SPONTANEITY OF PHYSICAL AND CHEMICAL CHANGES · PDF fileLectures 36-37: • Chemical...
Embed Size (px)
Transcript of SPONTANEITY OF PHYSICAL AND CHEMICAL CHANGES · PDF fileLectures 36-37: • Chemical...
Chem 1102 Semester 1, 2011
SPONTANEITY OF PHYSICAL AND CHEMICAL CHANGES
Lectures 36-37: Chemical Kinetics
Lecture 38: Spontaneity of physical and chemical changes
Spontaneity of physical and chemical changes
Thermodynamics Many manifestations of E: solar, geothermal, chemical
Nearly all E we depend on comes from the sun.
Connection between E and the extent of rxn.
Thermodynamics: study of E
Thermochemistry: study of relationship between E and the extent of chemical rxns.
Entropy provides an insight into why chemical and physical processes tend to favour one direction and not another.
Na(s) + H2O(l) NaOH(aq) + H2(g) NaOH(aq) + H2(g) Na(s) + H2O(l)
4Fe(s) + 3O2(g) 2Fe2O3(s)
Cellulose (C6H10O5)n + 5O2(g) 12CO2(g) + 10H2O(g)
Melting of ice at r.t.
A change for which the collection of Products is thermodynamically more stable than the collection of reactants under the given conditions is said to be product-favoured or spontaneous.
Library of Alexandria IV century AC
Mixing/Diffusion of gases is spontaneous, but no energy change is involved! The mixed state is more probable."
A spontaneous chemical reaction of physical change is one that can happen without any continuing outside influence.
Any spontaneous change has a natural direction.
The fact that a process is spontaneous does not mean that it will occur at an observable rate.
It may occur rapidly, at a modest rate or very slowly. The rate at which a reaction proceeds is addressed by kinetics.
Spontaneity of a chemical reaction or a physical process depends on two main groups of factors: energetic factors and order factors.
The 3 laws of thermodynamics as applied to chemical systems describe the nature and interplay of these two of factors on the spontaneity of natural processes. 6
Let us recall some basic definitions."The system: the portion of the universe being studied (e.g. a chemical
reaction or a physical process)."The surroundings: everything that is outside the system (e.g. the
immediate surroundings to the chemical reaction: reaction vessel and anything beyond it)."
The universe: the system "+ the surroundings."
Energy is commonly defined as the capacity to do w or to transfer q (J)"W is the energy used to cause an object with mass m to move against a
force F." ""Q is the energy used to cause the T of an object to increase. 7
E.g. Dissolve NaCl in water resulting solution gets cold. Dissolve LiCl in water resulting solution gets very warm.
1st process absorbs heat (q) from surroundings. 2nd process gives out q.
First Law Of Thermodynamics
First Law of Thermodynamics (qualitatively) The total amount of energy in the universe is constant.
Law of Conservation of Energy which is just another statement of the first law of TD:
Energy is neither created nor destroyed, but only transformed and transferred between the system and its surroundings.
What is it transformed into? Other forms of energy and/or work (w)! 8
Internal Energy Internal Energy (U) of a system: sum of all the kinetic and potential energies of all of its individual component particles. I.e. kinetic energy of motions of the gas molecules through space (their rotations, translations and internal vibrations) plus potential energies of electrostatic interactions between the nuclei and e-s.
E.g. Fuels burn and release E. Chemical E of these substances is due to the potential E stored in the arrangements of the atoms of the substance.
CH4(g) + 2O2(g) CO2(g) +2H2O(l) H= -890 kJ
One of the most important forms of potential E in chemistry is electrostatic potential E. Arises from the interactions between charged particles.
Internal Energy Kinetic energy of motions of the gas molecules through space (their rotations, translations and internal vibrations).
"Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways... their dancing is an actual indication of underlying movements of matter that are hidden from our sight... It originates with the atoms which move of themselves [i.e., spontaneously]. Then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible. - Lucretius, 60 BC, De Rerum Natura.
