CHAPTER 20 ENERGY AND DISORDER

20-1 Why reactions occur

exothermic reactions generally take place spontaneously.

endothermic reactions generally do not take place spontaneously

natural process: high energy-->low energy order->disorder

isothermal-constant temperature

isobaric-constant pressure

thermodynamics-study of energy flow

systems in nature tend to move toward states os low energy and

high disorder

20-2 State functions

a state function is one whose value depends only on the current

state of the system

P,V,&T are examples of state functions

the change in a state function is independent of the path and

depends only on the initial and final states

20-3 internal energy

internal energy(U) is a state function

2 ways to transfer energy into a system: heat it and by doing work on it.

delta U=q +w q=heat absorbed/w=work done on sys/U=internal energy

q and w are not state functions-U is

20-4 Enthalpy

enthalpy=H= U + PV

exothermic reaction: delta H<0 /negative because products are at

a lower enthalpy than reactants

endothermic reaction: delta H>0 /positive prod higher H reactants

20-5 Standard States

standard state refers to the enthalpy substances have at 298.15k

(25C or room temp) and 100 kPa

enthalpy of a free element= 0 kJ/mol

20-6 Enthalpy of formation

enthalpy of formation is the change in enthalpy when 1 mole of a

compound is produced from free elements in their standard states

thermodynamically stable compounds have large negative enthalpies

of formation

delta H_f=enthalpy of formation at 100.000 kPa and 25C

20-7 Calculation of enthalpy of reaction

the superscript "₀" indicates that the values given are at their

standard states(100 kPa and 25C)

a positive delta H⁰ indicates an endothermic reaction

a negative delta H⁰ indicates an exothermic reaction

enthalpy change, delta H_r0, for a reaction is the difference

between the enthalpies of the products and the reactants

20-8 Hess's Law

Hess's Law:the enthalpy change for a reaction is the sum of the

enthalpy changes for a series of reactions that add up to the

overall reaction

20-9 Entropy

a spontaneous process is one that occurs in a system left to itself. No external action is needed to make it happen

entropy means degree of disorder in a system

S=entropy delta S>0 increase in disorder

delta S<0 decrease in disorder

when a system increases in disorder, delta S is positive. In the

series of state changes for a substance, solid->liquid->gas,

entropy increases

20-10 Gibb's free energy

spontaneous reactions are exergonic/non spontaneous reactions are

endergonic

free energy determines if a reaction will occur or not

Gibbs free energy G=H-TS H=enthalpy/T=kelvin temp/S= entropy

endothermic reactions occur spontaneous when TdeltaS is large

Memorize table 20-1 pg 401

a spontaneous reaction proceeds toward equilibrium

chemical potential energy,G, is least at equilibrium

20-11 Gibbs free energy calculations

spontaneous reaction: delta G<0

the gibbs free energy decreases(delta G<0) in a spontaneous reaction because the system is changing to a more stable

state

decrease in disorder: delta S<0