Reaction rate — Chemistry, 11–13 years
Chemical reactions do not all happen at the same speed. Temperature, concentration, surface area and catalysts can change how often successful collisions occur.
Fast and slow reactions
Reaction rate describes how quickly reactants are changed into products. A fast reaction may finish in seconds, while a slow one may take days or years. The particles must meet in suitable ways, so changing their temperature, number or available surface can change the speed.
Why control the speed
Knowing that a reaction happens is not enough: a fire must spread quickly enough to warm a room, but food must not spoil quickly. Chemists therefore needed ways to speed useful reactions up and slow harmful ones down. Rate gives a way to compare these changes and design safer, more reliable processes.
Comparing magnesium strips
Put a 2 cm magnesium strip into acid and record the gas made each minute. Suppose it produces 12 mL in 1 minute and 30 mL in 3 minutes. Its average rate over those 3 minutes is 30 ÷ 3 = 10 mL per minute. A warmer acid may give a larger rate because particles move and collide more often.
Fast does not mean more
A reasonable mistake is to think that a faster reaction must make more product. Speed tells us how quickly change happens, not the final amount made. The same amount of magnesium can produce the same total gas in two experiments, even if one reaction finishes sooner.
Controlling real reactions
A refrigerator slows the reactions that spoil food, while crushing a tablet can give water more surface to reach it and make it dissolve faster. Catalysts help some industrial reactions happen efficiently without being used up. These examples are useful only when the particular reaction really responds to that change.
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