U = Ufinal-Uinitial
First Law of Thermodynamics (quantitatively): any energy lost by the system must be gained by its surroundings. In order to apply the first law quantitatively, we need to define the E of a system more precisely.
U = q + w The system can exchange energy with its surroundings as heat or work. U of a
system changes in magnitude as q is added to (+ve value) or removed from(-ve value) the system or as w is done on (+ve value) or by (-ve value) the system.
Exothermic Endothermic , ex, "outside" , endon, "within"
Internal Energy, a state function When q is added to a system or w is done on a
system, its U increases. The value of a state function depends only on the present state of the system, not on the path the system took to reach the state.
Heat and Work
q and w are not state functions.
I.e. the specific amount of q and w produced depend on the way in which the change is carried out.
Rxns can happen at: 1. Constant P (the vast majority) or 2. Constant V
At constant P, w = -P V
Pin prevents piston from moving
Rxn occurs, P builds up. P higher than atmospheric P.
Remove pin. Gas P goes to atm P. w done as piston moves.
e.g. Zn(s) + H+(aq) Zn2+(aq) + H2(g)
Enthalpy (H) From the Greek = to warm. Accounts for heat flow in processes occurring at constant P when no other forms of work are performed apart from P-V work. H=U + PV State function because U, P and V are state functions Enthalpy change H= U +PV Remember U= q + w and w= -P V; hence H= (qp + w)-w = qp E.g. 2H2(g) + O2(g) 2H2O(g) H= -484 kJ H2O(l) H2O(g) H= +44 kJ
Spontaneity is favoured when q is released during the process. However many chemical rxns and physical transformations that happen
spontaneously though accompanied by an increase of potential energy, ie an H increase. Endothermic reactions. E.g. dissolution in water of salts (NaCl, KCl, ), fusion of ice into liquid water at room temperature,
1850s: There is something missing maybe H is not the only factor affecting spontaneity of a process.
Spontaneity is also favoured when the change causes an increase in disorder. S another state function. From the Greek, - [en-] (in) and [trop]
(turn, conversion). In the first law: E is conserved in any process, which means that in the whole
universe the amount of total E is fixed. However the amount of S increases in any spontaneous process. In
spontaneous changes, the universe tends towards a state of greater disorder. S: measure of the disorder of the system.
In a spontaneous process, the entropy of the universe must increase:
Suniverse = Ssystem + Ssurroundings > 0
The Universe becomes more disordered. Like the First Law of Thermodynamics, the Second Law can never be violated. In a spontaneous process, a decrease in the S of the system can only occur if increases in the S of the surroundings outweigh them.
Second Law Of Thermodynamics
Rudolf Clausius, concept of entropy, 1865 17
Many definitions for S. -associated with extent of randomness in a system. -with the extent to which energy is distributed among the various
motions of the particles in the system. -the amount of energy not available for useful work in a
thermodynamic process. Also a state function. The more ways a particular state can be achieved, the greater is the likelihood of finding it in that state.
The black and white gas molecules have more freedom if they can move around the entire system. The system on the right is in a favoured state of entropy.
Entropy (S) - a property of bulk matter Must be connected to the behaviour of atoms and molecules. Statistical thermodynamics.
1 Mole of a gas: the molecules are in constant chaotic motion distribution of kinetic energies.
Take a photo at time t0. In that particular instant, each of the 6.03 x 1023 molecules travels at a particular speed and has a particular position. Take a photo at time t1, t2, t3 etc The speeds and positions of each one of those particles will be different. microstates of the thermodynamic system. a single possible arrangement of the positions and kinetic Es of the gas molecules when the gas is in a specific thermodynamic state.
Statistical thermodynamics defines S as a thermodynamic quantity that describes the number of arrangements that are available to the system in a given state.
Mathematically: S = k ln W S = k ln Wfinal - k ln Winitial =