Enthalpy, Entropy, and Heat Review

All the symbols!

• q - heat needed
• ΔT - Temperature change
• ΔH - Enthalpy change
  • ΔH° - in standard conditions
• ΔS - Entropy change
• R - Gas constant (8.314 J/Kmol)
• K - Dissociation rate
• ΔG - Gibbs free energy / available energy
  • ΔG° - in standard conditions

q (Heat)

• q = mcΔT
  • q - heat needed (J)
  • m - mass of object releasing or absorbing - usually water (4.184 J / gC)
  • ΔT - temperature change (C)

Molar Heat

• ΔH = q / moles

  • q - heat (J)
  • ΔH - Enthalpy change (J/mol)

• Products - reactants

  • Coefficient(enthalpy of formation) - coefficient(enthalpy of formation)
  • You are given the equation and enthalpy of formation for each.

  

• Meaning of ΔH

ΔG (Gibbs Free Energy)

• ΔG = ΔH - TΔS
  • ΔG - change in free energy (kJ)
  • ΔH - change in enthalpy (kJ)
  • T - absolute temperature (K)
  • ΔS - change in entropy (kJ/K)
  • Units can change. Make sure ΔH has same units as ΔS. ΔH is given in kJ/mole + ΔS in J/mole
• ΔG° = -RTln(K)
  • ΔG° applies to standard-state conditions while ΔG is gibbs free energy given certain condition
  • K - [C][D]/[A][B]
  • T - temperature (K)
  • R - constant (8.314 J / mole K)
• Meaning of ΔG
  • Gibb’s free energy.
  • ΔG < 0 is thermodynamically favored and exothermic.
  • ΔG > 0 is not thermodynamically favored and endothermic.

Hess’s Law

Hard to explain. Please click the title in order to watch a short video on Hess’s Law Problems.

ΔS - Change in Entropy

• Increases as you go from solid to aqueous to liquid to gas
• ΔS = Np(sum of products) - Nr(sum of reactants)
  • Np = coefficient of products
  • Nr = coefficient of reactants

Coupled Reactions

• Two reactions that share a common intermediate (a product of one reaction is the reactant of another).
• Usually combined with Hess’s law to determine free energy change, ΔG, for the coupled reaction.

Heat of reaction / heat of solution

• The heat of solution is the amount of heat absorbed or released when a solute dissolves in a solvent, while the heat of reaction is the amount of heat absorbed or released during a chemical reaction. The heat of solution is specific to the dissolution process, while the heat of reaction is specific to the chemical reaction taking place.
• Heat of solution
  • HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Enthalpy diagram with catalyst

• ΔH is the change between starting and ending energy.
• Energy of activation is the change in starting energy to the peak.
• Starting energy is where the graph starts.
• Endothermic reactions have a higher ending energy than starting energy. Exothermic reactions have a lower ending energy than starting energy.

Maxwell Boltzmann

• Higher temperatures move the graph peak right and down.

Lattice Enthalpy

• Bigger charge triumphs size. Smaller size wins if have same charge.

Drivers

• A reaction is favored if enthalpy decreases.

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