Chemistry · Both
Chemical Reactions & Rates
Types of reaction, balancing ideas, factors affecting rate and catalysts.
Chemical reactions make new substances. Mass is conserved. Rate depends on temperature, concentration, surface area, pressure (gases) and catalysts.
Common reaction types
• **Combustion** – reaction with oxygen, often releasing heat/light
• **Thermal decomposition** – compound breaks down when heated
• **Displacement** – more reactive element pushes out a less reactive one
• **Neutralisation** – acid + base → salt + water
• **Oxidation / reduction** – gain/loss of oxygen (or electrons at Higher)
Conservation of mass
In a closed system, total mass of reactants = total mass of products.
Atoms are rearranged, not created or destroyed.
If a gas escapes (e.g. open flask), measured mass can fall.
Rates of reaction
Rate increases when:
• **Temperature** rises → particles move faster, more frequent and energetic collisions
• **Concentration** (or pressure for gases) rises → more particles in the same volume
• **Surface area** of a solid increases (powder vs lump) → more collisions
• **Catalyst** is added → lowers activation energy, not used up
Collision theory: reactions happen when particles collide with enough energy (activation energy).
Worked example
Why does increasing temperature increase rate of reaction?
Solution
Particles have more kinetic energy, collide more often and a greater proportion of collisions have energy ≥ activation energy.
Practice questions
Try these without looking at the answers first.
Q1.What does a catalyst do to activation energy?
Show answer
Lowers it
Q2.Name three factors that increase rate of reaction.
Show answer
Temperature, concentration/pressure, surface area (also catalyst)
Q3.Acid + alkali → ?
Show answer
Salt + water