kinetics and equilibrium review ( items 114-132 of 200 ways ..)

Kinetics and Equilibrium review (Items 114-132 of 200 ways ..)

18.1

Kinetics deals with the rates of chemical reactions. In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time. Equilibrium refers to the condition where forward and reverse reactions are occurring at the same rates.

End Show© Copyright Pearson Prentice Hall

>Rates of Reaction

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Collision Theory

Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.

18.1


>Rates of Reaction

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Collision Theory

Effective Collision

18.1

According to collision theory, atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy and have the proper orientation.


>Rates of Reaction

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Collision Theory

Ineffective Collision

18.1

The minimum energy that colliding particles must have in order to react is called the activation energy.

18.1

An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier.

The activated complex is sometimes called the transition state.

Factors affecting Reaction Rates

What four factors influence the rate of a chemical reaction?

18.1

The rate of a chemical reaction depends upon

•temperature,

•concentration,

•particle size,

•the use of a catalyst.

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© Copyright Pearson Prentice Hall

>Rates of Reaction Factors Affecting Reaction Rates

Temperature

Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action.

18.1

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Concentration

a. In air, a lighted splint glows and soon goes out.

b. When placed in pure oxygen (higher oxygen concentration), the splint bursts into flame.

18.1

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Particle Size

The minute size of the reactant particles (grain dust), and the mixture of the grain dust with oxygen in the air caused the reaction to be explosive, destroying the grain elevator.

18.1

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Catalysts

18.1

A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium. The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction. At chemical equilibrium, no net change occurs in the actual concentration of the reactants and products – i.e. concentrations are constant.

(Note this does not mean that the concentration of reactants and products are equal – avoid this common mistake).

Reversible Reactions

How do the amounts of reactants and products change in a chemical system at equilibrium?

At chemical equilibrium, no net change occurs in the actual amounts or concentration of the reactants and products.

If the rate of the shoppers going up the escalator is equal to the rate of the shoppers going down, then the number of shoppers on each floor remains constant, and there is an equilibrium.


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At equilibrium, all three types of molecules are present.

SO2 and O2 react to give

SO3

SO3 decomposes to SO2 and O2


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Three stresses can cause a change in the equilibrium position of a chemical system?

•changes in the concentration of reactants or products,

•changes in temperature,

•changes in pressure (for equilibria involving gases).

18.2

The French chemist Le Châtelier proposed what has come to be called Le Châtelier’s principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.


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for Conceptual Problem 18.1


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Equilibrium ConstantsWhat does the value of the equilbrium constant, Keq , indicate about the equilibrium position of a reaction?

The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation.

aA + bB cC + dD

←


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Equilibrium Constants

A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.


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Kinetics and Equilibrium


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Physical Equilibria – Phases Equilibria

H2O (s) H2O (l)

At 0 oC (273 K) there exists a phase equilibrium between solid and liquid H2O.

At 100 oC (373 K) there exists a phase equilibrium between liquid and gaseous H2O.

H2O (l) H2O (g)


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for Sample Problem 17.4How many joules of heat are required to melt a 10.0 grams popsicle at 0 oC. Assume that the popsicle has the same heat of fusion as water.


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Heats of Vaporization and Condensation

The quantity of heat absorbed by a melting a solid is exactly the same as the quantity of heat released when the liquid freezes; that is,

∆Hfusion = –∆Hsolidification

The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, ∆Hvaporization = –∆Hcondensation


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Heats of Vaporization and Condensation

a. Enthalpy changes accompany changes in state.

17.3


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17.3


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Heating (Endothermic) and Cooling (Exothermic) CurvesHeating versus Cooling Curves

Physical Equilibria – Saturated Solutions

Saturated solutions are another example involving physical equilibrium. The term “saturated solutions” refers to a solution containing the maximum amount of solute that will dissolve at a given temperature. The rate at which a substance crystallizes out of solution is equal to the rate at which it dissolves.

pHET simulation for saturated solutions

http://phet.colorado.edu/en/simulation/soluble-salts

http://phet.colorado.edu/en/simulation/soluble-salts

Saturated Solution:

= contains the maximum amount of solute for a given quantity of solvent at a constant temperature & pressure

if additional solute is added, it will not dissolve; rather it will precipitate out

Unsaturated Solution:

= a solution that contains less solute than a saturated solution at a given temperature & pressure

if additional solute is added, it will dissolve

Supersaturated Solution:

= contains more solute than it can theoretically hold at a given temperature

crystallization will be initiate if a very small “seed crystal” of solute is added

**very unstable**


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Saturated Solution:

= contains the maximum amount of solute for a given quantity of solvent at a constant temperature & pressure

if additional solute is added, it will not dissolve; rather it will precipitate out

Unsaturated Solution:

= a solution that contains less solute than a saturated solution at a given temperature & pressure

if additional solute is added, it will dissolve

Supersaturated Solution:

= contains more solute than it can theoretically hold at a given temperature

crystallization will be initiate if a very small “seed crystal” of solute is added

**very unstable**

Sample Individual Solubility Curve- Potassium nitrite

http://www.stevespanglerscience.com/experiment/00000078



kinetics and equilibrium review ( items 114-132 of 200 ways ..)

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