Flashcards for Unit 12

Equilibrium Rate of forward process = Rate of reverse process

Conditions for Equilibrium 2-way process Closed System

Dynamic Equilibrium Macroscopic level – looks constant. Microscopic level – lots of action

Types of Equilibria 1)Physical: Phase & Solution 2)Chemical

A solution at equilibrium must be Saturated

Phase Equilibrium H 2 O(l)  H 2 O(g)

Solution Equilibrium Ionic Cmpd NaCl(s)  Na + (aq) + Cl - (aq)

Solution Equilibrium Covalent Cmpd C 6 H 12 O 6 (s)  C 6 H 12 O 6 (aq)

H 2 O(g)  H 2 O(l) Phase Equilibrium

NH 4 Cl(s)  NH 4 + (aq) + Cl - (aq) Solution Equilibrium

Chemical Equilibrium – Haber Synthesis N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat

LeChatelier’s Principle A system at equilibrium will respond to remove a stress

What’s a stress for a chemical system? Change in temperature, pressure, or concentration

Catalyst Substance that increases the rate of a chemical reaction without itself being consumed.

Catalyst Does not shift the equilbrium point – the equilibrium concentrations are the same. You just get to equilibrium quicker.

At what temperature can H 2 O(s) and H 2 O(l) exist in equilibrium? 0  C or 273K

At what temperature can H 2 O(g) and H 2 O(l) exist in equilibrium? 100  C or 373K

Pressure Changes Only affect systems that have a gas in them

An increase in pressure favors which side? CO 2 (g)  CO 2 (aq) When P , more CO 2 dissolves!

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Equilibrium shifts to right. Concentration of H 2 . Concentration of NH 3 . Temperature  Stress What happens?

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Shifts to the side with fewer moles of gas, in this case the right. How does this system respond to an increase in pressure? 4 moles 2 moles

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Shifts to the side with more moles of gas, in this case the left. How does this system respond to a decrease in pressure?

H 2 (g) + I 2 (g) + heat  2HI(g) This system has the same # of moles on each side. It cannot respond to pressure changes. How does this system respond to a decrease in pressure? 2 moles

NaCl(s)  Na + (aq) + Cl - (aq) This system has no gases. It does NOT respond to pressure changes. How does this system respond to a change in pressure?

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Equilibrium shifts to the left. Concentration of N 2 & H 2 . Temperature . Stress What happens?

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Equilibrium shifts to the right. Concentration of N 2 & H 2 . Temperature . Stress What happens?

N 2 (g) + 3H 2 (g)  2NH 3 (g) + heat Equilibrium shifts to the left. Concentration of N 2 . Concentration of NH 3 . Temperature . Stress What happens?

Exothermic Reaction Energy is released: It flows from system to environment. Temperature of environment increases.

Endothermic Reaction Energy is absorbed: It flows from environment to system. Temperature of environment drops.

Heat of reaction,  H H products - H reactants

The difference between the potential energy of the products & the potential energy of the reactants? H products – H reactants or  H

 H for exothermic rxn  H is negative. System has net loss in energy.

 H is negative? Exothermic reaction. Products have less potential energy than reactants.

 H is positive? Endothermic reaction. Products have more potential energy than reactants.

 H for endothermic rxn  H is positive. System has net gain in energy.

A + B  C + D + heat Exothermic Reaction. Heat term is on product side.

A + B + heat  C + D Endothermic Reaction. Heat term is on reactant side.

Which phase change is endothermic? a)Gas to liquid b)Gas to solid c)Solid to gas d)Liquid to solid

Which phase change is exothermic? a)Solid to gas b)Liquid to gas c)Solid to liquid d)Liquid to solid

A reaction occurs in water & the temperature of the water increases. Endo or Exo? Exothermic

A reaction occurs in water & the temperature of the water decreases. Endo or Exo? Endothermic

A + B  C + D.  H = -45 kJ. What is  H for C + D  A + B?  H = +45 kJ

A + B  C + D.  H = -45 kJ. What is  H for: 2A + 2B  2C + 2D?  H = -90 kJ

Entropy Degree of disorder or chaos in a system.

Entropy The more degrees of freedom a system has, the higher the entropy.

S Entropy

H Enthalpy or Chemical Potential Energy

0 entropy? A perfect crystal at 0K

SS Change in entropy S products – S reactants

Which phase has the greatest entropy? Gas phase – the molecules have more ways of moving & more places to be!

Which phase has the greatest entropy? S gas >> S liquid > S solid

Did the entropy increase or decrease? NaCl(s)  Na + (aq) + Cl - (aq) Increase!

Did the entropy increase or decrease? CO 2 (g)  CO 2 (aq) Decrease!

Did the entropy increase or decrease? 2H 2 O(l)  2H 2 (g) + O 2 (g) Increase!

Spontaneous Process Physical or Chemical change that occurs with no outside intervention. (Supplying activation energy does not count!)

Nature is lazy & disorganized. Nature favors: Negative value for  H: Net loss Positive value for  S: Net gain

When will a change never occur? When both factors go against nature: Positive value for  H: Net gain in energy. Negative value for  S: Net loss in chaos

Endo: starts low, ends high Endo or Exo?

Exo: starts high, ends low Endo or Exo?

Reactants Identify reactants, products, activated complex Activated Complex Products

Potential energy of reactants Identify the arrow

Activation energy of forward rxn Identify the arrow

Heat of reaction Identify the arrow

Activation energy of reverse reaction Identify the arrow

Potential energy of activated complex Identify the arrow

Potential energy of products Identify the arrow

Collision Theory Effective collisions between reacting particles must occur for a reaction to take place.

Effective Collisions Energetic Favorable Orientation

Rate of reaction The speed of the reaction. The faster it occurs, the greater the rate.

Breaking Bonds … Absorbs Energy

Making Bonds … Releases Energy

6 Factors that influence the rate of reaction Nature of reactants (ionic or covalent) Temperature Concentration Pressure (for gases) Surface area (for heterogeneous reactions) Presence of a catalyst

Raising the temperature increases the reaction rate by Increasing both the number of collisions and the efficiency of the collisions.

As the concentrations of the reactants , the rate of rxn … increases

As the pressure on gas-phase reactants , the rate of rxn … Increases.

Which 4.0-gram Zn sample will react faster with acid, and why? a)Lump b)Bar c)Powdered d)Sheet metal More surface area

How does a catalyst work? Provides an alternate reaction pathway with a lower activation energy.

Does a catalyst change  H? NO.

Which reacts faster: ions in solution or covalent molecules? Ions in solution.

Which reacts faster in water: 1 cm 3 of Na or 1 cm 3 of Ca? 1 cm 3 of Na - Nature of the metals

Which is usually slower, an organic reaction or an inorganic reaction? Organic, because organic particles contain covalent bonds.

Lowers the activation energy. Effect of a catalyst?

Potential energy of reactants Identify the arrow

Activation energy of forward rxn Identify the arrow

Activation energy of reverse rxn Identify the arrow

Potential energy of activated complex Identify the arrow

Heat of reaction Identify the arrow

Potential energy of products Identify the arrow