Chemistry: reaction rates and equilibrium (grade 10) · lesson

Collision theory

💡 Reactions go faster when particles collide more often and with more energy.

Particles must hit each other with enough energy to react. Anything that makes useful collisions more frequent speeds up a reaction.

Higher temperature makes particles move faster, so they collide more often and harder. Higher concentration puts more particles in the same space. More surface area, like a crushed tablet or powder, exposes more particles at once.

🧠 How to think about it

For each change, ask: does it make collisions more frequent or stronger? Cooling and a single big lump do the opposite and slow the reaction.

concentration: how much of a substance is in a given volume
surface area: how much of a solid is exposed

🔗 PhET: Reactions & Rates · PhET Interactive Simulations (free) ↗

Catalysts

💡 A catalyst lowers the energy needed to react, so it speeds a reaction without being used up.

Colliding particles must get over an energy barrier called the activation energy. A catalyst gives the reaction an easier path with a lower barrier.

A catalyst is not used up, and it does not heat the mixture. It speeds up the forward and reverse reactions equally, so it does not change the amounts at equilibrium.

🧠 Let's work it out

Picture a hill that particles must climb.
A catalyst digs a lower path through the hill.
More particles make it over, but the start and end points stay the same.

What do you think comes next?

activation energy: the energy barrier particles must get over to react

Reading a rate experiment

💡 Change one variable, keep the rest the same, and use one clear rule for measuring.

In the Fizz Race, temperature was changed in part one, with tablet size and water amount held the same. In part two, crushing was tested at a fixed 25 °C. The time to stop fizzing was the measured result.

A shared rule, like "no bubbles for five seconds," makes every trial end the same way. That makes the trials fair to compare.

🧠 Let's work it out

Read the trend: 10 °C took 90 s, 25 °C took 45 s, 40 °C took 20 s.
Warmer was always faster, so 55 °C should take less than 20 s.
And 90 ÷ 45 = 2, so 25 °C was twice as fast as 10 °C.

What do you think comes next?

rate: how fast something happens

🔗 Khan Academy: Reaction rates and equilibrium · Khan Academy (free) ↗

Equilibrium shifts

💡 A system at equilibrium shifts to partly undo any change made to it.

In a reversible reaction, the forward and reverse reactions keep going. At equilibrium they run at equal rates, so amounts stay steady. Nothing has stopped.

Add a reactant, and the system makes more product. Remove a product, and it makes more to replace it. Squeeze gases into a smaller space, and it shifts toward the side with fewer gas molecules.

Treat heat like a reactant or product. If the forward reaction releases heat, adding heat favors the reverse reaction.

🧠 Let's work it out

Name the change, then ask how the system could push back.
Opening a soda removes CO₂ gas, so dissolved CO₂ comes out to replace it.
The soda goes flat.

What do you think comes next?

equilibrium: a balance where forward and reverse reactions run at equal rates