Thermodynamics Flashcards
Enthalpy
The sum of the system’s internal energy and the product of its pressure and volume
The ‘internal heat content’
H = Hp - Hr
Per MOLE
If H is +ve = ENDOthermic
If H is -ve = EXOthermic
Entropy
A thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.
S = Sp - Sr
Per MOLE
Gibbs free energy equation
G = H - TS G = change in Gibbs Free Energy H = change in enthalpy T = temperature in Kelvin S = change in entropy
A reaction will only occur if Gp < Gr
G = Gp - Gr
Therefore reactions only occur if G is NEGATIVE
Equillibrium system
Phase - All parts of a system that are chemically and physically
Component - Minimum number of chemical species to describe all phases
Equilibrium - /\G = 0
Phase diagrams - P-T diagrams
Degrees of freedom
How many variables can be changed independently without changing the phase assemblage. P-T diagrams - Φ + F = C + 2 T-X diagrams - Φ + F = C + 1 Φ - Phases F - Degrees of freedom C - Components
Having 2 degrees of freedom is divariant, having 1 is univariant, have 0 is invariant.
Eutectic is a point which combines the solidus and liquidus, where the mixture seems to go from solid to liquid on an invariant.
Le Chatelier’s Principle
A system at eqilibrium experiencing a change in physical conditions will respond to lessen the effects of the change.
This means at higher temperatures a high enthalpy phase is favoured as is causes an endothermic reaction and releases some energy.
e.g. when kyanite decomresses to andalusite, andalusite takes up a larger volume to account for a pressure decrease.
Claperyon Equation
Describes the gradients of reaction boundaries.
/\P//\T = change in molar entropy/ change in molar volume
Steep (>1) - change in entropy is large so the pressure is not very important, higher entropy phase is favoured at high temperatures.
Shallow (<1) - Volume change is large, temperature not as important as pressure, pressure decrease leads to larger volume phase.
Around 1 - Both factors are equally important.
Curved usually means a vapour phase is compressed.
Rate equation
The rate of a reaction depends on the available amounts of reactants, therefore as a reaction progresses it slows down.
Rate = -dCr/dt = k x Ca x Cb k is the reaction constant, Ca and Cb are the concentrations of both reactants.
In a homogenous reaction where all mix into a phase this is simpler while in heterogeneous reaction this depends on factors surface area to volume ratios.
Arrhenius Equation
Higher temperature -> Higher KE -> More molecules above activation energy -> More successful collisions -> Faster reaction
Activation energy is the energy needed to break the bonds within a substance to allow it to react.
k - Reaction constant Ea - Activation energy A - Reaction variable T - Temp in Kelvins R - Gas constant -> 8.314 J mol-1 K-1
Using ln on both sides makes it a straight-line linear equation.
Therefore activation energy can be found from Ea = -Rm
Fick’s first law
This measures the rate of flow of diffusion along a chemical boundary. J - rate of flow, D - Diffusion coefficient, C - Concentration, x - distance
Rate of diffusion is proportional to concentration gradient
Diffusion has an activation energy, is faster at higher temp[eratures and is inversly proportional to time.
If it is under the activation energy for diffusion it cannot come to equilibrium and is known as a BLOCKING temperature.
This causes a disequilibrium texture.
Catalysts
- Reduce activation energy
