6/16- Chemical Kinetics Questions

Define activation energy.

Activation energy (Ea) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction.

State and explain factors that determines successful collision.

Species react as a result of collisions of sufficient energy and proper orientation.

-incorrect orientation = particles bounce apart = no reaction

-sufficient energy needs to be equal or greater than activation energy to overcome it and start the reaction.

How is the rate of expression measured? (its units?)

The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

How can change in concentration of a reaction be measured?

Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.

changes in volume (measurement of the rate at which CO2 is produced):

**mostly the reactions of metals and metal carbonates with acids such as HCl.

-higher [acid] = more gas produced = higher rate of reaction

changes in mass:

-the mass decreases as carbon dioxide is given off

-higher [acid] = more change in mass = higher rate of reaction

color:

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Sketch the Maxwell-Boltzmann distribution curve, and explain it in terms of population of molecules (gas, liquid, solution), and its spread of kinetic energies.

y axis: fraction of particles

x axis: kinetic energy

-area under each curve is the same because the number of particles is constant

-kinetic energy has increased (the highest point on the curve has moved to the right), for higher temperatures.

-lighter gaseous molecules (molar mass), on average, move faster than heavier gaseous molecules.

Explain what catalysts do.

catalyst (or an enzyme) speeds up a chemical reaction by providing an alternative reaction pathway of lower activation energy.

Describe how the rate of a reaction varies as the reaction proceeds and how this is illustrated graphically, and how it is measured?

-rate of reaction at a given time during the reaction can be deduced by drawing a tangent to the curve representing the course of the reaction.

Describe how the kinetics of a reaction can be followed.

Describe how the kinetics of a reaction can be followed by tracking the concentration of a particular reactant or product with time.

Explain how changing various conditions of a reaction alters the rate of a reaction.

concentration, surface area of a solid or pressure of a gas, temperature.

-alters the rate of a reaction by increasing the frequency of collisions between reacting particles.

[reactants]:

-more particles = higher frequency of collision = greater chance of a successful collision

surface area:

*reaction only occur on surface of solid

-more SA = more # of particles exposed at the surface = greater chance of a particle of the other reactant colliding with a particle

pressure:

-increasing pressure = less volume = higher concentration = collision frequency increases

temperature:

-the rate is increased because the particles collide more frequently;

-the particles have greater kinetic energy = a greater proportion of the collisions have energy exceeding the activation energy of the reaction.

Describe collision theory.

collision frequency;

two particles must collide;

particles must have sufficient energy to overcome the activation energy/E⩾EaE⩾Ea;

Concept of activation energy must be mentioned.

appropriate collision geometry/orientation;

What are the ways in which the progress of the reaction can be monitored. No practical details are required.

loss of mass «of reaction mixture/CO2»

«increase in» volume of gas produced

change of conductivity

change of pH

change in temperature

Explain the curve in graph showing product and time using the collision theory.

«initially steep because» greatest concentration/number of particles at start

OR

«slope decreases because» concentration/number of particles decreases

volume produced per unit of time depends on frequency of collisions

OR

rate depends on frequency of collisions

Define the term rate of reaction.

decrease in concentration/mass/amount/volume of reactant with time / increase in concentration/mass/amount/volume of product with time / change in concentration/mass/amount/volume of reactant/product with time;

Describe the role of catalysts.

Catalysts alter a reaction mechanism, introducing a step with lower activation energy.

Sketch the graphs showing the relationship between concentration and time of zero, first, and second order reaction.

Refer to notes.

Sketch the graphs showing the relationship between rate of reaction and concentration of zero, first, and second order reaction.

Refer to notes.

What is the relationship between rate contant (k) and temperature?

The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction.

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State the equation for rate expression, and overall order of reaction.

rate = k[A]^x[B]^y

x and y refers to the order of reaction with respected to A and B.

overall order = x + y

Describe the occurrence of rate determining step.

Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining step/RDS).

-Catalysts are involved in the rate-determining step.

**If the RDS is in the second step of the reaction, the reactants in the first step has to be included in the final equation.

State how to find the rate expression for an elementary step.

The molecularity of an elementary step is the number of reactant particles taking part in that step.

e.g.

A + B -> C

rate = k[A][B]

2A + 2B -> C

rate = k[A]^2[B]^2

Outline how catalysts work.

Catalysts alter a reaction mechanism, introducing a step with lower activation energy.