Topic 6 Flashcards

Rate of Reaction & Collision Theory

Rate of Reaction & Collision Theory: read all 5 questions and answers

1. What is the rate of reaction?
   A measure of how quickly reactants are used up or how quickly products are formed. Rate = amount of product formed ÷ time.
2. What two conditions must be met for a reaction to occur (collision theory)?
   1. Particles must COLLIDE with each other. 2. The collision must have SUFFICIENT ENERGY - at least the activation energy (Eₐ).
3. What is a successful collision?
   A collision where the particles have at least the activation energy. Only successful collisions lead to a reaction - most collisions just result in particles bouncing off each other.
4. What is activation energy?
   The minimum amount of energy that particles must have when they collide in order for a reaction to occur.
5. Give an example of a fast reaction and a slow reaction.
   Fast: an explosion, neutralisation. Slow: rusting of iron, weathering of rocks.

Factors Affecting Rate

Factors Affecting Rate: read all 6 questions and answers

1. How does increasing temperature affect rate? (Use collision theory)
   Particles move faster (more kinetic energy), so collisions are MORE FREQUENT and MORE ENERGETIC. A greater proportion of collisions exceed the activation energy → rate increases.
2. How does increasing concentration affect rate?
   There are more particles in the same volume, so collisions are MORE FREQUENT → more successful collisions per second → rate increases.
3. How does increasing pressure affect rate (for gases)?
   Gas particles are pushed closer together in a smaller volume, so collisions are MORE FREQUENT → rate increases. (Same logic as concentration but for gases.)
4. How does increasing surface area affect rate?
   Using smaller pieces or powders exposes more reactant particles on the surface. More particles are available for collisions → collisions are more frequent → rate increases.
5. What is the key structure for a 3-mark "explain" answer about rate factors?
   1. State how the factor affects particles (energy or collision frequency). 2. State the effect on successful collisions (more frequent collisions or a greater proportion with energy >= Ea; note that only catalysts lower Ea). 3. Conclude: more successful collisions per second → rate increases.
6. Why does a powder react faster than a lump?
   A powder has a much LARGER SURFACE AREA than a lump of the same mass. More particles are exposed on the surface for collisions, so collisions are more frequent → faster rate.

Catalysts

Catalysts: read all 5 questions and answers

1. What is a catalyst?
   A substance that increases the rate of a reaction by providing an ALTERNATIVE REACTION PATHWAY with a LOWER ACTIVATION ENERGY. It is NOT used up in the reaction.
2. Does a catalyst change the amount of product formed?
   No. A catalyst only makes the reaction faster. The same total amount of product is formed - it just forms more quickly.
3. What are biological catalysts called?
   Enzymes. They are proteins that catalyse specific reactions in living organisms. Example: amylase breaks down starch into sugars.
4. Name three industrial catalysts and their uses.
   1. Iron - Haber process (making ammonia). 2. Vanadium(V) oxide (V₂O₅) - Contact process (making sulfuric acid). 3. Manganese dioxide (MnO₂) - decomposition of hydrogen peroxide.
5. Why are catalysts important in industry?
   They can be reused, reduce energy costs (lower activation energy needed), and speed up production. However, they can be "poisoned" by impurities, which stops them working.

Measuring Rate & Required Practical

Measuring Rate & Required Practical: read all 6 questions and answers

1. Name three methods of measuring rate of reaction.
   1. Gas collection (gas syringe - measure volume over time). 2. Mass loss (balance - mass decreases as gas escapes). 3. Disappearing cross (time for precipitate to obscure a cross).
2. Describe the disappearing cross experiment.
   Sodium thiosulfate + HCl produces sulfur precipitate. Place flask on paper with a cross. Time how long until the cross is no longer visible. Shorter time = faster rate. Rate ∝ 1/time.
3. In the disappearing cross RP, what is the independent variable?
   The concentration of sodium thiosulfate (Na₂S₂O₃). You dilute it with water, keeping the TOTAL VOLUME constant.
4. Why must you keep the total volume constant in the disappearing cross RP?
   To ensure the depth of solution you look through stays the same. If the volume changed, it would take a different amount of sulfur to obscure the cross - making the comparison unfair.
5. How do you calculate mean rate from gas collection data?
   Mean rate = total volume of gas collected ÷ total time. E.g. 48 cm³ in 60 s → rate = 48 ÷ 60 = 0.80 cm³/s.
6. How do you calculate rate from a graph?
   Draw a TANGENT to the curve at the required time. Calculate the gradient of the tangent: rate = Δy ÷ Δx. The steeper the tangent, the faster the rate at that moment.

Reversible Reactions & Equilibrium

Reversible Reactions & Equilibrium: read all 6 questions and answers

1. What is a reversible reaction?
   A reaction that can proceed in both directions - products can re-form the original reactants. Shown by the ⇌ symbol.
2. What is the relationship between energy changes in forward and reverse reactions?
   If the forward reaction is exothermic (releases energy), the reverse reaction is endothermic (absorbs the same amount of energy). The energy changes are equal and opposite.
3. Give an example of a reversible reaction.
   Hydration of copper sulfate: CuSO₄ + 5H₂O ⇌ CuSO₄·5H₂O. Forward: white → blue (exothermic). Reverse: blue → white by heating (endothermic).
4. What is dynamic equilibrium?
   In a CLOSED SYSTEM, when the rate of the forward reaction EQUALS the rate of the reverse reaction. Concentrations of reactants and products remain constant, but both reactions continue.
5. What is a closed system?
   A system where no substances can enter or leave. This is essential for equilibrium - if products escape, the system can never reach equilibrium.
6. At equilibrium, are the concentrations of reactants and products equal?
   NOT necessarily. They are CONSTANT, but they can be at any ratio. The equilibrium position determines the relative amounts.

Le Chatelier's Principle (Higher Tier)

Le Chatelier's Principle: read all 7 questions and answers

1. State Le Chatelier's principle.
   If a system at equilibrium is subjected to a change in conditions, the position of equilibrium will shift to OPPOSE the change.
2. How does increasing temperature affect an exothermic forward reaction at equilibrium?
   Equilibrium shifts in the ENDOTHERMIC direction (reverse) to absorb the extra heat. This reduces the yield of product but increases the rate.
3. How does increasing pressure affect an equilibrium with fewer moles of gas on the product side?
   Equilibrium shifts to the side with FEWER moles of gas (the product side) to reduce pressure. This increases the yield of product.
4. How does adding more reactant affect equilibrium?
   Equilibrium shifts to the RIGHT (forward direction) to use up the extra reactant. This produces more product.
5. Does a catalyst affect the position of equilibrium?
   NO. A catalyst speeds up both the forward and reverse reactions equally. Equilibrium is reached FASTER, but the position (yield) does not change.
6. In the Haber process (N₂ + 3H₂ ⇌ 2NH₃, exothermic), why is 450°C used instead of a lower temperature?
   A lower temperature would give a higher yield (shifts equilibrium right), but the rate would be too slow. 450°C is a COMPROMISE - acceptable yield at a reasonable rate. An iron catalyst also helps speed up the reaction.
7. In the Haber process, why is 200 atm used?
   High pressure shifts equilibrium RIGHT (4 moles gas → 2 moles gas), increasing yield. 200 atm is a compromise - higher pressures would give better yield but are too expensive and dangerous to maintain.