Transcript CHAPTER 6 - RATES OF REACTION

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Rate of Reaction: speed at which a chemical
reaction occurs  usually expressed as the change
in concentration per unit of time.
 Product
being formed or reactant being consumed.
 Measurements can include mass, colour, conductivity,
volume, and pressure.
RATES OF REACTION
(CHAPTER 6)
AVERAGE AND INSTANTANEOUS
RATES OF REACTIONS
Reaction rates are not usually constant  change with
time.
If we use former equation, will only get an average
rate  no instantaneous.
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Instantaneous rate: rate of reaction at a particular
time.
 Do
so by drawing tangent line at the point in time on a
concentration-time graph.
 Slope of the tangent is the instantaneous rate of the
reaction.
Go over the ThoughtLab on page 270.
There may be something like this on the exam!
Reaction Rates in Terms of Products and
Reactants
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Just do ratios!
PPs, page 272, #1-4
6.2- The Rate Law: Reactant
Concentration and Rate
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In this section, we will study the reaction rates that
are not affected by the concentration of the
products.
Rate of reaction generally increases when the
concentrations of the reactants increase.
RATE LAW
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Rate at which a reaction occurs.
k: rate constant: different for each reaction at a given temperature.
m & n: must be determined by experiment.
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If m or n = 1, reaction is in ‘first order in this reactant.’
If m or n = 2, reaction is in ‘second order in this reactant.’
(m + n)  overall reaction order.
THE RATE CONSTANT, k
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Indicates the speed of a reaction.
As reaction proceeds, the reaction rate decreases
because the concentration of the reactants
decrease, but value of k remains constant.
Must use specific ks for each temperature.
DETERMINING A RATE CONSTANT
Be aware: the units associated with k may change depending on the order of
the equation! As long as you calculate using your units and crossing off
where applicable, you should end up with the right units!
PG. 7&8
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SR: #1, 3, 5
6.3- Theories of Reaction Rates
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5 BASIC FACTORS AFFECT REACTION RATES:
 chemical nature of reactants
 concentration of reactants
 temperature
 presence of a catalyst
 surface area
COLLISION THEORY: IN ORDER FOR A REACTION TO
OCCUR, REACTING PARTICLES MUST COLLIDE WITH
ONE ANOTHER.
CHEMICAL NATURE OF REACTANTS
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different elements react at different rates
e.g Au and Ag react slowly in air while Na and K react so
quickly in air that they are rarely found naturally in their
elemental state
similar elements in the same group tend to react similarly but at
different rates e.g. Zn, Fe and Pb all react with HCl acid to
produce H2 gas, even when all the other conditions are the same,
the rates of reaction are different
e.g. activity series
in homogeneous systems, such as reactions in aqueous solution,
most reactions with monatomic ions (e.g. Ag+ and Cl-) are
extremely fast, while reactions of molecular substances are often
slower (e.g. glucose (C6H12O6) and iron (II) ions (Fe2+) react with
purple permanganate (MMnO4-) ions very visibly: glucose reacts
slowly when compared to the reaction with Fe2+
CONCENTRATION/SURFACE AREA
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concentrated HCl vs. vinegar (acetic acid) or human
stomach acid shows how different concentrations can
cause different reaction rates
experiments suggest that if the initial concentration of a
reactant is increased, than the reaction rate generally
increases e.g. when one adds a metal to acid, a higher
initial concentration of acid increase the rate of gas
production
implies “concentrated is better & faster”
HOW DOES THIS RELATE TO THE COLLISION THEORY?
SURFACE AREA?
TEMPERATURE
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mixing ingredients for a cake and nothing happens until the
cake is placed in an oven
cooking uses increased temperature to make changes in food
happen more quickly
paint dries faster when the temperature of the system
increases, etc.
rule of thumb is that around SATP a 10°C rise in temperature
often doubles or triples the rate of a chemical reaction
lowering the temperature can likewise lower the rate of a
chemical reaction, useful for food storage
humans apply cold substances to burned skin to slow unwanted
physiological reactions (help minimize the effect of the burn)
Read pg. 290 – 294 and do SP on page 293.
PPs 13-15
SR #1, 3