الكيمياء (بالإنجليزية) الصف الثالث الثانوي، الأسبوع 12: Catalysts and light

Temperature, pressure, Kp and Le Chatelier’s principle

الأسبوع 12 · الباب الثالث (٣): تأثير درجة الحرارة ونظرية التصادم وطاقة التنشيط والضغط وثابت الاتزان Kp وقاعدة لوشاتيليه والعوامل الحفازة والضوء · 3 دروس

Catalysts and light

Chemistry Sec 3 Term 1, Chapter 3: pp. 61–65

GoalExplain how a catalyst changes the rate in both directions equally without changing the position or Kc, and say how light can drive a reaction.

New words

catalyst · a substance that changes the rate of a reaction, in a small amount and without being used up, and does not change the equilibrium positionenzyme · a protein molecule made in living cells that acts as a catalystphotosynthesis · the reaction in which plants use light to turn carbon dioxide and water into carbohydrates

Explanation

A catalyst gives a reaction an easier route with a lower activation energy. It lowers the activation energy of the forward and of the backward reaction by the same amount, so both rates rise equally. The system reaches equilibrium sooner, but the position, the yield and Kc are exactly the same as without the catalyst. A small amount is enough and it is not used up. Heating slow reactions costs a lot of fuel, so industry uses catalysts in most of its processes. Catalysts also work in car exhaust converters, which turn harmful gases into safer ones, and in living cells as enzymes. Light is the energy source of some reactions. In photosynthesis chlorophyll absorbs light, and carbon dioxide and water form carbohydrates and oxygen. In photographic film, light changes silver ions to silver, Ag⁺ + e⁻ → Ag, and more light gives more silver (paper only; no photographs here).

Worked examples

	Without	With catalyst
forward activation energy	50	25
backward activation energy	80	55
heat of reaction	−30	−30

Reading the energy sketch. Without a catalyst the peak is at 90 and the reactants at 40, so the forward activation energy is 90 − 40 = 50. With a catalyst the peak is 65, so it is 65 − 40 = 25. Backward: 90 − 10 = 80 without and 65 − 10 = 55 with. Both are lowered by 25. The products stay at 10 and the reactants at 40, so the heat of reaction is 10 − 40 = −30 in both cases. The catalyst changes the route, not the starting and finishing levels.

Same plateau, different time. Two runs of the same reversible reaction (invented data) are measured with and without a catalyst. The product reaches 0.40 mol/L at the end in both. With the catalyst it is there after 4 minutes, without it only after 8. The catalyst saved time, and the yield and Kc are unchanged.

Light and atoms. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with light absorbed by chlorophyll. Left: C 6, O 12 + 6 = 18, H 12. Right: C 6, H 12, O 6 + 12 = 18. Balanced. In photographic film, Ag⁺ + e⁻ → Ag: the charge is +1 − 1 = 0 on the left and 0 on the right, so the silver ion becomes a neutral atom. No light, no silver.

Class activity

Same balance, faster, with paper tokens and a die. Put 15 tokens in tray A and none in B. Slow rule: each round roll a die for every token in A, and move it to B on a 1; roll for every token in B and move it to A on a 1 or a 2. Fast rule: a token in A moves on a 1 or a 2, and a token in B moves on a 1, 2, 3 or 4. Play 8 rounds of each and write the counts. Both rules should settle near 10 in A and 5 in B, but the fast rule gets there in fewer rounds. Your dice results will differ. The fast rule is the catalyst.

Practice

A catalyst lowers the forward activation energy of an exothermic reaction from 80 kJ/mol to 50 kJ/mol. The heat of reaction is ΔH = −25 kJ/mol. Find the backward activation energy without and with the catalyst.

True or false, and correct if false: “A catalyst raises the yield of ammonia at equilibrium.”

A reaction has Kc = 0.50 at a fixed temperature without a catalyst. What is Kc at the same temperature with a catalyst?

Name one place where a catalyst is used in cars and one kind of catalyst found in living cells.