LITo explain how concentration and catalysts change reaction rate, using collision theory.
SC: I can:
Try these 2questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.
Two beakers contain the same mass of marble chips at the same temperature. Beaker A: filled with dilute hydrochloric acid. Beaker B: filled with concentrated hydrochloric acid (same volume). Which beaker reacts faster, and why?
You met all four in L11 (collision theory). L13 took the first two; today closes the block.
| Factor | Easy to control? | Everyday handle |
|---|---|---|
| Temperature | yes | fridge vs kitchen bench (L13) |
| Surface area | yes | whole vs crushed solid (L13) |
| Concentration (today) | yes | dilute vs concentrated acid |
| Catalyst (today) | needs the right substance | enzymes, catalytic converters |
Concentration is how many particles are packed into a given volume of solution or gas. Push concentration up and the rate goes up — here is the chain:
Figure 4.27 — Same volume, more particles. Higher concentration means particles collide more often, so the rate goes up.
A catalyst speeds up a reaction by lowering its activation energy — the energy hump particles must clear before they react. The catalyst itself is not used up. When writing the equation we put the catalyst above the arrow to show it is present but not consumed (Figure 4.31).
Catalysts are specific, which means that a catalyst increases the rate of some reactions but not others. Catalysts are important in industry because they reduce reaction times and enable chemical processes to take place at lower temperatures and pressures. Therefore, they reduce the cost of production of valuable products.
Enzymes are biological catalysts. They are proteins that help speed up chemical reactions in the body. For example, amylase in saliva catalyses the breakdown of carbohydrates in your mouth.
Wrong — diluting lowers the concentration of acid particles per unit volume, so collisions per second go down and the reaction goes slower.
If there are fewer acid particles in the same volume, there are fewer collisions with the marble chips each second, so the fizzing is slower.
Rule: dilution slows the reaction; concentration ↑ speeds it.
Wrong — the defining feature of a catalyst is that it is NOT consumed. In a chemical equation, the catalyst is often written above the arrow to show it is present but not one of the reactants or products.
A catalyst is regenerated at the end of every individual reaction it speeds up. Same amount before and after.
Rule: catalyst in = catalyst out. If it's used up, it's a reactant, not a catalyst.
Wrong — a catalyst changes how fast the reaction goes, not how much product you get.
The amount of product is limited by how much reactant is available. A catalyst only changes the rate — you reach the same final amount sooner.
It also doesn't change whether the reaction is exothermic or endothermic. Catalysts only lower Eₐ; they don't shift where the reactants and products sit on the energy axis (you'll see this in §4.7).
Rule: catalyst → faster, not bigger yield.
Fill in the blanks to complete the textbook definition of a catalyst.
One thing I now understand about concentration or catalysts that I didn't understand at the start of the lesson:
Next class (Wed 20 May, P5): §4.7 — exothermic and endothermic reactions and energy profile diagrams (the merged L15+L16 lesson).