Flashcards for Unit 12

Equilibrium

Rate of forward process=

Rate of reverse process

Conditions for Equilibrium

2-way processClosed System

Dynamic Equilibrium

Macroscopic level – looks constant.

Microscopic level – lots of action

Types of Equilibria

1) Physical: Phase & Solution2) Chemical

A solution at equilibrium must be

Saturated

Phase Equilibrium

H2O(l) H2O(g)

Solution EquilibriumIonic Cmpd

NaCl(s) Na+(aq) + Cl-

(aq)

Solution EquilibriumCovalent Cmpd

C6H12O6(s) C6H12O6(aq)

H2O(g) H2O(l)

Phase Equilibrium

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

(aq)

Solution Equilibrium

Chemical Equilibrium – Haber Synthesis

N2(g) + 3H2(g) 2NH3(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 H2O(s) and H2O(l) exist in equilibrium?

0C or 273K

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

100C or 373K

Pressure Changes

Only affect systems that have a gas in them

An increase in pressure favors which side?

CO2(g) CO2(aq)

When P , more CO2 dissolves!

N2(g) + 3H2(g) 2NH3(g) + heat

Equilibrium shifts to right.Concentration of H2 .

Concentration of NH3 . Temperature

StressStress What happens?What happens?

N2(g) + 3H2(g) 2NH3(g) + heat

Shifts to the side with fewer moles of gas, in this

case the right.

How does this system respond How does this system respond to an to an increaseincrease in pressure? in pressure?

4 moles 2 moles

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 How does this system respond to a to a decreasedecrease in pressure? in pressure?

H2(g) + I2(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 How does this system respond to a to a decreasedecrease in pressure? in pressure?

2 moles 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 How does this system respond to a to a changechange in pressure? in pressure?

N2(g) + 3H2(g) 2NH3(g) + heat

Equilibrium shifts to the left.

Concentration of N2 & H2 . Temperature .

Stress

What happens?What happens?

N2(g) + 3H2(g) 2NH3(g) + heat

Equilibrium shifts to the right. Concentration of N2 & H2 .

Temperature .

Stress

What happens?What happens?

N2(g) + 3H2(g) 2NH3(g) + heat

Equilibrium shifts to the left. Concentration of N2 . Concentration of NH3 .

Temperature .

Stress

What happens?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

Hproducts - Hreactants

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

Hproducts – Hreactants 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 liquidb) Gas to solidc) Solid to gasd) Liquid to solid

Which phase change is exothermic?

a) Solid to gasb) Liquid to gasc) Solid to liquidd) 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

Change in entropySproducts – Sreactants

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?

Sgas >> Sliquid > Ssolid

Did the entropy increase or decrease?

NaCl(s) Na+(aq) + Cl-(aq)

Increase!

Did the entropy increase or decrease?

CO2(g) CO2(aq)

Decrease!

Did the entropy increase or decrease?

2H2O(l) 2H2(g) + O2(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 lossPositive 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

EnergeticFavorable 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)TemperatureConcentrationPressure (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)Lumpb)Barc)Powderedd)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 cm3 of Na or 1 cm3 of Ca?

1 cm3 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