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14.1 Chemical Kinetics Study of how rapidly a reaction will occur. In addition to speed of reaction, kinetics also deals with the reaction mechanism (exactly how the reaction occurs) 1. Factors that affect reaction rate 1. Concentration of reactants – more particles means more frequent “effective” collisions occur 2. Temperature – higher temps, the molecules have more KE, are moving faster and are more likely to collide 3. Catalyst – chemical that speeds up a reaction by changing the reaction mechanism, not consumed in reaction 4. Physical state of reactants – (phase) 5. Surface area (for solids) – greater SA = faster reaction 14.2 Reaction Rate The change in concentration of reactants or products over time. The rate can be expressed as a rate of decomposition of reactants or rate of production of products. What is the average rate of decomposition of “A” st nd in the 1 20 seconds of the reaction? In the 2 20 seconds? Rate =- = - = 0.023 mol/sec Rate =- = 0.012 mol/sec What is the average rate of appearance of st “B” in the 1 20 seconds of the reaction? In nd the 2 2o seconds? Rate= = = 0.023 mol/sec Rate = = -0.012 mol/sec Chemical Equation showing mol ratio of A changing into B: 1A → 1B Mathematical Expressions for Reactions Reaction: N + 3 H → 2NH 2(g) 2(g) 3(g) ***If you want to compare the rates using different substances, you use the mole ratios The rate of reaction is always positive, and is usually measured in M/s Under certain conditions, the rate of formation of NH 3 is 0.28 M/s. What is the rate of change for N ? 2 0.28 M NH 1 mol N = 0.14 M/s 3 2 s 2 mol NH 3 Indicate how the rate of disappearance of each reactant is related to the rate of appearance of each product: 1. H O → H + O 2 2(g) 2(g) 2(g) -= = 2. 2SO + O → 2SO 2(g) 2(g) 3(g) (- = - = )2 - ∆[SO ]/∆t = -2∆[O ]/ ∆t = ∆[SO ]/∆t 2 2 3 *SO will disappear and SO will appear at twice the rate of 2 3 O 2
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