CHEM UNIT 4: RATES OF REACTION

  • REACTION RATES
  • ORDERS
  • EQUATIONS
  • MECHANISMS
  • HALOGENOALKANES
  • ACTIVATION ENERGY AND CATALYSTS
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  • Created by: Chynna
  • Created on: 11-03-13 09:12

REACTION RATES

Reaction rate - change in the amount of reactants and products per unit time

Ways to follow the rate of reaction

  • gas volume - collect it in a gas syringe and record how much you've got at regular time intervals (eg reaction between an acid and a carbonate in which CO2 is produced)
  • loss of mass - if gas is given off, system will lose mass so you can measure this at regular time intervals
  • colour change- track change using a colorimeter (eg reaction between propanone and iodine, brown colour of iodine fades)
  • clock reaction - sudden colour change when product reaches a certain conc. measure time it takes for the colour change to happen - shorter the time, the faster the rate
  • electrical conductivity - if no. of ions changes, so will the electrical conductivity

work out rate from a conc-time graph - gradient of the line will tell you the rate at that point on the graph. If the graph is a curve draw a tangent. - the steeper the line, the faster the rate. negative gradient if you're measuring the reactant conc. and positive if its the product conc.  

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RECATION RATES

Other graphs

  • absorbance vs time - bromine(aq) [orange/yellow] + methanoic acid followed using a colorimeter. percentage absorbance falls as bromine is used up - find the gradient of the graph to find the rate at that point
  • gas volume vs time - decomposition of hydrogen peroxide followed by recording the volume of oxygen produced at regular time intervals - volume should increase - find the gradient
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ORDERS OF REACTION

Order of reaction with respect to a reactant tells you how it's conc affects the rate

  • if you increase the conc by x and the rate stays the same, the order = 0
  • if you increase the conc by x and the rate increases by x, the order = 1
  • if you increase the conc by x and the rate increases by x^2, the order = 2
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RATE EQUATIONS

A + B --> C + D

m + n = overall order of the reaction

k = rate constant, the bigger it is the faster the reaction. rate constant is always the same for a certain reaction at a particular temp. - if you increase the temp, the rate constant rises too

Calculate rate constant from orders and rate of reaction

reaction rates can be used to calculate the half-life of a reaction

  •  zero order t(1/2) = [X] / 2k - the decomposition of X to Y and Z is zero order
  • first order t(1/2) = 0.69 / k - the decomposition of N2O5 to NO2 and O2 is first order
  • second order t(1/2) = 1 /k[X] - the decomposition of hydrogen iodide is second order

half lives can be used to figure out the rate equation - if the half life is contant then it is first order with respect to that reactant.

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DEDUCING ORDERS AND RATE EQUATIONS

Work out orders of reactions and rate equations by experimentation

  • do a series of experiments monitoring the rate of the reaction
  • in each separate experiment, vary the conc of only one reactant, keep everything else the same
  • plot each experiment on a conc-time graph and calculate the initial rate of reaction (gradient at time=0)
  • analyse the results to see how changing the conc affects the rate, work out rate equation
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RATES AND REACTION MECHANISMS

Rate determining step is the slowest step in a multistep reaction

Reactants in the rate equation affect the rate

  • if a reactant appears in the rate equation, it must be affecting the rate so this reactant must be in the rate determining step
  • if a reactant doesn't appear in the rate equation it must be involved in a faster step
  • rate determining step doesn't have to be the first step in a mechanism
  • reaction mechanism can't usually be predicted from just the chemical equation

You can predict the rate equation from the rate determing step

  • order of a reaction with respect to a reactant shows the no. of molecules of that reactant which are involved in the rate determining step
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HALOGENOALKANES AND REACTION MECHANISMS

-

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HALOGENOALKANES AND REACTION MECHANISMS

-

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ACTIVATION ENERGY AND CATALYSTS

Use the Arrhenius equation to calculate the activation energy

  • k = rate constant
  • Ea = activation energy (J)
  • T = temp
  • R = gas constant (8.31 JK^-1mol^-1)
  • A = another constant
  • as the Ea gets bigger, k gets smaller . So a large Ea will mean a slow rate
  • as temp. rises, k increases

you can use this equation to create an Arrhenius plot by plotting ln k against 1/T whch produces a graph with gradient of -Ea/R and rearrange to find Ea

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ACTIVATION ENERGY AND CATALYSTS

Catalysts lower the activation energy

catalyst - increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of reaction.

  • don't get used up in the reaction so you only need a little bit of catalyst to catalyse a huge amount of stuff. they all take part in the reaction but are remade
  • have high specifity - catalysts pick and choose which reactions they catalyse

homogeneous catalysts are in the same state as the reactants

heterogenous catalysts are in a different physical state from the reactants

  • solid - provide a surface for the reaction to take place on so usually a mesh or fine powder to increase surface area
  • can be easily separated from the products and leftover reactants
  • can be poisoned though - poison - subtance that clings to the catalyst's surface more strongly than the reactant, stopping the catalyst from getting involved in the recation it is meant to be speeding up  
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ACTIVATION ENERGY AND CATALYSTS

enthalpy profiles and maxwell-boltzmann distribution curves show why catalysts work

  • alternative pathway with lower Ea - more particles with enough energy to react when they collide so in a certain amount of time, more particles will react
  • saves a lot of money in industry because they mean reactions can happen at lower temps.   
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Comments

Jordan_H

I believe the 9th slide is wrong. The gradient of a lnK v 1/T graph is Ea/RT, you're getting it confused with a graph of ln1/t v 1/T which has the gradient of Ea/R 

Jordan_H

Woops... I just read the revision guide wrong, ignore that comment

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