9. Rate Equations Flashcards
What is rate of reaction?
Change in concentration of a substance per unit time
What is rate of reaction usually measured in?
mol dm⁻³ s⁻¹
What happens to the concentration of reactants and products as a chemical reaction progresses?
Concentration of the reactants decreases and the concentration of products increases
What is collision theory?
- for particles to react they must collide with sufficient energy and at the correct orientation
- the minimum energy particles need to react is called the activation energy
What factors affect the rate of reaction?
- concentration of a solution
- surface area of a solid
- pressure of gases
- temperature
- catalyst
Why does increasing concentration increase rate of reaction?
- more particles in a given volume
* more frequent collisions
Why does increasing the surface area of a solid increase the rate of reaction?
- more particles available to react
* more frequent collisions
Why does increasing the pressure of a gas increase the rate of reaction?
- more particles in a given space/same no. of particles in a smaller volume
- more frequent collisions
Why does increasing the temperature increase the rate of reaction?
- particles have more energy
- more particles with energy greater than or equal to Ea
- more frequent collisions
Why does adding a catalyst increase the rate of reaction?
Lowers Ea so more particles with energy greater than or equal to Ea
In terms of rate equations, what does the rate of a reaction depend on?
The temperature of the reaction and concentration of some or all of the reagents involved
What is the order of a reaction?
With respect to a given reactant, it is the power which the concentration of the reactant is raised in the rate equation
How can the order of the reaction be calculated?
Experimentally by measuring the change in concentration over time
What is the overall order of a reaction?
The sum of the powers of the concentration terms in the rate equation
What is the rate constant?
A constant that links the concentration of the reactants to the rate of a reaction
When is the rate constant the same?
For a certain reaction at a particular temperature
What happens when the rate constant is bigger?
It is a faster reaction
What is a zero order reactant?
One where the concentration has no effect on rate
What is a first order reactant?
One where doubling concentration doubles rate
What is a second order reactant?
One where doubling concentration quadruples rate
What does the units for k depend on?
The overall order of a reaction
When will the rate of a reaction increase without a change in concentration?
If the temperature increases or a catalyst is added
What does it mean for the rate equation, if there is a change in temperature or a catalyst is added?
The rate increases but concentration is constant, so the rate constant must increase
What happens to rate constant if the only change to a reaction is an increase in temperature?
It increases
What happens to rate constant if the only change to a reaction is an addition of a catalyst?
It increases
What is the equation for the relationship between k and temperature?
k = A e ^-Ea/RT
What does k stand for in k = A e ^-Ea/RT? Units?
Rate constant (units vary)
What does A stand for in k = A e ^-Ea/RT? Units?
Arrhenius constant (units vary)
What are the units for the Arrhenius constant?
They vary, but are the same as k
What does Ea stand for in k = A e ^-Ea/RT? Units?
Activation energy (J mol -1)
What does R stand for in k = A e ^-Ea/RT? Units?
Gas constant (J mol -1 K -1)
What does T stand for in k = A e ^-Ea/RT? Units?
Temperature (K)
What is the value of the gas constant?
8.31 J mol -1 K -1
Why does having a higher activation energy mean a slower rate?
There are fewer reacting particles with the activation energy, meaning very few collisions in a given time will result in a reaction
Why does a higher temperature mean a faster rate of reaction?
Reacting particles move faster as they have more energy, resulting in more collisions in a given time as they have the energy to do so
What does the natural log form of the Arrhenius equation equate to graphically?
The equation of a straight line graph - ln k against 1/T
What axes will the graph for the Arrhenius equation have?
ln k against 1/T
In the graph for the Arrhenius equation, what are the values of the y-intercept and gradient equal to?
- y-intercept = lnA
* gradient = -Ea/R
If rate of reaction is not affected by the concentration of a species, what order is the reaction with respect to that species?
Zero
If rate of reaction is directly proportional to the concentration of a species, what order is the reaction with respect to that species?
First
If rate of reaction is directly proportional to the square of the concentration of a species, what order is the reaction with respect to that species?
Second
How can the order of a reaction with respect to a given species be determined?
- initial rate data
* rate-concentration graphs
How to calculate order of reaction using initial rate data?
- Choose 2 experiments where the concentration of one of the reactants is the same
- For the same experiments, look at how the concentration of the other reactant changes
- See how the rate changes
- What is the relationship?
Why, after plotting a rate-concentration graph, is it usually necessary to plot a rate-time graph?
It is difficult to distinguish between first and second order using concentration-time
How can the rate of a reaction be found using a concentration-time graph?
By finding the gradient of the line at different points
What is the shape of a zero-order concentration-time graph?
Straight diagonal line
What is the shape of a first-order concentration-time graph?
Curve
What is the shape of a second-order concentration-time graph?
Curve
What is the shape of a zero-order rate-concentration graph?
Horizontal line
What is the shape of a first-order rate-concentration graph?
Straight diagonal line
What is the shape of a second-order rate-concentration graph?
Curve
What is the difference between first and second order concentration-time and rate-concentration graphs?
- concentration-time - both curves and hard to distinguish
* rate-concentration - first is straight line and second is a curve
How fast can the overall rate of reaction be?
Only as fast as the slowest step
What is the slowest step of a reaction?
The rate determining step
What is the rate determining step?
The slowest step of a reaction
What is the rate determining step used for?
- to determine the rate equation
* to determine the correct mechanism for a reaction
Does a step that occurs after the RDS affect rate?
No
Which steps in a reaction will not affect the rate?
Those after the rate determining step
What does it mean in terms of the rate equation, that steps after the rate determining step do not affect the rate?
They will not appear in the rate equation
Which steps in a reaction will affect the rate?
Those that are involved in the rate determining step or before, and will therefore appear in the rate equation
If you know the reactants involved in the rate determining step, what can be determined, as well as which step is the rate determining step?
The reaction mechanism
