6.3. why does the rate of a reaction increase with increased concentration of reactants increased...
TRANSCRIPT
Theories of Reaction Rates
6.3
Factors that Affect Reaction Rate
Why does the rate of a reaction increase with• Increased concentration of reactants• Increased temperature• Increased surface area
Collision Theory In order for a reaction to occur,
reacting particles (atoms, molecule, or ions) must collide with one another.
If collision is necessary for a reaction to occur, then it makes sense that the rate of the reaction will increase if there are more collisions per unit time.
Collision Theory and Surface Area
Increasing surface area of a solid-phase reactant speeds up a reaction.
With greater surface area, more collisions can occur. • Starting a fire: small twigs rather than logs.
Beyond Collision Theory Not every collision
between reactants results in a reaction.
Collision must be EFFECTIVE:• One that results in the
formation of products.
Orientation of Reactants Collision geometry: the correct
orientation of reactants relative to one another.
Activation Energy(1) Reactants must collide with energy that
is sufficient to begin to break the bonds in the reactants and to begin to form the bonds in the products.
In most reactions, only a small fraction of collisions have sufficient energy for a reaction to occur.
ACTIVATION ENERGY, Ea: the minimum collision energy that is required for a successful reaction.
Activation Energy(2)
Graph shows the distribution of kinetic energy in a sample of reacting gases at two different temperatures, T1 and T2, where T2>T1.
Important Observations1) At both temperatures, a relatively small fraction
of collisions have sufficient kinetic energy – the activation energy – to result in a reaction.
2) As the temperature of a sample increases, the fraction of collisions with sufficient energy to cause a reaction increases significantly.
Transition State Theory Examines the transition, or change, from
reactants to products. Kinetic energy of reactants is transferred
to potential energy as the reactants collide: law of conservation of energy. • Basketball analogy: kinetic energy of ball
converted to potential energy, which is stored in the deformed ball as it hits the floor. The potential energy is converted to kinetic energy as the ball bounces away.
Potential Energy Diagrams Diagram that charts the potential
energy of a reaction against the progress of the reaction.
The ‘hill’: Ea barrier. Notice the difference between exothermic and
endothermic reactions. There is no way to predict the activation energy
of a reaction from its enthalpy change: a highly exothermic reaction may be very slow because of a high activation energy.
Activation energy is determined by analyzing the reaction rate.
Activation Energies of Reversible Reactions
For an exothermic reaction, the activation energy of the reverse reaction, Ea(rev) equals Ea(fwd) + ∆H.
For an endothermic reaction, Ea(rev) equals Ea(fwd) - ∆H.
Transition State `Change-over`point Activated Complex: chemical species that
exists at the transition state• neither product nor reactant• Partial bonds highly unstable.
Activated complex: can either break down to form products or decompose to re-form the reactants. • Like a rock teetering on top of a mountain can
fall either way.
Tracing a reaction with a Potential Energy Diagram
Sample Problem – Drawing a Potential Energy Diagram
Seatwork/Homework PPs, page 294, #13-16
Read page 295 and make brief notes
Section Review, page 296, #1- 8.