5.4-5.8 Quiz Flashcards
What happens to a Maxwell-Boltzmann graph as the temperature increases?
Since the average kinetic energy increases, the curve will become broader and flatter. As a result of this change, a greater number of particles will be able to overcome the activation energy barrier.
What does the result to Products - Reactants tell you about a reaction?
If it is positive then it is endothermic. If it is negative, then the reaction is exothermic.
Activation energy
Minimum amount of energy that the reacting particles need in order to react. If it is not reached then no reaction will occur even if there are collisions. This energy is used to break bonds between the particles available.
Rate Determining Step(RDS)
The slower step between the elementary reactions
What are three ways to increase the rate of a reaction?
1) Increase moles of reactant(will lead to higher concentration and with more particles you will have more collisions)
2) Decrease volume of container( A decreased volume also increases concentration and with more particles you will have more collisions)
3) Increase heat of reaction(More particles will have energy greater than the activation energy now)
Why would a reaction rate increase?
If you heat a reaction because more particles present have energy greater than the activation energy and they can react now. HEAT DOES NOT AFFECT ACTIVATION ENERGY ITSELF EVER
A student repeats a lab with the same concentrations but the reaction occurs much slower than it did the first time. The student realizes the temperature was lower than the day before though. Give two explanations on why the reaction was slower.
1) The temp was lower on the second day, which decreases the average kinetic energy of the particles. So, there were less collisions with enough energy to react.
2) With a lower temp came a lower kinetic energy and average speed of particles. At lower speeds, the particles collided less frequently.
How do you create a rate law from an elementary step?
You create the rate law based on the rate-determining step. Instead of finding out the reaction order, you will use the coefficients to determine the order of each element.
Why wouldn’t you see a large increase in the reaction rate when the temperature is increased if the activation energy is low?
If you have a low activation energy, there is a large portion of particles that can already make product so raising the temperature won’t affect it as much
How do you create a net equation?
You cancel out the intermediates, catalysts, and anything reused(meaning goes from reactant to product) in both steps in the equation
What should you know about the enthalpy equation?
IT’S ON THE EQUATION SHEET
How do you write the rate laws when you have to do it for multiple steps?
Make the constant k(subscript 1) or subscript 2
What will be the relative rate of a reaction with a high transition state energy peak? A low energy peak?
It will be slower because fewer collisions will have enough energy to make it over that hump.
It will be faster because most collisions will have enough energy to make it over the hump.
Consider this reaction: 2 NO + O2 yields 2 NO2
Is it likely for this reaction to occur in one step and why?
No because there are three particles in this reaction and it is unlikely for them all to collide at the same time and in the correct orientation
Is a low or high activation energy better and why?
A low activation energy is better because high activation energy means molecules are slower and take up a lot of energy just to use a few
If you are given a question with a single-step equation and it asks you to find the rate law what can you assume?
That single step has to be the RDS because it is the only step
What makes something a valid mechanism?
If its net equation matches the overall balanced equation and if its rate law reflects a specific mechanism. if both are available it needs to satisfy both
When does a reaction occur? (Think about bonds)
When bonds are broken and remade
Exothermic reaction
It releases energy. It has products with lower potential energy than reactants.
Catalyst
Reactant in the slow step and a product in a later step. It will not appear in the overall balanced equation. IT HAS TO HAVE THE SAME COEFFICIENTS
When asked about the relative rates of the steps what do they want to know?
They want you to say that step __ is slower/faster because the rate law reflects it/ or something else
Molecularity of a reaction
Number of molecules that have to collide in an elementary step
Endothermic reaction
It absorbs energy. It has products with higher potential than reactants.
Activated complex/ Transition state
This is the activation energy shown on a reaction energy profile. It is going to be the peak of your graph. At this point in the graph, bonds are beginning to weaken in the reactants to start reactions and bonds are beginning to form in the products.
What factors affect “k”?
Everything except concentration and partial pressure because that affects the reactants
What must always be true of the net equation?
It must always be the same as the overall balanced equation
What makes a collision between particles successful?
When they have sufficient energy to overcome energy barriers and orientations that allow the bonds to rearrange properly
Purpose of catalysts
They lower activation energy by speeding up reactions and using less energy. However, potential energy does not change. You will still start with the same amount of reactants and end with the same amount of products.
Intermediate
Species that are formed during one step and used up in another. It is not a reactant or a product in the overall equation.
Practice writing net equations
- 5.4 #3
If you are shown a Maxwell- Boltzmann graph and given the point of activation energy, how do you interpret the graph?
Everything to the right of the activation energy point has enough energy to react (assuming everything else goes right).
Everything to the left will not react even if they collide “correctly”.
Collision Theory
Explains why different reactions occur at different rates
In order for a reaction to take place:
1) Particles must collide. (The rate of the reaction is dependent of the frequency of the collisions).
2) They must have sufficient energy(aka must reach activation energy threshold)
3) Particles must be in the correct orientation
What does a catalyzed reaction profile look like?
The activated complex will be lower than the original.
Elementary reaction
Single step within a reaction mechanism
How do you increase the number of collisions?
Increasing concentration of the particles and increasing surface area
What does the value of k account for in the rate law?
Every change in a reaction rate except for concentration. This includes temperature and catalyst.
What should you do if asked for the rate law of the overall reaction?
STILL USE THE RDS; NEVER CREATE THE RATE LAW FROM THE OVERALL REACTION
A scientist calculated that 10% of the collisions that occur for a specific reaction are in the correct
orientation for a reaction to occur, but they found experimentally that the actual number of reactions that occurred was much lower than their calculation. Which factor did they forget to account for?
They forgot to factor if the collisions had enough energy.
Given the reaction: 2 NO2 (g)→ N2O4 (g) the rate of reaction was slow with 2.00 moles of NO2 in a 2.00 L
container at 1 atm and 285K. Suggest three different ways in increase the rate for this reaction, and
explain, with reference to collision theory, how each will alter the rate of reaction.
Increasing the moles of NO2 and decreasing volume of the container will both increase
the concentration of NO2, therefore increasing collisions. Increasing temperature will
lead to an increase in the # of particles with enough energy to have effective collisions
and form product. E > Ea.
Methane (CH4), the gas used for Bunsen burners, doesn’t react with oxygen in the air until a striker/flame is
brought near to the Bunsen burner.
a) Explain the role of the striker/flame.
b) What is the energy used for in the reaction?
a) Supplies activation energy. b) breaking bonds in the CH4 and O2.
