to endure equilibrium is far to the right
A student investigates the decomposition of H2O2(aq) by measuring the volume of oxygen gas produced over time. All gas volumes are measured at room temperature and pressure.
The student uses 25.0 cm3 of 2.30 mol dm−3 H2O2.
From the results, the student determines the concentration of H2O2(aq) at each time. The student then plots a concentration–time graph.
Suggest a different experimental method that would allow the rate of this reaction to be followed over time.
(1 Mark)
Measure mass loss of the substance
