Rates of Reaction Factors
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is Rate of Reaction? Rate of reaction is the speed at which reactants are converted into products. It measures how fast or slow a chemical reaction occurs. Rate of reaction = Amount of reactant used OR product formed / Time taken A fast reaction (e.g., explosion) happens in seconds, while a slow reaction (e.g., rusting) can take years.
The Five Key Factors Affecting Rate The rate of a chemical reaction can be changed by altering these five factors: Temperature Concentration (for solutions) Pressure (for gases) Surface area (for solids) Catalyst 1.
Temperature Increasing temperature INCREASES the rate of reaction. Why does temperature affect rate? Particles move faster at higher temperatures More frequent collisions between particles Collisions have more energy More particles have energy ≥ activation energy More successful collisions per second Example: Food cooks faster at high temperature; food stays fresh longer in a refrigerator (lower temperature = slower reactions).
Rule of thumb: For many reactions, increasing temperature by 10°C approximately DOUBLES the rate. 2. Concentration (Solutions) Increasing concentration INCREASES the rate of reaction. Why does concentration affect rate?
Higher concentration = more particles in the same volume Particles are closer together More frequent collisions More successful collisions per second Example: Concentrated hydrochloric acid reacts faster with magnesium ribbon than dilute acid.
Concentration is measured in mol/dm³ (moles per cubic decimeter) or g/dm³ (grams per cubic decimeter). 3. Pressure (Gases) Increasing pressure INCREASES the rate of reaction (for gases). Why does pressure affect rate?
Higher pressure = gas particles forced closer together Same number of particles in smaller volume More frequent collisions More successful collisions per second Example: Increasing pressure in the Haber process (nitrogen + hydrogen → ammonia) increases the rate of ammonia production.
Note: Pressure only affects gases. It has no effect on solids or liquids. 4. Surface Area (Solids) Increasing surface area INCREASES the rate of reaction (for solids). Why does surface area affect rate?
Smaller pieces (powder) have more surface exposed than larger lumps More particles available for collisions at the surface More collisions can happen at once Faster reaction Example: Powdered calcium carbonate reacts faster with acid than marble chips (large lumps).
This is why flour dust can cause explosions but a bag of flour cannot! Form Surface Area Rate of Reaction Large lump Small surface area SLOW Small pieces Medium surface area MEDIUM Powder Large surface area FAST 5.
Catalysts A catalyst is a substance that increases the rate of a reaction without being used up. It provides an alternative pathway with lower activation energy. Key properties of catalysts: Speed up reactions NOT used up in the reaction (can be reused) Only a small amount needed Do NOT change the products or yield Do NOT change ΔH (enthalpy change) Lower the activation energy (Ea) Important: Catalysts do NOT make reactions happen that wouldn't normally happen - they just make existing reactions go faster!
Examples of Catalysts Catalyst Reaction Use Iron N₂ + H₂ → NH₃ Haber process (making ammonia) Vanadium(V) oxide (V₂O₅) SO₂ + O₂ → SO₃ Contact process (making sulfuric acid) Platinum or palladium Breaking down pollutants Catalytic converters in cars Manganese(IV) oxide (MnO₂) 2H₂O₂ → 2H₂O + O₂ Decomposition of hydrogen peroxide Nickel Adding H₂ to unsaturated fats Making margarine (hydrogenation) Enzymes (biological) Various reactions in living organisms Digestion, respiration, photosynthesis Comparing All Five Factors Factor How to Increase Rate Why It Works Temperature Increase temperature Particles move faster → more frequent & energetic collisions Concentration Increase concentration More particles in same volume → more frequent collisions Pressure Increase pressure (gases only) Particles closer together → more frequent collisions Surface area Use smaller pieces/powder More surface exposed → more collisions possible Catalyst Add a catalyst Lowers activation energy → more successful collisions Measuring Rate of Reaction Rate of reaction can be measured by monitoring: Volume of gas produced - use a gas syringe or collect over water Mass loss - measure mass on a balance as gas escapes Change in color/turbidity - time how long until solution changes color or becomes cloudy Change in concentration - measure reactant/product concentration over time Fast reaction: Steep gradient on graph, large amount of product in short time Slow reaction: Gentle gradient on graph, takes longer to produce same amount Key Points Summary Five factors affect rate: temperature, concentration, pressure, surface area, catalyst All five factors work by increasing frequency or success rate of collisions Catalysts lower activation energy without being used up Temperature has the biggest effect - often doubles rate per 10°C rise Surface are…
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