Rate Of Reactions Flashcards
How can a colorimeter be used to measure rate?
- A colorimeter measures the absorbance of light by a coloured sample.
- The more concentrated, the darker the colour
- more light is absorbed.
- Then plot a calibration curve
- known concentrations of (iodine) on x-axis.
- The absorption is measured for sample
- results are plotted
- then use curve and absorbance to find out [Iodine]
What is the initial rate?
the rate right at the start of the reaction when T=0
Describe a clock reaction
- You time how long it takes for a reaction to occur
- is used to simplify the initial rate method
- sit the reaction vessel on a paper with a cross
- Time how long is takes until you cant see the cross through the beaker
- There is a colour change
- This is the end point
- The quicker the clock reaction, the faster the initial rate is
What are 3 assumptions of the clock experiments?
- Temperature of the reaction remains constant
- no significant change in conc of reactants during the reaction
- Reaction has not proceeded too far when the end point is seen.
Describe the iodine clock experiment
- Add sodium thiosulfate and starch (indicator) to excess hydrogen peroxide + iodide ions in sol
- H2O2 + 2I- + 2H+ = 2H20 + I2
- Sodium thiosulfate reacts immediately with the Iodine produced
- 2S2O3(2-) + I2 = 2I- + S4O6(2-)
- iodine from first reaction used up in second
- when ST used up, some iodine left in solution
- starch indicator = blue/black
- end of clock reaction
What is an order?
- An order is the power to which a concentration is raised to in the rate equation
- Tells us how the concentration of the substance affects the rate
What is zero order on a rate-conc graph?
- Changes in concentration has no effect on rate
- If [A] doubles then rate doesn’t change .
- produces a horizontal straight line with no gradient
What is first order on a rate-conc graph?
- Changes in concentration has a proportional change to rate
- If [A] doubles then rate doubles.
- A positive line passing through the origin.
What is second order on a rate-conc graph?
- Changes in concentration has a squared proportional change on rate.
- If [A] double then rate x4
- Forms an exponential curve
How to get rate from second order graph?
- Rate-conc graph is a curve
- Plot a second graph of rate against the conc squared.
- Results in a straight line through the origin.
- The gradient of the straight line is equal to the rate constant k.
What does the value of ‘k’ link to rate mean?
- Larger the value of ‘k’ the faster the rate of reaction
Describe zero order on a rate-time graph
- straight line with a negative gradient
- reaction rate doesn’t change during the reaction
- gradient = k
Describe first order on a rate-time graph
- downward curve with a decreasing gradient
- time for the conc of the reactant to halve is constant
- half life = k
Describe second order on a rate-time graph
- downward curve like 1st
- a lot steeper at the start.
What is half life?
- time taken for half of a reactant to be used up.
How to calculate k for a first order reaction?
k = ln2 / t(1/2)
what is the rate determining step?
- the slowest step in a multi-step reaction.
- substances that aren’t in the rate equation can’t be in the rate determining step.
What is the arrhenius equation?
k = Ae^-Ea/RT k = rate constant Ea = activation energy (J mol -1) T = temperature (K) R = gas constant (8.31) A = pre exponential factor
What is the relationship between Ea and k?
- As Ea gets smaller, k gets bigger.
- Bc as Ea drops, ROR increases as particles have enough energy to react when they collide.
- large Ea = slow rate
Effect of increase in temp on k?
- Increase in temperature, increases k
- particles have more kinetic energy
more likely to collide with at least the Ea - ROR increases.
What is the log version of the Arrhenius equation
lnk = lnA - Ea / RT Ea = (lnA - lnk) x RT
How to use Arrhenius equation when given T and k table?
-Ea/R = gradient
lnA = y intercept
- first convert to 1/T and lnk using values in the table.
- Plot a graph of lnk(y) against 1/T(x)
- calculate the Ea/R from the -gradient of the straight line
- calc Ea by multiplying -gradient by R(8.31)
- Calculate lnA by extrapolating your line back to T=0/y - intercept.
- To calculate A use inverse of ln = e^lnA
what is ROR
- ways to measure
- change in the amount of reactants/products per unit time
- measure volume of gas given off
- measure loss in mass as gas is evolved
- colorimetry to measure colour change of a Rt
- pH change of a reaction
