Rates of Reaction

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Rates of Reaction What is a rate of reaction? The amount of change of a product or reactant in a given time. What affects the rate? a) Concentration of solution (more likely to collide as more in same vol.) b) Temperature (increases speed of molecules, more exceed EA) c) Surface area (same as concentration) d) Pressure of gasses (same as conc.) e) Catalyst (guide particles to correct orientation + lowers EA) f) Others e.g. light(light = increase in speed of particles) How are rates altered? Collision theory: Particles must collide with an energy exceeding the EA and they must have the correct orientation. A reaction rate increased by, - Increasing collision frequency - Increasing average kinetic energy of particles - Lowering activation energy - Giving molecules correct orientation Effect of concentration on rate rA = rate of reaction with respect to A E.g. RH+ = rate of reaction with respect to H+(aq) Rate of reaction = K [A]a [B]b [C]c Units of rates of reaction = mol dm-3 s-1 Units of concentration = mol dm-3 Indices a, b, c etc: order of reaction - what is the effect on the rate of doubling the concentration E.g. [H+]1 = 1st order = if conc. is doubled, rate is doubled [H+]2 = 2nd order = if con. is double, rate is quadrupled [H+]0 = zero order = if conc. is doubled, rate is unaffected The constant, K K = rate constant. This is a constant of proportionality Units may vary but must include s-1 rA = k[A]1[B]1 mol dm-3 s-1‑ = k (mol dm-3)(mol dm-3) for mol dm-3 s-1 to = mol dm-3 s-1 because k must = dm3 mol-1 s-1 because (mol dm-3)(mol dm-3) = mol2 dm-6, so to equal this out, k = dm3 mol-1 s-1 Expt. 11.2a the kinetics of the reaction between calcium carbonate and hydrochloric acid CaCO3(s) + 2HCl(aq) ® CaCl2(aq) + CO2(g) + H2O(l) Collect gas given off in a syringe. Create a table: Time (s) Volume of CO2 Vt (cm3) (Vfinal - Vt) cm3 Then to find the order, find the half - life if time periods are roughly the same between each half - life then it is first order. If it is second order the graph would increase and not be constant. Methods of following a reaction By titration E.g. reaction between iodine and propane in acid solution. Then on a graph time can be plotted against titre values, in this case a straight line is seen, suggesting the reaction to be zero order By colorimetry Can be used when one of the reactants or products have a colour, the intensity changing during the reaction. The intensity can be followed using a special colorimeter. By Dilatometry Where during the reaction the total volume of the solution changes.

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