Reactions in nature occur at different speeds. Some reactions are very slow, like the erosion of rocks , while other reactions can be very rapid like the burning of wood. The speed of a reaction is often critical to the use man makes of the reaction. It is important, for example that concrete set over a period of hours , not seconds or minutes. The regulation of the speed is also critical for many reactions. Many reactions that occur in living organisms like the removal of carbon dioxide from cells in our body to the blood stream or the oxidation of glucose need to be speed up if they are to be useful . An understanding of what affects the speed of a reaction and how it can be modified is therefore very important to a chemist.
Examine the picture below.
racing cars 1. What are some of the chemical reactions that occur in this picture?
2. How could you determine the speed of the cars if they are driving on an oval track ?
3. Can the same method be used for the rate of a chemical change?
4. Can we apply the same formula to calculating the speed of the car to the speed of chemical reactions ?
5. What problems might we encounter in determining the rate of a chemical reaction vs a physical object like the car ?
In Physics , speed is the defined as the "change of " distance over a certain of time. The following formula is often used to describe this calculation.
Speed = Distance


Time


Types of Speed (objects)
We can calculate the speed of objects from graphs of distance vs time of the motion of objects. Two types of speed can be calculated using a Distance vs Time graph. They are:
  • The average speed (this is the slope of the best fit line on the graph.)
  • Instantaneous speed. This can be in measured at a point on the line by taking the slope of the tangent line to this point
a) Speed is a nonvector quantity that has magnitude (size) but not direction.
b) In terms of speed there is two basic types of motion:
  • Uniform motion which occurs when the speed of the cars remains the same throughout the time period you measured it's motion.
  • Nonuniform motion which occurs if the speed of the car varies over the time intervals you measured.
The following activities give you practice in graphing an objects motion and determining it average and instantaneous speed.


Activities on Speed
Perform the following activities to test your understanding of speed
Assignment #1: Graphing the motion of object
Lab #1:
HTML: Measuring speed of cars.
pdf: Measuring speed of cars.