Reaction rates — Chemistry, 14–17 years
Reaction rate describes how quickly reactants are changed into products. Chemistry, 14–17 years.
How fast does a reaction happen?
Reaction rate measures how much a reactant disappears or a product appears in a certain time. Particles must collide in a suitable way and with enough energy for a reaction to occur. More successful collisions each second mean a faster reaction. Rate is about speed, not about how much product exists at the end.
Why control reaction speed?
Some reactions must be slowed down, such as food spoilage or corrosion, while others must be fast enough to be useful, such as fuel burning. Chemists investigate rate to choose safe temperatures, concentrations and catalysts. Without this idea, a reaction could be chemically possible but too slow, too violent or too difficult to control.
Measuring a reaction rate
Magnesium reacts with acid and produces 30 cm³ of hydrogen gas in 15 s. To find the average rate of gas production, divide volume by time: 30 cm³ ÷ 15 s = 2.0 cm³/s. If another experiment produces 30 cm³ in 30 s, its rate is 1.0 cm³/s. The first reaction was twice as fast under those conditions.
A catalyst does not make more product
People often think a catalyst increases the final amount of product because the reaction finishes sooner. This is understandable: a quicker process looks as if it has done more. A catalyst provides a different pathway with lower activation energy, so it changes the rate. It does not change the quantities fixed by the reaction or make the catalyst disappear.
Reaction rates around us
Refrigerators slow the reactions that spoil food, and paint or coatings can slow corrosion. Engines use controlled rapid combustion, while industrial catalysts speed up useful reactions without being consumed. These examples involve other factors too, such as heat transfer and mixing, so rate is one part of the explanation, not the whole story.
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