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reactants products

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Kinetics Branch of chemistry that studies the speed or rate with which chemical reactions occur. Some reactions do not occur in one simple step. Some occur in several more complex steps. Kinetics includes studying the steps, or mechanism, of a chemical reaction.

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Identify variables used to monitor reaction rate. Define reaction rate. Measure the average rate of a chemical reaction. Relate the rate to the reaction stoichiometry. reactants products

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Rate - refers to the speed of the reaction How fast reactants are used or products form. A (s) + B (l) C (g) + heat exothermic A (s) + heat C (g) + B (l) endothermic

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(°C/min) (kPa/s or mmHg/s) (g/min) Colour, pH, conductivity (over time). Rate can be measured using different methods depending on the reaction:

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A (s) + B (l) C (g) + heat *We will only use concentration change with time.

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Rate = Δ[A] Δt Rate = [A] final - [A] initial t final - t initial Average Rate is described as total change in concentration of reactant or product over time. Reactions often start quickly, but slow down over time.

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Concentration Time Instantaneous rate - rate at a specific time. Determined by calculating the slope of the line tangent to a point on the curve.

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Calculating Average Rate A B Time (s) Concentration of B (mol/L) 0.0 10.00.30 20.00.50 30.00.60 40.00.65 50.00.67

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a. What is the average rate over the entire 50 seconds?

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b. What is the average rate for the interval 20 s to 40 s?

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Decomposition of nitrogen dioxide produces nitrogen monoxide and oxygen: 2 NO 2 (g) 2 NO (g) + O 2 (g) Time (s) [NO 2 ] (mol/L [NO] (mol/L) [O 2 ] (mol/L) 0.00.1000.00 1000.0660.0340.017 2000.0480.0520.026 3000.0380.0620.031 4000.0300.0700.035

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a. Ave. rate of decomposition of NO 2 over 400 s. Actual value is 1.75 x 10 -4 mol/L·s Rate is always expressed as a positive value.

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b. Ave. rate of production of O 2 over 400 s.

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Rates can be predicted from reaction stoichiometry. 2 NO 2 (g) 2 NO (g) + O 2 (g)

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rate of decomposition of NO 2 is equal to the production of NO - molar coefficients are 1:1. = Δ[O 2 ] Δt Δ[NO] Δt 2 = Δ[O 2 ] Δt Δ[NO 2 ] Δt ½ 2 NO 2 (g) 2 NO (g) + O 2 (g) rate of production of O 2 is half of the NO - molar coefficients are 1:2.

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What is the rate of production of C and the rate of disappearance of B if A is used up at a rate of 0.60 mol/Ls? 2 A + B 3 C 0.60 mol/L·s A 2 mol A 1 mol B = 0.30 mol/L·s B 0.60 mol/L·s A 2 mol A 3 mol C = 0.90 mol/L·s C

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rate is a change in the concentration of a reactant or product over a period of time. rate can be determined by measuring many changes in properties. rate is related to stoichiometry of the reaction's balanced equation. average rate is determined over an interval, while instantaneous rate is the rate at a certain moment.

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6.1 - Expressing Reaction Rates

6.1 - Expressing Reaction Rates

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