# 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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## 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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