1.11 Rate Equations Flashcards
Define reaction rate
Change in amount of reactant or product per unit time
State the units for reaction rate
Mol dm-3 s-1
State the generalised rate equation
rate = k[A]m[B]n
rate = k[A]m[B]n
What does m and n represent?
orders of the reaction
rate = k[A]m[B]n
What does k represent?
k = rate constant
What do orders of reactions tell you?
Tell you how reactant concentrations affect the rate
(e.g. m tells you how concentration of reactant A affects rate and n tells you same for reactant B)
Find the overall order of reaction
rate = k[A]m[B]n
m + n
How can you only find the orders of reaction?
From experiments
If [A] changes and the rate stays the same, what is the order of the reaction?
The order of reaction with respect to A is 0
If the rate is proportional to [A], what is the order of the reaction?
The order of reaction with respect to A is 1
If the rate is proportional to [A]2, what is the order of the reaction?
The order of reaction with respect to A is 2
The rate constant relates…
reactant concentrations to rate at a particular temperature
Bigger value of k = ____ __ ______
faster the reaction
k is always _____ for certain reaction at a particular temperature
the same
Why does the rate constant increase as temperature increases?
- ∵ ↑ temp. = rate of reaction ↑
- ∵ increasing no. of collisions between reactant molecules + energy of each collisions
- But conc. of reactants and orders of reaction stay the same
- So k must increase for rate equation to balance
Define the initial rate of a reaction
Rate right at the start of the reaction
Describe how you can find the initial rate of a reaction (from graph)
Find it from a concentration-time graph by calculating the gradient of the tangent at time = 0
What is the initial rates method used for?
Used to create rate equations
Describe the initial rates method
- Repeat an experiment several times using different initial concentrations of the reactants
- Usually only change 1 conc. of at time
- Calculate initial rate for each experiment
- See how initial concentration affects initial rates and figure out the order for each reactant
Name 3 methods of measuring the initial reaction rate
- 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
Describe the iodine clock reaction
- Add dilute sulfuric acid and starch solution to beaker
- Add sodium thiosulfate to reaction mixture
- Add potassium iodide solution
- Add hydrogen 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
Iodine Clock Reaction
State the equation for how iodine is produced
Iodine Clock Reaction
State the equation for how iodine reacts with thiosulfate ions
Name 2 methods you can use to measure rate of reaction
- Measuring Initial Reaction Rate
- Continuous Monitoring
Describe continuous 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
Name 4 examples of continuous monitoring methods
- Loss of Mass
- Colour Change
- Gas Volume
- Change in pH
What does a colorimeter do?
Measures absorbance
Colorimeter
Higher the absorbance =
More concentrated the colour of the solution is
Describe how you can measure the rate of reaction by continuous monitoring when there’s a colour change
Can measure change in absorbance:
- Plot calibration curve
- Graph of known concentrations of coloured solution (e.g. I₂) plotted against absorbance
- During experiment, take small samples from your reaction solution at regular intervals and read the absorbance
- Use calibration curve to convert absorbance at each time point into a concentration
Describe how you can measure the rate of reaction by continuous monitoring when there’s a loss of mass
- If gas is given off, system will lose mass
- Can measure this at regular intervals with a balance
- Use mole calculations to work out how much gas you’ve lost
- & thus how many moles of reactants are left
Describe how you can measure the rate of reaction by continuous monitoring when there’s a gas volume
- If gas given off, could collect it in gas syringe & record how you’ve got at regular time intervals (e.g. every 15s)
- e.g. acid + carbonate = CO₂
- e.g. magnesium ribbon + HCl
- Find conc. of reactant at each time point
- Use ideal gas equation to work how many mole of gas
- Then use molar ratio to work out conc. of reactant
Describe how you can measure the rate of reaction by continuous monitoring when there’s a change in pH
If reaction produces or uses H+ ions, can measure pH of solution at regular intervals & calculate the conc. of H+
How can you construct a rate-concentration graph?
Can use data from concentration-time graph to construct it
What can a rate-concentration graph tell you?
Reaction order
Describe how you can construct a rate-concentration graph
- Find gradient at various points on graph
- Gives you rate at that particular concentration
- For curve, need to draw tangents
- Plot each point on new graph with axes rate and concentration
- Then draw smooth line or curve through points
- Shape of line will tell you the order of the reaction with respect to that reactant
State order of the reaction with respect to reactant [X]
State order of the reaction with respect to reactant [X]
State order of the reaction with respect to reactant [X]
In a multi-step reaction, each step can have a ___ ___
different rate
State what is meant by the rate determining step
Slowest step in a multi-step reaction
What is the overall rate decided by?
By the step with the slowest rate = rate determining step (aka rate-limiting step)
Explain how you know reactants in rate equations affect the rate
If a reactant appears in the rate equation, it must affect the rate. ∴ this reactant, or something derived from it, must be in the rate determining step.
Rate determining step doesn’t have to be the ___ step in a mechanism
first
Reaction mechanism can’t usually be predicted from just the ____ _____
chemical equation
What does the order of a reaction with respect to a reactant show?
Shows the number of molecules of that reactant that are involved in the rate determining step
Determine the rate equation from these equations
Rate determining step can sometimes involve an _____ that isn’t in the full equation
intermediate
rate = k[(CH3)2CBr]. State which mechanism is correct.
What does the Arrhenius Equation show?
How rate constant (k) varies with temperature (T) and activation energy (Ea)
State what each letter represents and their units
- k = rate constant
- Ea = activation energy (J)
- T = temperature (K)
- R = gas constant (8.31 J K-1 mol-1)
- A = the Arrhenius constant
Explain why as Ea increases, the rate constant decreases?
- Large Ea = slow rate
- If reactions has high Ea, not many reactant particles have enough energy to react
- ∴ few of collisions will result in reaction occurring & rate will be slow
Explain why as temperature increases, the rate constant increases?
- Higher temperatures mean reactant particles move around faster with more energy
- More likely to collide and more likely to collide with E ≥ Ea
- So reaction rate ↑
What can you use Arrhenius plot to find?
Ea or Arrhenius constant
How can you use Arrhenius equation to create an Arrhenius plot?
By plotting lnk against 1/T
State the gradient of an Arrhenius plot
-Ea/R
