ACER 10/21 Lecture #16: Phase Changes and Spontaneity

Is breaking bonds exothermic or endothermic? What is the enthalpy change value?

Is forming bonds exothermic or endothermic? What is the enthalpy change value?

Breaking bonds is endothermic, ∆H > 0

Forming bonds is exothermic, ∆H < 0

What equation can you use to calculate enthalpy change using bond energies?

∆H_rxn ≈ ∑(E_{bonds broken}) - ∑(E_{bonds formed})

This can be used to estimate the enthalpy change, but does not provide an exact value.

Can we measure lattice enthalpy?

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No, the lattice enthalpy of solids cannot be directly measured.

Instead we use the Born-Haber Cycle, a specific kind of Hess cycle. This method combines the enthalpies that we can measure (since enthalpy is a state function) to find the lattice enthalpy.

What do the intercepts on the dotted line in this graph mean? Use water as an example.

What is the triple point? Where is it on this graph?

The intercepts with the dotted line is the melting point (0°C for water) and the boiling point (100°C for water).

The triple point is the intercept between the curves. It is the point where the substance exists in all three phases (solid, liquid, gas) simultaneously (meaning the substance is freezing and boiling at the same time).

What are the 6 main state changes? (Some of them are called multiple names; include all of them)

Describe the sign of their enthalpy and how it is denoted. For some of them, compare their enthalpies.

• Heat of Fusion (Melting) is a change from Solid to Liquid.

  • Enthalpy change is positive: ∆H_fus > 0

• Heat of Solidification (Freezing) is a change from Liquid to Solid.

  • Enthalpy change is negative: ∆H < 0

  • Enthalpy change is the negative of ∆H_fusion

• Heat of Vaporization (Boiling/Evaporation) is a change from Liquid to Gas.

  • Enthalpy change is positive: ∆H_vap > 0

• Heat of Condensation is a change from Gas to Liquid.

  • Enthalpy change is negative: ∆H < 0

  • Enthalpy change is the negative of ∆H_vaporization

• Heat of Sublimation is a change from Solid to Gas.

  • Enthalpy change is positive: ∆H_sub > 0

• Heat of Deposition is a change from Gas to Solid.

  • Enthalpy change is negative: ∆H > 0

  • Enthalpy change is the negative of ∆H_sublimation

When is a process spontaneous?

What are two functions that can determine whether a process is spontaneous?

If it has a tendency to occur without being driven by an external influence

Determined by total entropy ∆S_total > 0; spontaneous reactions always result in greater total entropy of the universe

∆G < 0; negative Gibbs Free Energy

Does a spontaneous reaction need to occur quickly?

No.

What is entropy?

a measure of disorder—how many different ways a molecule can be arranged and have the same energy.

Entropy increases when… (4 pts)

Include an exception for two of these patterns

Volume or temperature increases (as long as the other is held constant)

• Volume changes actually result in a negligible entropy change for solids and liquids

Pressure decreases

• Pressure changes actually result in a negligible entropy change for solids and liquids

Gases are formed from solids or liquids, or when liquids are formed from solids

more moles are produced

What general equation can you use to calculate entropy?

Explain each variable.

∆S = q_rev/T

• q_rev is the heat transferred to/from a system during a reversible process. The greater the heat transferred, the greater the change in entropy.

• T is the temperature. The lower the temperature T, the greater the change in entropy ΔS.

What are the signs for entropy and enthalpy during melting or vaporization?

What are the signs for entropy and enthalpy during freezing or condensation?

melting/vaporization: ∆S > 0, ∆H > 0

freezing/condensation: ∆S < 0, ∆H < 0

How do you calculate entropy change at phase changes? (2 equations)

What three conditions apply?

∆S° = ΔH_fus°/T or ∆S° = ΔH_vap°/T

• the temperature remains constant as heat is added

• the heat transfer is reversible (meaning it is always at equilibrium)

• pressure is constant (meaning q_rev = ΔH)

How do you calculate the standard molar enthalpy of a reaction?

∆S^o_rxn = ∑(n.∆S°_product) - ∑(n.∆S°_reactant)

The second law of thermodynamics states…

The second law of thermodynamics states that processes always tend toward greater entropy of the system and the surroundings (the total entropy of the ”universe” increases).

What equation gives total entropy change (∆S_total)?

What equation gives entropy change of the surroundings (∆S_surr)?

What equation gives Gibbs Free Energy (∆G)?

How can you combine these equations to get a new equation for ∆G?

• What can you use this final equation for?

∆S_total = ∆S_surr + ∆S_sys

∆S_surr = -∆H_rxn/T

∆G = -T∆S_total

∆G = ∆H - T∆S

• This final equation can be used to predict spontaneity.

What does it mean if Gibbs Free Energy (∆G) is positive, negative, or zero?

If ∆G < 0, then the reaction is spontaneous.

If ∆G > 0, then the reaction is non-spontaneous.

If ∆G = 0, then the reaction is at equilibrium.

Determine under what conditions a reaction is spontaneous when ∆H > 0 and ∆S > 0.

High temperatures

Determine under what conditions a reaction is spontaneous when ∆H > 0 and ∆S < 0.

No temperatures; the reaction is non-spontaneous under all temperatures.

Determine under what conditions a reaction is spontaneous when ∆H < 0 and ∆S < 0.

Low temperatures

Determine under what conditions a reaction is spontaneous when ∆H < 0 and ∆S > 0.

All temperatures

Enthalpy, entropy, and Gibbs Free Energy are all…

state functions

Can Gibbs Free Energy change? What about Standard Gibbs Free Energy?

Standard Gibbs Free Energy ∆G° is fixed and does not change as the reaction proceeds (at a specific temperature).

Gibbs Free Energy ∆G continuously changes as the reaction proceeds (∆G depends on the concentrations of reactants and products).