Catalysts and enzymes: decomposing hydrogen peroxide sim

What the sim shows

2H₂O₂(aq) → 2H₂O(l) + O₂(g). The flask holds 25 cm³ of 0.30 mol/dm³ hydrogen peroxide: 0.0075 mol, which gives 0.00375 mol of oxygen, or 90 cm³ at room temperature and pressure (24 dm³ per mole). We use 0.30 mol/dm³, not 0.5, so that all the gas fits in the 100 cm³ syringe; 0.15 mol/dm³ gives half as much, 45 cm³. Weigh 0.50 g of catalyst, tip it in, start the clock and read the syringe every 10 s. Each press of Read syringe jumps the clock 10 s (played in about 1 s) and the reaction waits while you read; nothing moves between presses. The catalyst only changes how fast the gas comes: given long enough, every catalyst gives the same final volume, because that is set by the amount of hydrogen peroxide. Compare initial rates from the gradient of a tangent at 0 s (gradient = V_final × k). At 20 °C, manganese(IV) oxide is fastest, then liver, then copper(II) oxide, then potato; with no catalyst there is almost no reaction. (Warm the liver to 40 °C and it beats manganese(IV) oxide.) Filter, wash and dry the manganese(IV) oxide and it still weighs 0.50 g: a catalyst is not used up. It speeds up the reaction by giving a route with a lower activation energy. Liver and potato contain catalase, an enzyme: a protein that acts as a biological catalyst. Its rate about doubles for each 10 °C rise up to an optimum near 40 °C; above that it falls sharply as the enzyme is denatured (the shape of its active site changes), and boiled liver stays inactive even when cooled. A glowing splint relights in the gas, so it is oxygen; we test it in the flask, because the syringe first fills with air pushed out of the flask. Model and limits: V(t) = V_final(1 − e^(−kt)), first order, with illustrative school-typical rate constants (k = 0.050, 0.020, 0.0040 and 0.0020 s⁻¹), not measured ones; hot catalase also loses activity during the run. Readings are to the nearest 1 cm³ and scatter by ±1 cm³ while the plunger is moving. The liver or potato is cut as a 0.50 g cube, about 8 mm across, so every catalyst has the same mass (a 1 cm cube of liver would weigh about 1.05 g). The figure ignores the flask warming, oxygen dissolving in the water, the plunger's friction and the foam that real liver makes pushing gas out.

• Plunger: the oxygen pushes it out
• Catalyst, 0.50 g (powder, or a cube of tissue about 8 mm across)
• Glass: flask, delivery tube, syringe

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