rates and kp Flashcards
What does the order of reaction mean?
power of concentration term in rate of reaction
What is the effect of temperature on K (rate constant)?
increases the temperature increases the value of rate constant
What is the Arrhenius equation and what is its purpose?
What is the rate determining step?
the slowest step
it is in the rates equation
When given a series of steps, how do you know which one is the rate-determining step?
species in the rate-determining step are in the same ratio/proportion as in the rate equation
Describe the iodine clock reaction
- Add dilute sulfuric acid andstarch solution to beaker
- Add sodium thiosulfate to reaction mixture
- Add potassium iodide solution
- Addhydrogen peroxide solution
- Sodium thiosulfate reacts with iodine being formed
- Once all sodium thiosulfate is used up = any more iodine formed remains in solution
- Turns starch indicator blue-black
- Varying conc. of iodide or hydrogen peroxide while keeping everything constant = different times for colour change
◦ Used to work out reaction order
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What are the two equations for the two reactions in the iodine clock reaction and explain how they relate to the experiment
Hydrogen peroxide reacts with iodide ions to form iodine and the thiosulfate ion immediately reacts
with iodine
When the I2 produced has reacted with all of the limited amount of thiosulfate ions present, excess
I2 remains in solution. Reaction with the starch then forms a dark blue-black colour.
By varying the concentration of iodine you can determine the order of reaction with respect to I ions.
What are the different ways to measure the initial rate of a reaction
- Iodine clock reaction
- Reactions that produce precipitates
- Measure time it takes for mark underneath reaction vessel to disappear
- Other reactions
- Measure time taken for small amount of product to be formed
- calculated from taking the gradient of a continuous monitoring conc vs time
graph at time = zero
Describe continous monitoring
- Can follow reaction all way to its end by recording amount of product (or reactant) you have at regular time intervals
- Use results to work out how rate changes over time
What are the 4 examples of continuous monitoring methods
- loss of mass
- colour change
- gas volume
- ## change in pH
A series of experiments was carried out using concentrations of propanone approximately 100 times the concentrations of iodine and hydrochloric acid. Suggest the rate equation under these conditions. Explain you answer
As propanone is so large it is effectively constant therefore rate = k[H+]
Why does a large concentration of a reactant make it effectively constant
If the reaction were A + B -> C
If you had 1 million A molecules and 1 million of B, by the time half of the A had been used up, half of the B would also be used up.
But, if you had 1 million A and 1 billion B, by the time half of the A had been used up, a fraction of a percent of B had been used up. [B] effectively is constant.
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Explain qualitatively why doubling the temperature has a much greater effect on the
rate of the reaction than doubling the concentration
The reaction occurs when molecules have E>Ea
Doubling T by 10 °C causes many more molecules to have this E
Whereas doubling concentration only doubles the number with this reactant
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State the effect, if any, on the value of the rate constant k when the temperature is lowered but all other conditions are kept constant. Explain your answer.
Lowered
fewer particles/collisions have energy > Ea
OR
fewer have sufficient (activation) energy (to react)
What is partial pressure?
In a mixture of gases, each gas exerts its own pressure
