Presentation on theme: "Title: Lesson 1 Rates of Reaction"— Presentation transcript:
1Title: Lesson 1 Rates of Reaction Chapter: 6 KineticsTitle: Lesson 1 Rates of ReactionLearning Objectives:Understand the term ‘rate of reaction’Meet three possible approaches for measuring rates of reaction
2The Rate of ReactionsIn simple terms, the rate of a reaction can be thought of as its speedSome reactions are very fastSome reactions are very slowDiscuss:Think of examples of ‘fast’ and ‘slow’ reactionsWhy is ‘speed’ not a good word to use in this context?
3Chemical reaction rates What does “rate of reaction mean?A → [B]Product[A] decrease with time as [B (product)] increases.
4Chemical reaction rates Predict the general shape of the graph you would expect forA → ProductWhat do reaction rate graphs look like?
5Chemical reaction rates Predict the shape of the graph for the reactionA + 2B →CWhat do reaction rate graphs look like?Describe what is happening in this graph in terms of reaction rate and concentrations
6Chemical reaction rates The steeper the curve the faster the rate of reactionWhy 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
7Chemical 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.
8Chemical 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.
9Defining 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 timeDefinition – 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.
10Using 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 yChange in xRate 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.
11Finding the rate Reaction Rates Consider the chemical reaction: A B t = 20. min5.0 mol A5.0 mol Bt = 40. min2.0 mol A8.0 mol BHow do you find reaction rates?Time = 0.10. mol A
12Finding the rateCh 1.1A2If the number of moles of A and B are measured and plotted, a graph such as this one can be obtainedThis data can be used to find the reaction rate.How do you find reaction rates?
13Finding the rate In this reaction: Average rate of Ch 1.1A2In this reaction:Average rate ofappearance of B = change in # of moles of Bchange in time= D [mol B]D tHow do you find reaction rates?We can calculate the average rate for any time interval involved in the reaction.
14Finding the rateCh 1.1A2t = 20 min5.0 mol A5.0 mol BTime = 010 mol Athe rate of appearance of B over the first 20 minutes of reaction:Average rate ofappearance of B = D [mol B]D t= 5.0 mol B – 0.0 mol B20. min – 0. min= 0.25 mol/minHow do you find reaction rates?
15Finding the rateCh 1.1A2t = 40. min2.0 mol A8.0 mol Bt = 20 min5.0 mol A5.0 mol BThe average rate of appearance of B during the second 20 minutes of the reaction:Avg. rate = 8.0 mol B – 5.0 mol B40. min – 20. min= 0.15 mol/minHow do you find reaction rates?
16Finding the rateCh 1.1A2The 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 → BHenceDB/Dt = - DA/DtHow do you find reaction rates?
17Finding the rate For reactions with 1:1 stoichiometry: Avg. rate = D (moles product)D t= - D (moles reactant)How do you find reaction rates?
18Finding the rateCh 1.1A2For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or productAverage Rate = D [Product] = - D [Reactant]D t D tWith concentration in mol dm-3How do you find reaction rates?
19Measuring Reaction Rates TechniqueApparatus/NotesCollecting gasUsing a gas syringe or inverted measuring cylinder filled with waterCollecting gas (low solubility in water)Using displacement of water from an inverted buretteMass lossReaction conducted on a balance…if it produces a gas the mass will decreaseColour changeMeasured using a colorimeterConductivityConductivity will change depending on the concentration of the ions and charges in the reactants/products.pHMonitored using a pH probeTitration*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’...