Chemistry kinetics collision theory

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Chemistry kinetics collision theory

Chemistry

Chemistry kinetics collision theory

Collision Theory:

How fast will a reaction go? The reaction rate is the number of atoms, ions, or molecules that react in a given time to form products.

Reactions proceed at different rates. Collision theory provides an explanation for why reactions proceed at different rates.

Most chemical reactions involve the transfer of atoms from one molecule to another. For any reaction involving two or more reactants, the reacting particles must collide (contact between the reactants is very important). The more often the particles collide, the faster the reaction should go. The colliding particles must also have enough energy for bond breaking and bond making. Otherwise, the particles collide without reacting.

Activation energy is the minimum energy colliding particles must have in order to react. (A barrier that the reactants must cross to be converted into products.)

During a reaction, particles that are neither reactants nor products form momentarily. An activated complex or transition state is the arrangement of atoms at the peak of the activation barrier. The very unstable activated complex is equally likely to return to the reactants, as it is to go to products.

Factors that Affect the Rate of a Reaction:

Nature of reactants
the nature of the reactants will determine what kind of reaction will occur (fast or slow)
for example: ionic reactions do not involve electron transfer and occur rapidly because of the attraction of opposite charges and frequent collisions; reactions that require electron transfers and bond rearrangements take much longer ( i.e. neutral molecules)
Temperature
raising the temperature speeds up a reaction because at higher temperatures, the frequency and energy of the collisions become greater
more colliding molecules become energetic enough to slip over the activation energy barrier to become products

Concentration
the number of reacting particles in a given volume affects the rate at which reactions occur; cramming more particles into a fixed volume increases the collision frequency, therefore increasing the concentration increases the reaction rate

Particle Size
the smaller the particle size, the larger the surface area for a given mass; this increases the collision frequency

Catalyst
a catalyst is a substance that speeds up the rate of a reaction without being used up in the process
it is written above the yield sign, or appears on both sides of the equation (a catalyst is used in one step and produced in a later one)
catalysts provide a path requiring lower activation energy (with a lower activation energy, more reactants can form products)
a substance that interferes with a catalyst is called an inhibitor

Reaction Mechanism:

In an elementary reaction, reactants are converted into products in a single step (one intervening activated complex)

Most reactions do not take place in a single step and consist of a number of elementary reactions

In general, each step only involves two particles colliding. It is not likely that three or more particles would collide in the right position and have the right energy for a proper reaction to occur.

In a reaction involving several steps, one of the steps will always proceed slower than the others ( called the rate determining step)

The faster steps do not affect the rate of a reaction

The series of steps (elementary reactions) that must occur for the reaction to be complete is called the reaction mechanism.

For complex or multi-step reactions, intermediates form as products that immediately become reactants in the next reaction. Intermediates are shown in a reaction mechanism, but not in the overall net reaction. (Therefore just looking at an overall reaction tells us nothing about the actual reaction mechanism). They have real ionic or molecular structures and some stability. They are reactive enough, however, to react further to eventually give the final product for a reaction.

Potential energy diagram and activation energy diagram (with and without the use of a catalyst)

Example:

For the reaction mechanism below:

list the intermediate(s), if any
indicate the rate determining step
write the overall equation for the mechanism

2NO ® N₂O₂ (fast)

N₂O₂ + H₂® H₂O + N₂O (slow)

N₂O + H₂® N₂ + H₂O (fast)

CHEMISTRY 12 – KINETICS ASSIGNMENT

Refrigerated food stays fresh for long periods. The same food stored at room temperature quickly spoils. Explain.

a) What is meant by the rate of a reaction?
How does each of the following factors affect the rate of a chemical reaction?

- concentration

- particle size

- inhibitor

a) Using the collision theory, predict why it is unlikely that the

reaction: 2C₂H₆ + 7O₂® 4CO₂ + 6H₂O represents an elementary

reaction.

Does every collision between reacting particles lead to products? What other factor is involved?

How is the rate of a reaction influenced by a catalyst? How do catalysts make this possible?

5) For the reaction mechanisms below,

list the intermediate(s), if any
indicate the rate determining step
write the overall equation for each mechanism

Br₂® 2Br^- (slow)

Br^- + H₂® HBr + H⁺ (fast)

H⁺ + Br₂® HBr + Br^- (fast)

ii) I₂® 2I^- (fast)

H₂ + 2I^-® 2HI (slow)

6) Sketch & label a potential energy diagram for this reaction mechanism:

2NO ® N₂O₂ (fast)

N₂O₂ + O₂® 2NO₂ (slow)

Write the balanced equation for the overall reaction.

Source : http://www.horton.ednet.ns.ca/staff/coldwell/Chemistry%20(4%20folders%20-%20Anita)/AP%20CHEMISTRY/Kinetics/CHEM%2012%20NOTES-%20kinetics.doc

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