Laws of Thermodynamics

Geochemistry-BS GeologyDepartment of Earth & Environmental Sciences, Bahria University, Islamabad

ThermodynamicsThermodynamics is the study of the heat, energy and effects of work on a system

It is a theory of the relations between heat and mechanical energy, and of the conversion of either into the other.Thermodynamics is only concerned with macroscopic (large-scale) changes and observations

Volume, Temperature, Pressure, Heat Energy, Work.

Importance in Geology

In geology, thermodynamics relates to the stability of rocks and minerals under varying heats and pressures.

Thermodynamics not only allows us to predict what minerals will form at different conditions (forward modeling), but also allows us to use mineral assemblages and mineral compositions to determine the conditions at which a rock formed (thermobarometry).

Terminologya system:Some portion of the universe that you wish to study

The surroundings:The adjacent part of the universe outside the system

Changes in a system are associated with the transfer of energy

Natural systems tend to attain states of minimum energy or equilibrium

a Phase: a mechanically separable portion of a system

MineralLiquidVapor

a Reaction: some change in the nature or types of phases in a system

Gibbs Free Energies of Phases

All phases, whether mineralogical or not, have an associatedGibbs Free Energy of Formationvalue abbreviatedas Gf.

TheGfvalue describes the amount of energy that is released or consumed when a phase is created from other phases.

Gibbs Free Energy of ReactionsTheGibbs free energy (Grxn)tells us whether a reaction will take place.Grxnis the Gibbs Free Energy of the right hand side of a reaction, minus the Gibbs Free Energy of the left hand side.

IfGrxn< 0, the reaction will proceed to the right; if it is > 0, the reaction will proceed to the left.

Consider the example of Enstatite. The Gibbs Free Energy of Formation for Enstatite (MgSiO3) from pure elements Mg, Si and O (Gfenstatite, elements) is about -1,460.9 J/mole at room temperature and pressure.

The Gibbs Free Energy of Formation for enstatite from oxides (MgO and SiO2) =Gf(enstatite, oxides) is about -35.4 J/mole at room temperature and pressure.Negative values indicate that enstatite is more stable than, and will form from, the separate elements or separate oxides at room P,T.

Enthalpy of ReactionTheenthalpy of reaction (Hrxn)tells us how much heat will flow in or out of the system in a chemical reaction.

IfHrxn< 0, the reaction is exothermic it releases heat. For example, combustion of carbon based compounds (C + O2= CO2) gives off a lot of heat.

IfHrxn> 0, the reaction isendothermic it consumes heat. Melting ice [H2O (ice) = H2O (water)] is endothermic.

Entropy of a ReactionTheentropy of a reaction (Srxn)tells us whether the products or the reactants are more disordered.

For example, the reaction of liquid water to steam (boiling) has a large associated entropy.

The steam molecules are more dispersed, are less well bonded together, and have greater kinetic energy.

Internal Energy & TemperatureInternal energy (also called thermal energy) is the energy an object or substance have due to the kinetic and potential energies associated with the random motions of all the particles that make it up.

The hotter something is, the faster its molecules are moving or vibrating, and the higher its temperature. Temperature is proportional to the average kinetic energy of the atoms or molecules that make up a substance

Internal Energy & HeatThe term heat refers is the energy that is transferred from one body or location due to a difference in temperature. Heat is internal energy when it is transferred between bodies.

Technically, a hot potato does not possess heat; rather it possesses a good deal of internal energy on account of the motion of its molecules. If that potato is dropped in a bowl of cold water, we can talk about heat: There is a heat flow (energy transfer) from the hot potato to the cold water; the potatos internal energy is decreased, while the waters is increased by the same amount.

Thermal EquilibriumTwo bodies are said to be at thermal equilibrium if they are at the same temperature. This means there is no net exchange of thermal energy between the two bodies.

Types of ProcessesIsobaric Processes:

Transformations at constant pressure dp = 0

Isochoric Processes:

Transformations at constant volume dV = 0

pVifpVif

Types of ProcessesIsothermal Processes:

Transformations at constant temperature dT = 0

Adiabatic Processes:

Transformations without the exchange of heat between the environment and the system dQ = 0

pVif

The Zeroth Law of ThermodynamicsIf object A is in thermal equilibrium with object C, and object B is separately in thermal equilibrium with object C, then objects A and B will be in thermal equilibrium if they are placed in thermal contact.

The First Law of ThermodynamicsThe first law of thermodynamics is a statement of the conservation of energy. Energy neither can be created nor destroyed.

If you burn a piece of wood, you change the potential chemical energy of the wood into thermal (heat) energy, and the wood transforms from a log into smoke and ashes. You have altered the wood but not destroyed it.

The First Law of ThermodynamicsIf a systems volume is constant, and heat is added, its internal energy increases.

The First Law of ThermodynamicsIf a system does work on the external world, and no heat is added, its internal energy decreases.

The First Law of ThermodynamicsCombining these gives the first law of thermodynamics. The change in a systems internal energy is related to the heat Q and the work W as follows:It is vital to keep track of the signs of Q and W.

The Second Law of ThermodynamicsIt states that the amount of energy doesn't stay the same in a transfer of energy or energy conversion.

For example, it takes a certain amount of energy to charge a battery. But the amount of energy that the charged battery can produce (to power a flashlight, for example) is less than the amount of energy needed to charge it in the first place.

The Second Law of ThermodynamicsWe observe that heat always flows spontaneously from a warmer object to a cooler one, although the opposite would not violate the conservation of energy. This direction of heat flow is one of the ways of expressing the second law of thermodynamics:When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object. Spontaneous heat flow never proceeds in the reverse direction.2nd law is also called as Law of Increased Entropy

EntropyIt is a measure of number of specific ways in which a system can be arranged or a measure of a system s progression towards thermal equilibrium.

Entropy is a state function ( have certain value at a particular stage of system)

Entropy of a reversible system remain conserved

Entropy of a irreversible system increases

The total entropy of the universe increases whenever an irreversible process occurs.

The total entropy of the universe is unchanged whenever a reversible process occurs.

The entropy is maximum when a system reaches thermal equilibrium

Since all real processes are irreversible, the entropy of the universe continually increases

EntropyIf we look at the ultimate fate of the universe in light of the continual increase in entropy, we might envision a future in which the entire universe would have come to the same temperature. At this point, it would no longer be possible to do any work, nor would any type of life be possible. This is referred to as the heat death of the universe.

The Third Law of ThermodynamicsThird Law of Thermodynamics states that since heat produces motion and heat does not exist at absolute zero. Therefore, at absolute zero an object has neither potential energy nor molecular disorder.

Absolute zero is a temperature that an object can get arbitrarily close to, but never attain.

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