Flashcards for Unit 12

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Flashcards for Unit 12
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Equilibrium

• Rate of forward process
• Rate of reverse process

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Conditions for Equilibrium

• 2-way process
• Closed System

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Dynamic Equilibrium

• Macroscopic level looks constant.
• Microscopic level lots of action

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Types of Equilibria

1. Physical Phase Solution
2. Chemical

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A solution at equilibrium must be

• Saturated

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Phase Equilibrium

• H2O(l) ? H2O(g)

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Solution EquilibriumIonic Cmpd

• NaCl(s) ? Na(aq) Cl-(aq)

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Solution EquilibriumCovalent Cmpd

• C6H12O6(s) ? C6H12O6(aq)

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H2O(g) ? H2O(l)

• Phase Equilibrium

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NH4Cl(s) ? NH4(aq) Cl-(aq)

• Solution Equilibrium

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Chemical Equilibrium Haber Synthesis

• N2(g) 3H2(g) ? 2NH3(g) heat

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LeChateliers Principle

• A system at equilibrium will respond to remove a
  stress

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Whats a stress for a chemical system?

• Change in temperature, pressure, or concentration

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Catalyst

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

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Catalyst

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

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At what temperature can H2O(s) and H2O(l) exist
in equilibrium?

• 0?C or 273K

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At what temperature can H2O(g) and H2O(l) exist
in equilibrium?

• 100?C or 373K

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Pressure Changes

• Only affect systems that have a gas in them

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An increase in pressure favors which side?

• CO2(g) ? CO2(aq)

When P ?, more CO2 dissolves!
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N2(g) 3H2(g) ? 2NH3(g) heat

• Equilibrium shifts to right.
• Concentration of H2 ?. Concentration of NH3 ?.
  Temperature ?

What happens?
Stress
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N2(g) 3H2(g) ? 2NH3(g) heat

• Shifts to the side with fewer moles of gas, in
  this case the right.

4 moles
2 moles
How does this system respond to an increase in
pressure?
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N2(g) 3H2(g) ? 2NH3(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?
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H2(g) I2(g) heat ? 2HI(g)

• This system has the same of moles on each side.
  It cannot respond to pressure changes.

2 moles
2 moles
How does this system respond to a decrease in
pressure?
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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?
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N2(g) 3H2(g) ? 2NH3(g) heat

• Equilibrium shifts to the left.
• Concentration of N2 H2 ?. Temperature ?.

Stress
What happens?
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N2(g) 3H2(g) ? 2NH3(g) heat

• Equilibrium shifts to the right.
• Concentration of N2 H2 ?. Temperature ?.

Stress
What happens?
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N2(g) 3H2(g) ? 2NH3(g) heat

• Equilibrium shifts to the left.
• Concentration of N2 ?. Concentration of NH3 ?.
  Temperature ?.

Stress
What happens?
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Exothermic Reaction

• Energy is released It flows from system to
  environment. Temperature of environment
  increases.

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Endothermic Reaction

• Energy is absorbed It flows from environment to
  system. Temperature of environment drops.

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Heat of reaction, ?H

• Hproducts - Hreactants

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The difference between the potential energy of
the products the potential energy of the
reactants?

• Hproducts Hreactants or ?H

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?H for exothermic rxn

• ?H is negative. System has net loss in energy.

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?H is negative?

• Exothermic reaction. Products have less
  potential energy than reactants.

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?H is positive?

• Endothermic reaction. Products have more
  potential energy than reactants.

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?H for endothermic rxn

• ?H is positive. System has net gain in energy.

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A B ? C D heat

• Exothermic Reaction. Heat term is on product
  side.

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A B heat ? C D

• Endothermic Reaction. Heat term is on reactant
  side.

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Which phase change is endothermic?

1. Gas to liquid
2. Gas to solid
3. Solid to gas
4. Liquid to solid

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Which phase change is exothermic?

1. Solid to gas
2. Liquid to gas
3. Solid to liquid
4. Liquid to solid

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

• Exothermic

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

• Endothermic

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A B ? C D. ?H -45 kJ.

• What is ?H for C D ? A B?
• ?H 45 kJ

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A B ? C D. ?H -45 kJ.

• What is ?H for
• 2A 2B ? 2C 2D?
• ?H -90 kJ

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Entropy

• Degree of disorder or chaos in a system.

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Entropy

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

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S

• Entropy

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H

• Enthalpy
• or
• Chemical Potential Energy

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0 entropy?

• A perfect crystal at 0K

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?S

• Change in entropy
• Sproducts Sreactants

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Which phase has the greatest entropy?

• Gas phase the molecules have more ways of
  moving more places to be!

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Which phase has the greatest entropy?

• Sgas gtgt Sliquid gt Ssolid

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Did the entropy increase or decrease?

• NaCl(s) ? Na(aq) Cl-(aq)

Increase!
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Did the entropy increase or decrease?

• CO2(g) ? CO2(aq)

Decrease!
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Did the entropy increase or decrease?

• 2H2O(l) ? 2H2(g) O2(g)

Increase!
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Spontaneous Process

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

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Nature is lazy disorganized.

• Nature favors
• Negative value for ?H Net loss
• Positive value for ?S Net gain

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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

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• Endo starts low, ends high

Endo or Exo?
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• Exo starts high, ends low

Endo or Exo?
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Activated Complex

• Reactants

Products
Identify reactants, products, activated complex
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• Potential energy of reactants

Identify the arrow
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• Activation energy of forward rxn

Identify the arrow
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• Heat of reaction

Identify the arrow
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• Activation energy of reverse reaction

Identify the arrow
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• Potential energy of activated complex

Identify the arrow
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• Potential energy of products

Identify the arrow
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Collision Theory

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

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Effective Collisions

• Energetic
• Favorable Orientation

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Rate of reaction

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

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Breaking Bonds

• Absorbs Energy

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Making Bonds

• Releases Energy

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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

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Raising the temperature increases the reaction
rate by

• Increasing both the number of collisions and the
  efficiency of the collisions.

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As the concentrations of the reactants ?, the
rate of rxn

• increases

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As the pressure on gas-phase reactants ?, the
rate of rxn

• Increases.

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Which 4.0-gram Zn sample will react faster with
acid, and why?

1. Lump
2. Bar
3. Powdered
4. Sheet metal

More surface area
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How does a catalyst work?

• Provides an alternate reaction pathway with a
  lower activation energy.

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Does a catalyst change ?H?

• NO.

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Which reacts faster ions in solution or
covalent molecules?

• Ions in solution.

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Which reacts faster in water 1 cm3 of Na or 1
cm3 of Ca?

• 1 cm3 of Na -
• Nature of the metals

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Which is usually slower, an organic reaction or
an inorganic reaction?

• Organic, because organic particles contain
  covalent bonds.

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• Lowers the activation energy.

Effect of a catalyst?
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• Potential energy of reactants

Identify the arrow
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• Activation energy of forward rxn

Identify the arrow
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• Activation energy of reverse rxn

Identify the arrow
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• Potential energy of activated complex

Identify the arrow
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• Heat of reaction

Identify the arrow
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• Potential energy of products

Identify the arrow