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Factors Affecting Reaction Rate

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What are the chances of a fender-bender on the highway?

Driving on a crowded freeway can get exciting. Lots of cars, drivers who aren’t paying attention, people who speed, people who drive too slow – the chances of a collision are rather high. A lot of cars in a particular amount of space equals a high car concentration and many opportunities for unwanted connections with other cars.

Factors Affecting Reaction Rates

By their nature, some reactions occur very quickly, while others are very slow. However, certain changes in the reacting conditions can have an effect on the rate of a given chemical reaction. Collision theory can be utilized to explain these rate effects.

Concentration

An increase in the concentration of one or more reacting substances results in an increase in the rate of reaction. When more particles are present in a given amount of space, a greater number of collisions will naturally occur between those particles. Since the rate of a reaction is dependent on the number of collisions occurring between reactants, the rate increases as the concentration increases.

Pressure

When the pressure of a gas is increased its particles are forced closer together, decreasing in the process the amount of empty space between the particles. Therefore, an increase in the pressure of a gas is also an increase in the concentration of the gas. For gaseous reactions, an increase in pressure increases the rate of reaction for the same reasons as described for an increase in concentration. Higher gas pressure leads to a greater number of collisions between reacting particles.

Surface Area

A large log placed in a fire will burn relatively slowly. If the same mass of wood were added to the fire in the form of small twigs, they would burn much more quickly. This is because the twigs provide a greater surface area than the log does. An increase in the surface area of a reactant increases the rate of a reaction. Surface area is larger when a given amount of a solid is present as smaller particles. A powdered reactant has a greater surface area than the same reactant as a solid chunk. In order to increase the surface area of a substance, it may be ground into smaller particles or dissolved into a liquid. In solution, the dissolved particles are separated from each other and will react more quickly with other reactants.

Temperature

Raising the temperature of a chemical reaction usually results in a higher rate of reaction. When the reactant particles are heated, they move faster and faster. This results in a greater frequency of collisions. A more important effect of the temperature increase is that the collisions occur with a greater force and are thus more likely to surmount the activation energy barrier and go on to form products. Increasing the temperature of a reaction increases the number of effective collisions between reacting particles, so the reaction rate increases.

Summary

  • Factors affecting reaction rate are:
    • concentration of reactants
    • pressure (if gas)
    • surface area
    • temperature

Practice

Watch the video at the link below and answer the following questions:

http://www.youtube.com/watch?v=UWkzS-HaxZE

  1. What factor was demonstrated by adding oxygen to the fire?
  2. What was shown with the aluminum powder?
  3. What factor was mentioned, but not demonstrated?

Review

  1. How does an increase in concentration of reactant increase rate?
  2. Why would rates increase with a larger surface area of reactants?
  3. What effect does temperature have on reaction rate?

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