Lesson 4

Equilibrium basics

<p>Learn about Equilibrium basics in this comprehensive lesson.</p>

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Why This Matters

Imagine you're on a seesaw with your friend. Sometimes you're up, sometimes they're up. But what if you both weigh exactly the same and sit at just the right spot? The seesaw stops moving, right? It's balanced! That's a bit like **equilibrium** in chemistry. It's when a chemical reaction seems to stop, but it's actually still happening in both directions at the same speed. It's super important because it explains why some reactions don't go all the way to completion and how we can control them in factories to make useful stuff like ammonia for fertilisers. Understanding equilibrium helps us predict what happens when we change things like temperature or pressure, which is a powerful tool for chemists!

Key Words to Know

01
Equilibrium — A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction.
02
Dynamic Equilibrium — The continuous movement of reactants to products and products to reactants, even when the overall concentrations remain constant.
03
Reversible Reaction — A chemical reaction that can proceed in both the forward direction (reactants to products) and the reverse direction (products to reactants).
04
Forward Reaction — The reaction where reactants are converted into products.
05
Reverse Reaction — The reaction where products are converted back into reactants.
06
Rate of Reaction — How quickly reactants are used up or products are formed over a period of time.
07
Closed System — A system where no matter (reactants or products) can enter or leave, though energy can be exchanged.
08
Concentration — The amount of a substance present in a given volume, often measured in moles per cubic decimeter (mol dm⁻³).

What Is This? (The Simple Version)

Think of it like a tug-of-war that never ends. You have two teams pulling on a rope. At the start, one team might be much stronger and pull the rope quickly to their side. But then, maybe the other team gets a burst of energy, or the first team gets tired. Eventually, if both teams pull with the exact same strength in opposite directions, the rope stops moving, right? It's not that they've stopped pulling; they're just pulling equally hard!

In chemistry, equilibrium (say: ee-kwih-LIB-ree-um) is when a chemical reaction reaches a point where the forward reaction (reactants turning into products) and the reverse reaction (products turning back into reactants) are happening at the same speed.

This means the amounts of reactants and products stay constant, even though the reactions are still going on. It's a dynamic (meaning 'always moving') state, not a static (meaning 'still') one. Like our tug-of-war, the teams are still pulling, but the rope isn't going anywhere!

Real-World Example

Let's imagine a busy shop with two doors: an entrance and an exit. At the start of the day, lots of people come into the shop (the 'forward reaction'). Soon, people start leaving through the exit (the 'reverse reaction').

  1. Beginning: Many people enter, few leave. The shop gets fuller.
  2. Middle: More people are now leaving, but still more are entering. The shop is still filling up.
  3. Equilibrium: Eventually, the number of people entering the shop per minute becomes exactly the same as the number of people leaving the shop per minute. The total number of people inside the shop stays constant, even though individual people are still moving in and out.

This is a perfect example of dynamic equilibrium! The shop isn't empty, it's not overflowing; it's just stable with a constant number of shoppers, even though there's constant movement.

How It Works (Step by Step)

Let's break down how a reaction reaches equilibrium:

  1. Start: You mix your reactants (the starting chemicals) together. The forward reaction (reactants turning into products) begins quickly because there's lots of reactant.
  2. Products Form: As products are made, the reverse reaction (products turning back into reactants) also starts, but slowly at first because there aren't many products yet.
  3. Rates Change: The speed of the forward reaction slows down as reactants are used up. The speed of the reverse reaction speeds up as more products are formed.
  4. Rates Equalize: Eventually, the speed of the forward reaction becomes exactly equal to the speed of the reverse reaction.
  5. Equilibrium Achieved: At this point, the amounts (concentrations) of reactants and products no longer change. They stay constant, even though both reactions are still happening.

Key Characteristics of Equilibrium

When a reaction hits equilibrium, it has some special features:

  • Dynamic: As we said, it's not static! Reactions ...
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Common Mistakes (And How to Avoid Them)

Here are some common traps students fall into and how to steer clear of them:

  1. Mistake: Thinking the reaction ha...
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Exam Tips

  • 1.Always use the double arrow (⇌) for reversible reactions when discussing equilibrium.
  • 2.Clearly state that equilibrium is 'dynamic' and explain what that means (rates are equal, not stopped).
  • 3.Remember that concentrations are constant at equilibrium, not necessarily equal.
  • 4.Practice drawing graphs showing how reactant and product concentrations change over time until equilibrium is reached.
  • 5.Understand that equilibrium can only be achieved in a closed system; if a gas escapes, it's not equilibrium.
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