# Topic 13.2 - Energetics II: Entropy Flashcards

What is the equation for the energy change (J) of a reaction?

Q = mc(delta)T

Q - energy in Joules, J

m - mass of solution, g (equivalent to the volume of solution as we assume density = 1)

(delta)T - change in temperature, K

What is the equation for the enthalpy change of reaction when you know the energy change of the reaction?

(delta) H = Q/n

(delta)H - Enthalpy change of reaction, kJmol(-1)

Q - energy in Kilojoules, kJ

n - Number of moles of limiting reagant

What is the equation for the enthalpy change of reaction when you know the enthalpy change of formation for your products and reactants?

(delta) H reaction = (delta) H products - (delta) H reactants

All units are kJmol(-1)

What is the equation for the standard total entropy change of a reaction?

(delta) S total = (delta) S system + (delta) S surroundings

All units JK(-1)mol(-1)

Under standard conditions

What is the equation for the standard entropy of the system?

(delta) S system = (delta) S products - (delta) S reactants

All units JK(-1)mol(-1)

Under standard conditions

What is the equation for the standard entropy of the surroundings?

(delta) S surroundings = - (delta)H reaction/ T

(delta) S - Entropy change of surroundings, JK(-1)mol(-1)

(delta) H - Enthalpy change of reaction, convert to Jmol(-1)

T - Temperature, K

What is the standard equation for Gibbs Free energy?

(delta) G = (delta) H reaction - T(delta) S system

What is the equation for Gibbs Free energy that includes the equilibrium constant of a reaction?

(delta) G = -RTlnKc

(delta) G - Gibbs free energy of the reaction, KJmol(-1)

R - Gas constant, 8.31 Jmol(-1)K(-1)

T - Absolute temperature, K

Kc - Equilibrium constant, from this equation you cant know the units of Kc

Under standard conditions

What is the equation for Gibbs Free energy that includes the Ecell value of a reaction?

(delta) G = -nFEcell

(delta) G - Gibbs Free energy change, KJmol(-1)

n - the number of electrons involved in the redox reaction

F - Faraday constant, 9.65 x10^4 Cmol(-1)

Ecell - Standard cell potential, V

What is the equation for the standard total change in entropy that includes the Ecell value of the reaction?

(delta) S total =(nFEcell)/T

(delta) S total - Total entropy change of the reaction, JK(-1)mol(-1)

n - the number of electrons involved in the redox reaction

F - Faraday constant, 9.65 x10^4 Cmol(-1)

Ecell - Standard cell potential, V

What is the definition of bond enthalpy?

The energy required to break 1 mole of that bond

What is the equation for the standard total enthalpy change of a reaction when you are given the bond energies?

(delta) H total = (sum of) Bonds broken - (sum of ) Bonds made

What sign does entropy always have? Why?

+ve, as particles are always in constant motion so there is always a degree of disorder

What is the definition of Entropy?

Entropy, S, is a quantitative measure of the degree of disorder of a system

When is the only time that entropy of a system = 0?

At Absolute Zero

What is the definition of a Spontaneous reaction?

A reaction that does not require any external input of energy in order to occur

What is the definition of a Feasible reaction?

A reaction that naturally tends to happen, even if it is very slow as the activation is high

What is required, in terms of entropy, for a reaction to be feasible?

- For (delta) S total > 0

Are spontaneous reactions always feasible?

Yes

Are feasible reactions always spontaneous?

No - A reaction can be thermodynamically feasible but not occur due to a very high activation energy (kinetics)