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Chapter: 6 Kinetics Title: Lesson 1 Rates of Reaction Learning Objectives: – Understand the term ‘rate of reaction’ – Meet three possible approaches for measuring rates of reaction

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Main Menu The Rate of Reactions In simple terms, the rate of a reaction can be thought of as its speed Some reactions are very fast Some reactions are very slow Discuss: Think of examples of ‘fast’ and ‘slow’ reactions Why is ‘speed’ not a good word to use in this context?

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Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? A → [B]Product [A] decrease with time as [B (product)] increases. A → [B]Product [A] decrease with time as [B (product)] increases.

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Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Predict the general shape of the graph you would expect for A → Product

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Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Predict the shape of the graph for the reaction A + 2B →C Describe what is happening in this graph in terms of reaction rate and concentrations

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Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? The steeper the curve the faster the rate of reaction Why is the initial slope of the graph the steepest? The rate of reaction is highest here because there is more likelihood of collision

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Chemical reaction rates Ch 1.1 A2 What does “rate of reaction mean? Why does the slope of the reaction decrease with time? The rate of reaction slows down over time because there are fewer reactant particles left.

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Chemical reaction rates Ch 1.1 A2 What do reaction rate graphs look like? Why does [B] fall faster than [A]? For each molecule or atom of A, two B particles of B are used up.

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Main Menu Defining the Rate of Reaction The rate of reaction is: Where: ∆[R] is change in concentration of reactants ∆[P] is change in concentration of products ∆t is change in time Definition – Rate of reaction is the change in concentration of reactants or products per unit time. Units: mol dm -3 s -1 (moles per decimetre cubed per second) The negative sign in the reactant expression indicates the reactant concentration is decreasing, but by convention, rate is expressed as a positive value.

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Main Menu Using a graph to find rate of reaction Steepness/gradient is the measure of the change in the concentration per unit time, or the rate of reaction. The gradient for the curve is not constant and so can only be given for a particular value of time. Note: Even though the graph gradient is negative, the rate is expressed as positive. Gradient = Change in y Change in x Rate is greatest at the start when the reactant concentration is greatest. As rate varies from this point onwards, it is common to compare initial rates of reactions by taking the tangent at t = 0.

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Reaction Rates Consider the chemical reaction: A B Time = mol A t = 20. min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B Finding the rate Ch 1.1 A2 How do you find reaction rates?

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Finding the rate Ch 1.1 A2 How do you find reaction rates? If the number of moles of A and B are measured and plotted, a graph such as this one can be obtained This data can be used to find the reaction rate.

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Finding the rate Ch 1.1 A2 How do you find reaction rates? In this reaction: Average rate of appearance of B = change in # of moles of B change in time = [mol B] t In this reaction: Average rate of appearance of B = change in # of moles of B change in time = [mol B] t We can calculate the average rate for any time interval involved in the reaction.

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Finding the rate Ch 1.1 A2 How do you find reaction rates? the rate of appearance of B over the first 20 minutes of reaction: Average rate of appearance of B = [mol B] t = 5.0 mol B – 0.0 mol B 20. min – 0. min = 0.25 mol/min the rate of appearance of B over the first 20 minutes of reaction: Average rate of appearance of B = [mol B] t = 5.0 mol B – 0.0 mol B 20. min – 0. min = 0.25 mol/min Time = 0 10 mol A t = 20 min 5.0 mol A 5.0 mol B

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Finding the rate Ch 1.1 A2 How do you find reaction rates? The average rate of appearance of B during the second 20 minutes of the reaction: Avg. rate = 8.0 mol B – 5.0 mol B 40. min – 20. min =0.15 mol/min The average rate of appearance of B during the second 20 minutes of the reaction: Avg. rate = 8.0 mol B – 5.0 mol B 40. min – 20. min =0.15 mol/min t = 20 min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B

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The rate of a reaction can also be expressed as the disappearance of A as a function of time. For this particular reaction, when 1 mole of B is formed, 1 mole of A must disappear. A → B Hence B/ t = - A/ t The rate of a reaction can also be expressed as the disappearance of A as a function of time. For this particular reaction, when 1 mole of B is formed, 1 mole of A must disappear. A → B Hence B/ t = - A/ t Finding the rate Ch 1.1 A2 How do you find reaction rates?

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For reactions with 1:1 stoichiometry: Avg. rate = (moles product) t =- (moles reactant) t For reactions with 1:1 stoichiometry: Avg. rate = (moles product) t =- (moles reactant) t Finding the rate Ch 1.1 A2 How do you find reaction rates?

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For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or product Average Rate = [Product] = - [Reactant] t With concentration in mol dm -3 For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or product Average Rate = [Product] = - [Reactant] t With concentration in mol dm -3 Finding the rate Ch 1.1 A2 How do you find reaction rates?

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Main Menu Measuring Reaction Rates TechniqueApparatus/Notes Collecting gasUsing a gas syringe or inverted measuring cylinder filled with water Collecting gas (low solubility in water) Using displacement of water from an inverted burette Mass lossReaction conducted on a balance…if it produces a gas the mass will decrease Colour changeMeasured using a colorimeter ConductivityConductivity will change depending on the concentration of the ions and charges in the reactants/products. pHMonitored using a pH probe Titration*Titration of reactants/products against a known standard. Cannot be done continuously, only at set time intervals. Obscured cross*Useful if reaction produces a precipitate (for example sodium thiosulphate and acid) Iodine clock*Produces a sudden colour change from colourless to black…. Monitors any reaction producing iodine * These reactions are not continuous or have an ‘end point’...

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Solutions

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Review Rates determined by monitoring a change

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