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NAME: _______________________________
UNIT #10: Reaction Rates
Heat/Energy in Chemical Reactions
Le Chatlier’s Principle
Potential Energy Diagrams
1. REACTION RATES
a) The speed of a chemical reaction determined by the change in concentration of a
reactant or product per unit time, expressed as mol/(L∙s).
b) Reaction rates are determined experimentally by measuring the concentrations of
reactants and/or products in an actual chemical reaction.
c) Factors Effecting Reaction Rates
1. Nature of Reactants: The more reactive a substance, the faster the reaction rate.
2. Concentrations: Increased concentration of reactants increases reaction rate by
increasing the chance of particle collisions; likewise, decreasing concentration of
reactants decreases reaction rate.
3. Surface Area: Increased surface area increases reaction rate due to more exposed
particles to react.
Ex. Pulverizing (grinding) a solid reactant will increase its reaction rate;
granulated sugar will dissolve faster in water than a sugar cube.
4. Temperature: Generally, increasing temperature increases reaction rate by
increasing the average kinetic energy of reactants; thus, collisions between
particles are more frequent and occur with more energy, increasing the chance
that product will be formed.
5. Catalysts: Catalysts are compounds introduced to a reaction that are not
consumed in the reaction. The activation energy of a reaction is the minimum
amount of energy required to start a reaction. Catalysts increase reaction rates
by decreasing activation energy and therefore, more particle collisions have
sufficient energy to initiate product formation.
2. HEAT/ENERGY IN CHEMICAL REACTION
a) The Law of Conservation of Energy states that in any chemical or physical process,
energy is neither created or destroyed but instead, is conserved.
b) In a chemical reaction, the bonds between atoms of the reactants are broken.
Breaking bonds requires an input of energy. Energy contained by reactants is
expressed as the Hreactants.
c) Atoms rearrange and form new bonds, resulting in the product. When bonds are
formed, energy is released. Energy contained by products is expressed as the Hproducts.
d) The heat of reaction is an expression of the net energy involved in a chemical reaction
and is expressed as ΔHreaction = Hproducts - Hreactants
e) An endothermic reaction is one that requires a net input of energy, meaning it takes
more energy to break bonds of reactants than is released in the formation of product.
Since energy is transferred and conserved, the resulting products contain more energy.
Since the ΔHreaction = Hproducts - Hreactants, the ΔHreaction for an endothermic reaction is
always positive.
f) An exothermic reaction is one that has a net release of energy, meaning more energy
is released in the formation of products than is required to break the bonds of the
reactants. Since energy is transferred and conserved, the resulting products contain
less energy. Since the ΔHreaction = Hproducts - Hreactants, the ΔHreaction for an exothermic
reaction is always negative.
g) Summary:
Endothermic reaction Reactants + Energy → Products
Hproducts is greater than Hreactants and energy appears on the reactant side of the
equation. ΔHreaction is positive.
Exothermic reaction Reactants → Products + Energy
Hproducts is less than Hreactants and energy appears on the product side of the equation.
ΔHreaction is negative.
3. LE CHATELIER’S PRINCIPLE
a) Many chemical reactions occur in both a forward and a reverse direction. This is
depicted as Reactants ↔ Products.
b) Chemical equilibrium is a state in which the rate of the forward reaction is equal to
the rate of the reverse reaction.
c) Le Chatelier’s principle states that if a stress is applied to a system at equilibrium, the
system shifts in the direction that relieves the stress.
1. Stress can be a change in concentration, volume and pressure or temperature.
2. Change in Concentration
a. Addition of reactants: system compensates by using up additional reactants
to form product. System shifts right towards products.
b. Removal of reactants: system compensates by reversing reaction to replace
reactants. System shifts left towards reactants.
c. Addition of products: system compensates by reversing reaction to use up
additional products. System shifts left towards reactants.
d. Removal of products: system compensates by producing more product.
System shifts right towards products.
3. Change in Volume and Pressure
a. Changes in volume and pressure effect only a system that contains reactants
and products in a gaseous state.
b. Changes in volume and pressure affect a system in equilibrium only if the
total number of moles of gaseous reactants is different than the total
number of moles of gaseous products. Coefficients of the balanced equation
are used to determine moles of reactants and products.
c. Changes in volume and pressure are interrelated because decreasing the
volume of a reaction vessel at constant temperature increases the pressure
inside. Conversely, increasing the volume decreases the pressure.
d. Decreasing volume/increasing pressure will shift a gaseous reaction towards
the side of the equation with fewer moles.
e. Increasing volume/decreasing pressure will shift a gaseous reaction toward
the side of the equation with more moles.
4. Changes in Temperature
a. In an endothermic reaction, energy is on the reactant side of the equation
and predictions of equilibrium shift are equivalent to changes in
concentration of reactants.
1. Increase in temperature: system compensates by using up additional
energy. System shifts right towards products.
2. Decrease in temperature: system compensates by producing more
energy. System shifts left towards reactants.
b. In an exothermic reaction, energy is on the product side of the equation and
predictions of equilibrium shift are equivalent to changes in concentration of
products.
1. Increase in temperature: system compensates by using up additional
energy. System shifts left towards reactants.
2. Decrease in temperature: system compensates by producing more
energy. System shifts right towards products.
4. POTENTIAL ENERGY DIAGRAMS
a) A potential-energy diagram is used to depict the shift in energy as a chemical reaction
progresses. Specifically, it shows the changing potential energy between the reactants
and products.
b) A potential energy diagram shows:
1. The potential energy of the reactant.
2. The potential energy of the activated complex. The activated complex is an
intermediate structure of atoms that exists as bonds are breaking and new bonds
are forming. It occurs temporarily before the final product is formed.
3. The potential energy of the product.
4. The activation energy of the forward reaction, which is the minimum energy
required to form an activated complex. On the diagram, activation energy is the
difference between the PE of the reactants and the PE of the activated
complex.
5. The heat of reaction, which is the difference between the PE of the product and
the PE of the reactant.
c) In an endothermic reaction, the Hproduct is greater than the Hreactants due to an input of
energy into the reaction. Therefore, the potential energy of the products will be
higher than the reactants on the diagram.
d) In an exothermic reaction, the Hproduct is less than the Hreactant due to a release of
energy. Therefore, the potential energy of the products will be lower than the
reactants on the diagram.
e) Potential energy diagrams can also depict the energy shift of a catalyzed reaction in
which the activated energy is reduced and consequently, the reaction is accelerated.
UnitINoteQuizQuestions
Unit 10.1: Collision Theory
1. a
2. a
3. a
4. a
5. a
6. Which shows the activation energy of the
reaction?
7. Which letter is the enthalpy of the reaction?
8. Which letter shows the potential energy of CH4
and O2?
9. Is the reaction endo/exothermic?
10. a
Unit 10.2: Le Châtelier's principle
1. a
2. A
3. A
4. A
5. A
6. A
7. A
8. A
9. A
10. A