Rates Flashcards
How do we calculate rate from a graph
Gradient is equal to rate on a graph
Either by finding gradient of a straight line or drawing a tangent
What is the equation for rate of a reaction
rate = k[A]ᵃ[B]ᵇ
Where:
k is the rate constant
a and b are orders
[] are concentrations in the following equation A + B –> C + D
What is the units for rate
molmd⁻³s⁻¹
What does the order of a species in the rate equation tell us
Order is equal to the power
Tells us how the concentration of a substance affects the rate
If a substance has zero order what does this mean about its effect on the rate
Increasing or decreasing the concentration of this substance doesn’t affect the rate
If a substance has first order what does this mean about its effect on the rate
Changes in concentration has a proportional change on rate
e.g. if concentration doubles, rate also doubles
If a substance has second order what does this mean about its effect on the rate
Changes in concentration has a squared proportional change on rate
e.g. if concentration doubles, rate quadruples
What is the rate constant
A number that allows us to equate rate and concentration
What is the rate constant dependent on
Temperature
If the temperature increases so does the rate constant
What does a larger value of k (rate constant) tell us about the rate of a reaction
The larger the value of k, the faster the rate of the reaction
Explain why as temperature increases so does the rate constant
As we increase temperature particles have more kinetic energy and collide more often, therefore rate increases. However, concentrations of substances remain the same, so to balance the equation k must increase
What are the practical steps in calculating the rate equation of the reaction A+ B + C —> D + E
Perform a series of experiments, changing the concentration of one reactant (A, B, or C) at a time while keeping the others constant.
Measure the initial rate of reaction for each experiment by monitoring a property that changes with time (e.g. gas volume, colour, pH), plotting a graph of that property against time, and drawing a tangent at t = 0 to calculate the initial rate.
Record the concentrations of A, B, and C and their corresponding initial rates in a table.
Determine the order of reaction with respect to each reactant by observing how the rate changes when the concentration of that reactant is altered.
Use the results to write the rate equation in the form:
Rate = k[A]^m[B]^n[C]^p, where m, n, and p are the orders with respect to each reactant and k is the rate constant.
What is the equation for the iodine clock reaction
H2O2(aq) + 2H+(aq) + 2I-(aq) –> 2H2O + I2(aq)
How do we determine the order of reaction for a reactant in the iodine clock reaction
- Add a fixed amount of sodium thiosulphate and starch
- Thiosulphate ions react immediately with I2 produced in the iodine clock reaction
- When there is no more thiosulphate left, I2 then reacts with starch to give a deep blue-black colour
- We can then vary then concentration of iodine or hydrogen peroxide and keep everything else constant and see how that affects the time taken for the blue-black colour to appear (completely cover a cross on a piece of paper through the top of a beaker )
Graphically we can:
Plot a graph of concentration against time taken for colour to appear
Then take tangents at different points on that graph and calculate gradient to find rate
Then plot a graph of rate against concentration and analyse the shape of the line or curve to determine that species’ order
What 4 ways can rate be measured
Change in pH
Amount of mass lost
Volume of gas produced
Change in colour
How can we analyse a change in pH to measure the rate of a reaction
- pH may change if H+ are being used up or produced
- Measure initial pH and [H+]
- We can use a pH meter to calculate the pH of a reaction at regular intervals, then we can calculate H+ concentration
- Then rate = change in concentration/time
How can we analyse a change in mass to measure the rate of a reaction
-For reactions that produce a gas, place the reaction vessel on a balance and measure the mass of gas lost
-Perform a moles calculation to work out the number of moles of gas lost and hence the moles of reactants left and their concentration (SRE)
- Then rate = change in concentration/time
How can we analyse the volume of gas produced to measure the rate of a reaction
- Measure the amount of gas is produced using a gas syringe and measure this over a specific time
- Then use the ideal gas equation to calculate the number of moles of gas produced and then the moles of reactants left and their concentration
- Then rate = change in concentration/time
How can we analyse a change in colour to measure the rate of a reaction
- Use a colorimeter
- Plot a calibration curve using different concentrations of a reactant measuring its absorbance
- Then to calculate rate: measure the absorbance of the initial sample and use calibration curve to find concentration of reactant, then the absorbance of the final sample after the reaction has taken place and use calibration curve to find final concentration of reactant
- Then rate = change in concentration/time
What does a rate-concentration graph look like for a reactant with 0 order
What does a rate-concentration graph look like for a reactant with 1st order
What does a rate-concentration graph look like for a reactant with 2nd order
What is on the x and y axis of a rate concentration graph
X- Concentration of species
Y- Rate
What is the rate determining step
The slowest step in a multi-step reaction
