Module 5: How Fast? V1 Flashcards
How could you follow the rate of the following reaction? Include the apparatus.
Measure decreasing light absorbance as orange Br2 decolourises using a colorimeter ✓
Measure the decreasing pH due to increasing H+ and Br- ions using a pH meter ✓
Measure increasing conductivity due to more H+ and Br- ions using a conductivity meter ✓
Measure the increasing volume of CO2 gas produced using a gas syringe ✓
Measure the decreasing mass of the solution until constant mass using a balance ✓
Define what is meant by a “Reaction order”
How the rate of reaction will be affected if the reactant’s concentration is changed ✓
Define what is meant by:
zero order
first order
second order
Zero order: The rate is unaffected by any changes in concentration. ✓
First order: The rate changes the same as the change in concentration. ✓
Second order: The rate changes by the square of the change in concentration. ✓
Sketch a concentration-time graph, with labelled axis, for a 0, 1st and 2nd order concentration-graph
State a key feature for each one
Sketch a rate-concentration graph, with labelled axis, for a 0, 1st and 2nd order concentration-graph
State a key feature for each one
Define what is meant by “half-life” and with which order can calculate a value for the rate constant for
The half-life (t1/2) of a reactant is the time taken for its concentration to halve ✓
For reactant A, the concentration-time graph has a constant half-life. For reactant B, the rate-concentration graph is a horizontal line.
Determine the order for reactants A and B.
For reactant C, the concentration-time graph has a constant gradient. For reactant D, the rate-concentration graph also has a constant gradient with the line passing through the origin.
Determine the order for reactants C and D.
Show that the order is 2 wrt [A] and 0 wrt to [B].
In experiments 1 and 2, [A] X2 whilst [B] is constant and rate X4. ✓
2nd order wrt [A] ✓
In experiments 2 and 3, [B] X2 whilst [A] is constant and rate does not change. ✓
0 wrt [B] ✓
Overall order is 2nd + 0 = 2nd order ✓
The following data was obtained for the bromine clock reaction.
i) Determine the order with respect to Br -.
(ii) Determine the order with respect to BrO3-.
(iii) Determine the order with respect to H+.
(iv) Determine the overall order.
i) 1 ✓
(ii) 1 ✓
(iii) 2 ✓
(iv) 4 ✓
For a reaction involving the reactants A, B and C where the order is 2, 1 and 0 respectively.
Give the rate equation
State the overall order
Rearrange the rate equation for k and work out its units.
Order 2 ✓
Look at the units. The order is based on what the units of the rate constant is.
A reaction is first order with respect to a reactantX.
Which rate–concentration graph for reactantXis the correct shape?
A ✓
A reaction is zero order with respect to a reactantA.
Which concentration–time graph for reactantAis the correct shape?
A ✓
20 minutes ✓
Find the order for the reactants A, B, C & D
i) [A] x2 and rate x2, order wrt [A] is…
ii). [B] x2 and rate does not change, order wrt [B] is…
iii) [C] x3 and rate x9, order wrt [C] is…
iv) [D] x3 and [A] x3, rate x9, order wrt [D] is…
i) First ✓
ii) Zero ✓
iii) Second ✓
iv) First ✓
The half-life of a first order reaction is 300 seconds.
Calculate the rate constant for the reaction and give its units.
Describe the initial rates method
Start by mixing equal amounts of each solution and find the rate of reaction. ✓
Repeat the experiment, diluting each solution in turn with distilled water. ✓
Change the concentration of only one solution at a time. ✓
Keep the total volume the same for each experiment. ✓
Put the results in a table and check how changing the concentration affects the rate of reaction. ✓
State what happens to the rate constant k, as temperature increases.
Include a sketch of a graph in your answer
Increases ✓
State the effect on k and hence the rate of reaction if the following are increased:
If the pre-exponential factor increases
If activation energy increases
If temperature increases
If the pre-exponential factor increases, k increases
If activation energy increases, k decreases
If temperature increases, k increases
In the Arrhenius equation, state what each term, stands for in terms of y=mx+c
y - y coordiantes = ln(k)
x - x coordiantes = 1/T
c - y Intercept = lnA
m - Gradient = -Ea/R
Activation energy in the Arrhenius equation is always given and used in what unit?
The rate determining step for a reaction between X and Y is given as 2 X → Z.
Predict the rate equation for this reaction.
Define what is meant by the “rate of reaction”
change in concentration of reactant or product per unit time for the change to take place ✓
Describe how you would draw a rate-concentration graph
Use the continuous monitoring method as for determining a concentration-time graph ✓
Find the rate (from the gradient) at various points along the concentration-time curve ✓
You would then plot a rate-concentration graph ✓
For reactant A, the concentration-time graph has a constant half-life. For reactant B, the rate-concentration graph is a horizontal line. Determine the
A: First order ✓
B: Zero order ✓
For reactant C, the concentration-time graph has a constant gradient. For reactant D, the rate-concentration graph also has a constant gradient with the line passing through the origin. Determine the order for reactants C and D.
c: Zero order ✓
B: First order ✓
Define what is meant by the “Rate determining step”
Slowest step in the reaction mechanism ✓
A reaction can be shown as being first order wrt [H+] by continuous monitoring
State how you would use the graph to show this first order relationship
Calculate multiple half-lifes ✓
Constant half life shows first order ✓
There would be a x125 increase in rate ✓
If you were to follow the reaction by measuring the mass lost, how would you know the reaction had gone to completion?
Weigh to constant mass ✓
What apparatus would you use to follow a reaction with a colour change?
Colorimeter ✓
What effect does an increase in pressure have on the reaction rate and rate constant
Rate of reaction increases and rate constant k increases ✓
There are more molecules per unit volume ✓
There are more frequent collisions ✓
What effect does a decrease in volume have on the reaction rate and rate constant
Pressure increases, rate of reaction increases and rate constant k increases ✓
There are more molecules per unit volume ✓
There are more frequent collisions ✓
What effect does an increase in temperature have on the reaction rate and rate constant
Rate of reaction increases and rate constant k increases ✓
More molecules have energy above the activation energy ✓
There are more frequent collisions ✓
In rates experiment, a student is trying to find out the order wrt [Y]
In the reaction, X reacts with Y.
He thinks that both reactants are first order, wrt to [X] and [Y]
How can the student modify the experiment to show that the reaction is first order wrt [Y]
Use a large excess of [X] ✓
So its concentration is kept constant and monitor [Y] against time ✓
20 minutes ✓
First order, so half life is constant
+37.6 ✓
Remember to convert from J to kJ
