AH Chemistry - Physical Chemistry - (b) - Reaction Feasibility

What is the standard enthalpy of formation?

The standard enthalpy of formation, ∆H°, is the enthalpy change when one mole of a substance is formed from its elements in their standard states.

What is the standard state of a substance?

The standard state of a substance is its most stable state at a pressure of 1 atmosphere and at a specified temperature, usually taken as 298K.

How can the enthalpy of a reaction be calculated?

The standard enthalpy of a reaction can be calculated from the standard enthalpies of the formation of the reactants and the products.

What is the formula for calculating the standard enthalpy of formation, ∆H°?

The formula for calculating the standard enthalpy of formation, ∆H°, is:

∆H°=∑∆H°_f (products) - ∑∆H°_f (reactants)

What is the entropy of a system?

The entropy of a system (S) is a measure of the degree of disorder of the system.

What does a greater degree of disorder result in?

The greater degree of disorder, the greater the entropy.

What degree of disorder do solids have?

Solids have low disorder.

What degree of disorder do gases have?

Gases have high disorder.

What happens to entropy as temperature increases?

As temperature increases, entropy also increases.

When are there a rapid changes in entropy?

There is a rapid increase in entropy at the melting point of a substance and an even more rapid and larger change in entropy at the boiling point.

What does the second law of thermodynamics state?

The second law of thermodynamics states that the total entropy of a reaction system and its surroundings always increases for a spontaneous process.

What does heat energy released by the reaction system into the surroundings increase?

Heat energy released by the reaction system into the surroundings increases the entropy of the surroundings.

What does heat energy absorbed by the reaction system from the surroundings decrease?

Heat energy absorbed by the reaction system from the surroundings decreases the entropy of the surroundings.

What does the third law of thermodynamics state?

The third law of thermodynamics states that the entropy of a perfect crystal at 0K is zero.

What is the standard entropy of a substance?

The standard entropy of a substance is entropy value for the standard state of the substance.

How can the change in standard entropy for a reaction system be calculated?

The change in standard entropy for a reaction system can be calculated from the standard entropies of the reactants and products.

What is the formula used to calculate the change in standard entropy for a reaction?

The formula used to calculate the change in standard entropy for a reaction is:

∆S°=∑∆S°(products) - ∑∆S°(reactants)

What is the change in free energy for a reaction related to?

The change in free energy for a reaction is related to the enthalpy and entropy changes.

What is the formula for calculating free energy, ∆G, using enthalpy changes and entropy changes?

The formula for calculating free energy, ∆G, using enthalpy changes and entropy changes is:

∆G = ∆H - T∆S

If the change in free energy, ∆G, between reactants and products is negative what may occur and why is this?

If the change in free energy, ∆G, between reactants and products is negative, a reaction may occur and the reaction is said to be feasible. This is because the equilibrium composition favours the products over the reactants. This does not give any indication of the rate of the reaction.

What is a feasible reaction?

A feasible reaction is one that tends toward the products rather than reactants.

What is the formula for calculating the standard free energy change for a reaction from the free energies of formation of the products and the reactants?

The formula for calculating the standard free energy change for a reaction from the free energies of formation of the products and the reactants using the relationship:

∆G = ∑∆G(products) - ∆G = ∑∆G(reactants)

How can the feasibility of a chemical reaction under standard conditions be predicted?

The feasibility of a chemical reaction under standard conditions can be predicted from the calculated value of the change in standard free energy.

How can the temperatures at which a reaction may be feasible be estimated?

The temperatures at which a reaction may be feasible can be estimated by considering the range of values of T for which ∆G° < 0.

Under non-standard conditions any reaction is feasible is ∆G is what?

Under non-standard conditions any reaction is feasible is ∆G is negative.

At equilibrium ∆G equals what?

At equilibrium ∆G equals 0.

A reversible reaction will proceed spontaneously until the composition is reached where ∆G = what?

A reversible reaction will proceed spontaneously until the composition is reached where ∆G = 0.

What kind of reaction is endothermic?

Bond breaking is endothermic (∆H + ve)

What kind of reaction is bond making?

Bind making is exothermic (∆H - ve)