 # Title: Lesson 1 Rates of Reaction

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Title: Lesson 1 Rates of Reaction
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

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?

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

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

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

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

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

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

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.

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.

Finding the rate Reaction Rates Consider the chemical reaction: A B
t = 20. min 5.0 mol A 5.0 mol B t = 40. min 2.0 mol A 8.0 mol B How do you find reaction rates? Time = 0. 10. mol A

Finding the rate Ch 1.1 A2 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. How do you find reaction rates?

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

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

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

Finding the rate Ch 1.1 A2 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 DB/Dt = - DA/Dt How do you find reaction rates?

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

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

Measuring Reaction Rates
Technique Apparatus/Notes Collecting gas Using 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 loss Reaction conducted on a balance…if it produces a gas the mass will decrease Colour change Measured using a colorimeter Conductivity Conductivity will change depending on the concentration of the ions and charges in the reactants/products. pH Monitored 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’...

Solutions

Review Rates determined by monitoring a change