How Fast

Flashcards for the module Rates, Equilibrium and pH

Focussing specifically on 5.1.1 How Fast?

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  • Created by: Samantha
  • Created on: 21-04-14 16:58

Key Terms

Rate of Reaction: the change in concentration of a reactant or product, per unit time. Units: mol dm^-3

Order: the power to which the reactant is raised in the rate equation

Rate constant: k, the constant that links the rate of reaction with the concentration of the reactants raised to the powers of their orders in the rate equation

Half-life: the time taken for the concentration of a reactant to reduce by half

Rate-determining step: the slowest step in a any multi-step reaction

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Rate Equation

Rate = (change in conc of reactant/product) / (time for change to occur)

Units = mol dm^-3 s^-1

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Concentration Against Time

A a reaction proceeds:

  • concentration of reactants decrease
  • fewer collisions per time unit between reactant particles
  • rate slows down

Concentration against time graphs

At any point in time, the rate = slope of curve (slope measured by drawing a tangent at that point).Gradient of tangent = rate

At t=0, the rate is at its peak, highest concentration of reactants, more chance of collisions

Concentration of products against reactants

Increasing curve 

Concentration of reactants against products

Decreasing curve

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What do the graphs show?

Zero Order Reaction Graph

The concentration decreases at a constant rate. Half-life decreases with time

First Order Reaction Graph

Constant half-life. The half-life is the same regardless of the concentration

Second Order Reaction Graph

Concentration decreases rapidly, then rate of decrease slows down. Half-life increases with time.

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Rate Against Concentration Graphs

Zero Order

Constant horizontal line. No effect on the rate of reaction if the concentration is changed. Rate is independent of [A].

Rate = k[A]^0 therefore Rate = k moldm^-3s^-1

First Order

Straight line through origin. [A] is directly proportional to rate.

Rate = k[A]^1 therefore Rate = k[A], k = Rate / [A] s^-1

Second Order

Rate is proportional to [A]^2

Rate = k[A]^2 therefore k = rate / [A]^2 mol^-1 dm^-3 s^-1

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Determining Order using the Initial Rates Method

Initial rate of reatction calculated by measuring the time for a certain amount of product to be formed

Clock reactions measure the time from the start of the reaction to a visual change (precipitate, disappearance of a solid, change in colour)

Approximation: the initial rate is proportional to 1 / t

Carry out a series of clock reactions, varying each reactant in turn.

Plot a graph of the initial rate against the initial concentration for each reactant

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The Effect Of Temperature On The Rate Constant

Slow reaction, small value of k

Fast reaction, large value of k

Temperature affects the rate by:

  • increasing the speed of particles therefore more collisions
  • increasing the kinetic energy of particles therefore more successful collisions (collisions in which the energy exceeds Ea)

If the rate of reaction increases with temperature, when the concentration of reactants remains the same, the rate constant must increase with temperature.

  • increasing the temperature speeds up the rate of most reactions by increasing k
  • in many reactions, the rate doubles for each 10 degree celcius increase in temperature because there are more reactant particles with energy above Ea
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Multi-Step Reactions

If a reactant is in the rate equation it must be in or before the rate-determining step.

The overall equation does not tell you anything about the reaction mechanism.

This is determined by rate experiments.

The order with respect to the reactants in the rate equation tells you how many particles of the reactant are in the rate-determining step.

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