Causes of Change

Causes of ChangeThermochemistry

Thermodynamics Thermodynamics is the study of the flow

or exchange of energy. System Surroundings

The First Law of Thermodynamics

The law of Conservation of Energy The total amount of energy in the

universe is constant The change in energy of the system +

the change in energy of the surroundings equals zero

“You can’t get something for nothing”

The movement of heat Heat is always transferred from

hot to cold

Enthalpy Enthalpy (H) is the heat content of a

system at a constant pressure. H or the change in enthalpy is what we

calculate H = Hproducts - Hreactants

Molar Heat Capacity The molar heat capacity of a substance

is the amount of energy needed to raise 1 mole of a pure substance by one degree Celcius or Kelvin. C = q / mol x T Just like specific heat, except you are

using moles instead of mass

Molar Enthalpy The change in energy per mole is

referred to as Molar Enthalpy q/mol=H

So, let’s change the molar heat capacity equation . . . If C=q/mol x T, and q/mol = H, then C=H/T or H=CT Remember that T= Tfinal - Tinitial

What does H mean? Chemical changes and changes in physical

states either release or absorb energy. Exothermic reactions release heat so heat is

a product, let’s look at water freezingH2O(l, 292K) H2O(s, 273K) + 459.8 J

T = final – initial = 273K – 292K = -19KH=CT=(24.2J/molK)(-19K)= -459.8J/mol

What does H mean? Endothermic reactions absorb heat so it

feels cold, let’s look at ice meltingH2O(s, 273K) + 459.8 J H2O(l, 292K)

T = final – initial = 292K – 273K = 19KH=CT=(24.2J/molK)(19K)= 459.8J/mol

Reaction Pathways Reaction pathways are used to illustrate the

amount of energy throughout the reaction Reactants must absorb some amount of

energy in order for a reaction to proceed (unless it is a spontaneous reaction) This amount of energy is referred to as

Activation Energy Once the energy is absorbed, the reaction

can proceed These compounds are now referred to as an

Activated Complex

Exothermic Pathway In an exothermic

reaction, the reactants absorb the activation energy but the resulting products lose energy (H)

H= a negative number

Endothermic Reaction In an endothermic

reaction, the reactants absorb the activation energy and so the resulting products have more energy (H)

H= a positive number

So to review, Endo or Exo? A positive change in enthalpy would be

an endothermic reaction. A negative change in enthalpy would be

an exothermic reaction

Enthalpy (cont.) What would happen if the temperature

of a sample decreased? The kinetic energy of the sample also

decreases! If the kinetic energy of the sample

decreases, the enthalpy (H) decreases

Entropy The measure of the randomness or

disorder in a system is called entropy (S) and is measured in J/K Aha! What are the variables in entropy? You guessed it, energy and temperature!

The entropy change in a system is the measure of the entropy of the products minus the reactants ΔS = Sproducts - Sreactants

What is disorder? Disorder is any system which is not

ordered a solid has very ordered particles a liquid has slightly less ordered particles a gas has very unorderly particles your locker is a very disorderly!

• In any spontaneous change the entropy of the universe must increase

• ΔSsys + ΔSsurr > 0

• ΔSreaction = ΔSproducts - ΔSreactants

• “You can’t break even”

The Second Law of Thermodynamics

Entropy

Randomness of the system

Increasing order

Increasing entropy

Trends in Entropy What happens if . . .

Temperature increases, disorder increases Change in phase toward gas, disorder increases Concentration decreases, disorder increases # of products increases, disorder increases A solute is dissolved in a solvent, disorder

increases

Chemical EntropyChemical Entropy

Lower Entropy Higher Entropy

Entropy ProblemsWill the entropy change for each of the following be positive or negative?

Sugar dissolves in tea Air is pumped into a tire Acetone evaporates from nail polish CaCO3(s) CaO(s) + CO2(g)

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

Gibb’s Energy Gibb’s Energy is the measure of the

available energy in a system, the energy available to do work often referred to at “free” energy

The change in Gibb’s Energy is, ΔG = Gproducts - Greactants

Gibb’s is Not alone Gibb’s Energy is a function of enthalpy

(H), entropy (S) and temperature (K) ΔG = ΔH - TΔS

So, what happens when the temperature goes up? Kinetic energy increases, entropy

increases and temperature increases . . . Now what!

Let’s Talk Spontaneous A reaction is spontaneous (proceeds

without any help) if the change in Gibb’s Energy is negative That would mean that the measure of the

temperature times the change in entropy is a large number

This will help predictH S G Spontaneous?

- + - Yes at all temperatures

- - + / - Yes, at low temp.

+ + + / - Yes, at high temp.

+ - + No

2NO2(g) 2N2(g) + O2(g) The change in enthalpy is negative What is the change in entropy?

Positive Is this spontaneous or not?

Yes, it is spontaneous

H2O(l) H2O(s) Is the reaction endothermic or exothermic?

Exothermic Is the enthalpy positive or negative?

Negative Does entropy increase or decrease?

Decrease Is this spontaneous?

Only at low temperatures

2NH3(g) N2(g) + 3H2(g) The change in enthalpy is positive. What is the change in entropy?

It increases. Is this reaction spontaneous?

Yes, as long as the temperature is high

3O2(g) 2O3(g) What would you predict the change in

enthalpy would be? (endo or exo?) Endothermic, so a positive change

What do you predict the entropy would be? More ordered, so decreased

So . . . Spontaneous? No, not at any temperature

Hess’s Law Well, Hess’s law states that the total

amount of energy lost or gained in a given reaction is the sum of each step in the reaction.

Some reactions must occur in 2 or more steps.

Apply Hess’s Law Guess what is conserved according to

Hess’s Law?

Yeah! Energy!

Add Each Step Let’s decompose water a little bit

3H2O(g) 3H2O(l) ΔH = -132 kJ

3H2O(l) 3H2(g) + (3/2)O2(g) ΔH = +858 kJ

The total change in enthalpy is 858 kJ - 132 kJ = +726 kJ