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Flashcards · J248 · Topic 5
Topic C5 – Monitoring and Controlling Chemical Reactions flashcards
24 cards on Topic 5, subtopics C5.1 to C5.3. Pick a subtopic or study them all.
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C5.1 Monitoring Chemical Reactions
- Titration technique (PAG C6)
- Pipette a measured volume into a conical flask with a few drops of indicator; fill a rinsed burette, read the bottom of the meniscus at eye level; add while swirling, drop by drop near the end point; repeat until titres are concordant.
- Concordant titres
- Accurate titres that agree closely, usually within 0.10 cm³. Only these are averaged; the rough titre is left out.
- Colours of phenolphthalein and methyl orange
- Phenolphthalein: pink in alkali, colourless in acid. Methyl orange: yellow in alkali, red in acid.
- Why is the actual yield less than the theoretical yield?
- The reaction is incomplete or reversible; product is lost during filtering, transferring or crystallising; side reactions form unwanted products.
- Atom economy: formula, and why it matters
- (total Mr of desired product ÷ total Mr of reactants) × 100, from the balanced equation. High atom economy means less waste. A reaction with one product has 100%.
- 2.92 g of HCl (Mr 36.5) is dissolved to make 400 cm³. Concentration in mol/dm³?Higher
- 2.92 ÷ 36.5 = 0.0800 mol; 0.0800 ÷ 0.400 = 0.200 mol/dm³.
- 25.0 cm³ of KOH needs 16.00 cm³ of 0.125 mol/dm³ HNO₃ (1 : 1). Concentration of KOH?Higher
- HNO₃: 0.125 × 0.01600 = 0.00200 mol = KOH. 0.00200 ÷ 0.0250 = 0.0800 mol/dm³.
- Molar gas volume at RTP. What volume does 0.0250 mol of oxygen occupy?Higher
- 24 dm³ (24 000 cm³) per mole at room temperature and pressure. 0.0250 × 24 = 0.600 dm³.
- Factors to weigh when choosing a reaction pathwayHigher
- Atom economy, percentage yield, rate, equilibrium position, and the usefulness of by-products.
C5.2 Controlling Reactions
- Three ways to measure the rate of a reaction that gives off a gas
- Gas volume at set times (gas syringe, or upturned measuring cylinder over water); mass loss on a balance (good for a heavy gas such as CO₂, poor for hydrogen); or time for a fixed change (1/t).
- 105 cm³ of gas forms in 70 s. Mean rate?
- 105 ÷ 70 = 1.5 cm³/s
- A tangent passes through (10 s, 28 cm³) and (50 s, 76 cm³). Rate at that time?
- (76 − 28) ÷ (50 − 10) = 48 ÷ 40 = 1.2 cm³/s
- Which factors change only collision frequency, and which change collision energy too?
- Concentration, pressure and surface area: more frequent collisions only. Temperature: more frequent and more energetic collisions, so more reach the activation energy.
- Surface area to volume ratio of a 5 mm cube, and of it cut into 1 mm cubes
- 5 mm cube: 150 mm² ÷ 125 mm³ = 1.2 : 1. 1 mm cubes: 125 × 6 = 750 mm² ÷ 125 mm³ = 6 : 1. Smaller pieces react faster.
- Characteristics of a catalyst, and how it works
- Increases the rate, is not used up (same mass at the end), does not change the amount of product; needed in small amounts. It provides an alternative pathway with a lower activation energy.
- Enzyme
- A biological catalyst: it speeds up reactions in living things and is not used up.
- Run X takes 20 s and run Y takes 80 s for the same change. How many times faster is X?
- Rate ∝ 1/t. 1/20 = 0.050 s⁻¹ and 1/80 = 0.0125 s⁻¹, so X is 4 times as fast.
C5.3 Equilibria
- Heating hydrated cobalt(II) chloride, then adding water to the product
- Heat: pink → blue as water is driven off (forward, endothermic). Add water: blue → pink, heat released (reverse, exothermic).
- Dynamic equilibrium
- In a closed system, the forward and reverse reactions happen at equal rates, so the concentrations stay constant. Both reactions continue.
- At equilibrium, are the concentrations of reactants and products equal?
- No. They are constant, rarely equal. It is the rates of the two reactions that are equal.
- Le Chatelier's principleHigher
- A change made to a system at equilibrium makes the position shift to oppose the change.
- PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). Effect of raising the pressure?Higher
- The position shifts left, to the side with fewer gas molecules (1 on the left, 2 on the right). Count gas molecules, not atoms.
- A forward reaction is endothermic. What happens to the amount of product when the temperature is lowered?Higher
- The position shifts in the exothermic (reverse) direction, so there is less product.
- CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g), forward exothermic. Conditions for the most methane?Higher
- High pressure (4 gas molecules → 2), low temperature (forward is exothermic), and excess hydrogen.