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PRELIMINARY COURSE ASSESSMENT CHEMISTRY: REACTION RATES
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HOMEPAGE
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Page: 2 of 10 (* currently answering part (a) )
Reaction Rate and Concentration of Reactants
Doubling the concentration of a reactant can double the reaction rate since there will be twice as many possibilities for a collision to occur. Doubling the concentration of two reactants can quadruple the reaction rate since each reactant has twice the chance of colliding. Chemical reactions ordinarily occur as a result of collisions between reacting particles. Consider the following reactions:
Example 1 CO(g) + NO2(g) ® CO2(g) + NO(g) Seeing that doubling the concentration of CO and holding NO2 constant, the number of collisions in a given time doubles. Also doubling the concentration of NO2 and holding CO constant, has the same effect. In general, the number of collisions per unit time is directly proportional to the concentration of CO or NO2. The fact that the rate is directly proportional to these concentrations implies that reaction occurs as a direct result of collisions between CO and NO2 molecules. Not every collision leads to a reaction. Consider a mixture of CO and NO2 at 700 K and a concentration of 0.10 mol/L. Theoretically; every molecule should collide with about a billion other molecules in one second, and if every collision resulted in a reaction, then the whole mixture would be reacted in a fraction of a second. But the actual reaction takes about 20 seconds.
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