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
- Repeat the reaction but changing concentrations of A B and C one at a time
- Work out the initial rate for each experiment (tangent at 0)
- Record the concentrations of reactants used for each experiment and their initial rates in a table
- Work out the orders of each reactant and form an equation
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 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 )
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
What does the whole rate of a reaction depend on the most in a multi step reaction
How quick the rate determining step is
Does a reactant in the rate determining step have to appear in the rate equation of a reaction
Yes
How do we find the order of a reactant in the rate determining step
The order of a reactant in the rate determining step is equal to the coefficient of that species in the rate determining step
e.g.
Rate determining step:
2C –> D
So C is in the rate equation with order 2
How do we find the order of reactants other than the one in the rate determining step in a multi step equation
The other species’ which the reactant in the rate determining step is derived from is also in the rate equation
You divide the order of the reactant in the rate determining step by the number of species it is derived from to get their order respectively
How do we calculate the rate determining step from a rate equation
How do we find what the most likely mechanism is for a reaction is from its rate equation
What is the Arrhenius equation
State what each component is and its units
k = Ae⁻ᴱᵃ/ᴿᵀ
k - Rate constant
A - Arrhenius constant
e- exponential
Ea - Activation energy (J)
R - Gas constant (8.31JK⁻¹mol⁻¹)
T - Temperature (K)
Explain the relationship between activation energy and the rate constant
As activation energy lowers, the rate constant increases.
As activation energy drops, the rate of reaction increases due to many more particles having enough energy to react when they collide (greater frequency of successful collisions due to more particles having energy greater than or equal to Ea)
Explain the relationship between temperature and the rate constant
As we increase the temperature, the rate constant increases.
Particles have more kinetic energy and are more likely to collide with energy greater than or equal to Ea. Hence, a greater frequency of successful collisions. Therefore the rate of reaction increases.
What is the Arrhenius equation rearranged for Ea
Ea = (lnA - lnk) x RT
Write the rearranged Arrhenius equation which we use to plot a graph
lnk = lnA - Ea/RT
When we plot a graph using the Arrhenius equation, what is on the x and y axis
X - 1/T
Y - lnk
What is the gradient represented by when we plot a graph using the Arrhenius equation
(-Ea/R)
How do we work out the Arrhenius constant from plotting a graph with the equation
Substitute the value of the gradient and the coordinates of any point into the equation: lnk = lnA - Ea/RT, and rearrange for A
State which components of the Arrhenius equation follow the form y =mx + c
lnk = (-Ea/R) X 1/T + lnA
y = m X x + c
y - lnk
m - (-Ea/R)
x - 1/T
c - lnA
