The Rate and Extent of Chemical Change
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Rate of reaction
Rate of reaction measures how fast reactants are converted into products. It is measured by the quantity of reactant used or product formed per unit time.
Rate tells you how quickly a reaction happens, not how much product you get overall.
Rate of reaction — Key Knowledge
- Rate of reaction quantity of reactant used or product formed per unit time
- Faster rate more product formed in less time
Collision theory
For a reaction to occur, particles must collide with sufficient energy and the correct orientation. The minimum energy needed is called the activation energy.
Any factor that increases the frequency or energy of collisions will increase the rate.
Collision theory — Key Knowledge
- Collision theory particles must collide to react
- Activation energy minimum energy needed for a successful collision
- Correct orientation particles must collide the right way round
Effect of temperature
Increasing temperature makes particles move faster. Collisions become more frequent, and a greater proportion of particles have energy equal to or greater than the activation energy.
The increased proportion exceeding activation energy is the more important effect — not just faster movement.
Effect of temperature — Key Knowledge
- Higher temperature particles move faster, more kinetic energy
- More frequent collisions particles collide more often
- More energetic collisions greater proportion exceed activation energy
Effect of concentration and pressure
Increasing the concentration of a solution (or pressure of a gas) means more particles in a given volume, leading to more frequent collisions.
Pressure only applies to reactions involving gases.
Effect of concentration and pressure — Key Knowledge
- Higher concentration more solute particles per unit volume
- Higher pressure gas particles closer together
- More frequent collisions more particles in the same space
Effect of surface area
Breaking a solid into smaller pieces increases its surface area. More particles are exposed to the other reactant, so collisions are more frequent.
Crushing marble chips into powder makes them react faster with acid — same total mass, more surface exposed.
Effect of surface area — Key Knowledge
- Greater surface area smaller pieces or powder
- More exposed particles more contact with other reactant
- More frequent collisions faster rate
Catalysts
A catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy. It is not used up and does not appear in the overall equation.
A catalyst changes the rate but not the amount of product formed.
Catalysts — Key Knowledge
- Catalyst speeds up reaction, not used up
- Alternative pathway lower activation energy
- Not consumed recovered chemically unchanged after the reaction
Calculating rates from graphs
On a graph of product formed against time, the rate at any point equals the gradient of the line. A steeper gradient means a faster rate; a flat line means the reaction has finished.
Rate = change in quantity / change in time
Drawing a tangent to the curve at a given point gives the rate at that moment.
Calculating rates from graphs — Key Knowledge
- Rate = gradient of the line steeper = faster
- Steep section fast rate, plenty of reactant
- Flat section reaction complete, limiting reactant used up
Reversible reactions
In a reversible reaction, the products can react to re-form the original reactants. The ⇌ symbol shows the reaction can go in both directions.
Many industrial processes involve reversible reactions, such as the Haber process.
Reversible reactions — Key Knowledge
- Reversible reaction products can re-form reactants
- ⇌ symbol indicates reaction goes both ways
Energy changes in reversible reactions
If the forward reaction is exothermic, the reverse reaction is endothermic by exactly the same amount of energy, and vice versa.
You cannot get energy for free — what the forward reaction releases, the reverse reaction absorbs.
Energy changes in reversible reactions — Key Knowledge
- Forward exothermic = reverse endothermic same energy, opposite direction
- Energy transfer is equal same amount in both directions
Dynamic equilibrium
Dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. The concentrations of reactants and products remain constant but are not necessarily equal.
"Equilibrium" means equal rates, not equal amounts — a common misconception.
Dynamic equilibrium — Key Knowledge
- Closed system nothing enters or leaves
- Equal rates forward rate = reverse rate
- Constant concentrations amounts of reactants and products stay the same
- Both reactions still occurring not stopped, just balanced
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The Rate and Extent of Chemical Change
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