Rate

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  • Created by: ava.scott
  • Created on: 23-04-15 14:55

TERMS: Rate

Rate of reaction:

How fast a reaction is e.g. how quickly reactants are turned into products.

Rate constant:

Quantified rate of reaction. Rate[A][B]

Half life:

The time taken for the concentration of a reactant to decrease by half.

Rate determining step:

The slowest step in a multi-step reaction. ALWAYS 1ST STEP IN OCR.

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TERMS: order

Order:

The order of a reaction in respect to a reactant is the index to which its concentration term in the rate equation is raised.

Overall order of reaction:

All orders in the rate equation added together.

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Concentration-time graphs

conc/time graphs

Rate= change in concentration/ change in time

  • Draw a tangent from where your chosen time meets the line.
  • Concentration minus concentration of where your line meets the axes.
  • Time minus time where your line meets the axes.
  • UNITS= MOLDM-3 SEC-1

Half-life for 1st order

  • Gradient is rate.
  • 1st order reactions- half life is constant!
  • Time at initial concentration- time at half of initial concentration.

CAN BE DRAWN USING CONTINUOUS MEASUREMENTS

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Drawing/sketching concentration time graphs

  • ensure all reactants plateau at the same point.
  • Gradient=rate

0 ORDER

  • straight line down- as rate doesnt change with concentration
  • half life decreases

1st ORDER

  • goes down relatively steeply.
  • half life is constant

2nd Order

  • Decreases more steeply and will decrease by double the amount of any 1st order reactants.
  • Plateaus at the same TIME value as 1st order.
  • half life increases
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WAYS TO MEASURE RATE: Acid +Base

pH probe during reaction

  • 10^(-pH) will give you H+ concentration.
  • [HA]= [H+]
  • change in concentration of HA/ change in time.

Titration

  • Gives you mols of fluid used until equilibira point
  • convert mols to concentrations.
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WAYS TO MEASURE RATE: involving gas

GAS

Measure change in volume

  • volume produced= vol/24 = mol
  • convert mol to conc

Measure change in mass of reactants (as gas is produced)

  • should decrease
  • giving you reacted mols
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WAYS TO MEASURE RATE: when there's visual change

  • Measure mass of precipitate
  • Use colorimeter to measure colour change
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Conc-time graphs: Half-life: zero order

Graph

  • Gradient is the same as rate
  • So straight negative correlation.
  • HALF LIFE DECREASES.
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HALF-LIFE: first order

The time take for the concentration of a reactant to decrease by half.

First order half lives examples:

  • Radioactive chemicals
  • Removal of drugs from the body

Graph

  • Half life is constant for 1st order
  • Curve will steep decrease and plateau.
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Conc-time graphs: Half life: 2nd order

Graph

  • Very steep decrease
  • HALF LIFE INCREASES.
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ORDERS: assumptions + determination

  • Only 2 step reactions
  • The first step is the rate determining step

Determination

  • Order is determined by the molar ratio in the rate-determining step (NOT overall equation!!)
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Rate-concentration graphs

zero order

  • line is straight but does not meet y axis
  • concentration doesnt effect rate

1st order

  • line is straight positive correlation
  • concentration is proportional to rate
  • conc doubles; rate doubles

2nd order

  • line increases steeply
  • concentration is proportional to rate-squared.

REQUIRES SEPARATE EXPERIMENTS WITH DIFFERENT CONCENTRATIONS of reactants to DRAW.

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Determing order from experiments PHRASE

  • Between experiements ___ and ____
  • The concentration of A__
  • increases by a factor of ___
  • and the initial rate of reaction increases by a factor of _____
  • Therefore the order of reaction in respect to _A is ______.

DO PAST PAPER QUESTIONS M8.

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Determining rate from experimental data

DETERMINING RATE FROM DATA

  • Find Orders
  • Check headings and use 'x10^-y' values

When using data where two reactants change in concentration

  • Use one order you've already worked out
  • Times factor of concentration change by order worked out.
  • Use this value to times the rate of smaller rate.
  • Divide larger rate by this value.
  • Compare to factor of change in concentration of unknown reactants.

do questions.

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Temperature and K

Temperature increase will increase the rate constant and therefore the rate of reaction.

  • More heat
  • Particles have more energy
  • more successful collisions
  • leading to more reaction
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