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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 2: When Molecules Collide

For two molecules to react, they must come in contact with each other; they must collide. But not all collisions result in a reaction; the molecules must collide with the correct orientation. In the reaction between HCl and C2H4, a productive collision only happens when the hydrogen of HCl approaches the double bond in C2H4. In other collisions, no reaction will occur. (Figure 12-2)

The reaction rate will increase if productive collisions happen more frequently. One way to increase collision frequency is to pack more molecules into a smaller space: Increase the concentration. An acid can quickly cause chemical burns if its concentration is high; even the acetic acid in harmless vinegar is hazardous when highly purified. The tanks of concentrated oxygen for medical use carry warning labels due to the risk of fire, and liquid oxygen is so concentrated that it can support the combustion of a diamond.

Collisions and Chemical Reactions

Figure 12-2. Collisions and Chemical Reactions

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Collisions and Chemical Reactions

Figure 12-2. Collisions and Chemical Reactions

Not all collisions between reactants result in a chemical reaction. This figure shows four collisions in which the reactants have different orientations; only the first orientation allows for a productive collision.

Rate of Reaction and Surface Area

Figure 12-3. Rate of Reaction and Surface Area

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Rate of Reaction and Surface Area

Figure 12-3. Rate of Reaction and Surface Area

The reaction on the right happens faster because the surface area of the reactants (in this case, baking soda) is greater.

Another way to speed up a reaction is to increase the surface area of the reactants. For example, a large chunk of baking soda will react slowly with vinegar because the two substances can only react where they make contact with each other—at the surface. If the baking soda is broken into small particles, the vinegar will make contact with a lot more of the baking soda and the reaction will happen faster. (Figure 12-3)

A substance that is normally safe and inert can become dangerously reactive in powdered form. A pile of flour exposed to a flame will become singed on the surface but will not catch fire. But if the flour is dispersed as a cloud in the air, it becomes extremely flammable due to the increased contact between atmospheric oxygen and flour particles. Fires and explosions used to be major hazards in flour mills. After the Great Mill Disaster in Minneapolis in 1878 killed 18 people, flour mills began using precautions, such as ventilation and spark prevention.

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