Thermochemistry
What is the system in thermochemistry?
The part of the universe being studied, usually the chemical reaction.
What does it mean when a system is at equilibrium?
The forward and reverse reaction rates are equal.
What is the Arrhenius definition of an acid?
A substance that increases the concentration of H⁺ ions in solution.
What is the Arrhenius definition of a base?
A substance that increases the concentration of OH⁻ in solution.
What does Gibbs free energy (ΔG) tell us?
Whether a reaction is spontaneous (ΔG < 0) or not.
What is the difference between endothermic and exothermic reactions?
Endothermic absorbs heat (ΔH > 0); exothermic releases heat (ΔH < 0).
What happens to the amount of reactants and products when a reaction reaches equilibrium?
They stay constant, but are not necessarily equal.
What is the pH of a solution with [H⁺] = 1.0×10^-3?
pH = 3
What is the pOH of a solution with [OH⁻] = 1.0×10^-4?
pOH = 4
What is the equation for Gibbs free energy?
ΔG = ΔH - TΔS
Define enthalpy (ΔH).
It is the heat content of a system at constant pressure.
What does it mean if a reaction has more products than reactants at equilibrium.
The reaction favors the products.
Define a strong acid and give an example
A strong acid completely ionizes in water; example: HCl.
What is the relationship between pH and pOH?
pH + pOH = 14
If ΔH is negative and ΔS is positive, is the reaction spontaneous?
Yes, always spontaneous.
State Hess’s Law.
The total enthalpy change of a reaction is the sum of the enthalpy changes for individual steps.
What happens to equilibrium when pressure is increased in a gaseous system?
The system shifts toward the side with fewer gas molecules.
What is a conjugate base?
The species formed when an acid donates a proton (H⁺).
Name a strong base and explain why it is strong.
NaOH; it dissociates completely in water.
Define entropy (ΔS).
A measure of disorder or randomness in a system.
Calculate the heat released when 25.0 g of water cools from 85.0°C to 25.0°C. (c = 4.18 J/g·°C)
q = mcΔT = (25.0)(4.18)(60.0) = 6270 J or 6.27 kJ
Explain Le Châtelier’s Principle with an example.
If a stress is applied to a system at equilibrium, the system shifts to relieve the stress. Example: Adding more reactant shifts the equilibrium toward products.
The species formed when an acid donates a proton (H⁺).
pH = -log(0.025) ≈ 1.60
Calculate the [OH⁻] in a solution with a pH of 11.
pOH = 14 - 11 = 3 → [OH⁻] = 1.0×10^-3 M
A reaction has ΔH = 100 kJ and ΔS = 250 J/mol·K. At what temperature does it become spontaneous?
ΔG = 0 when T = ΔH/ΔS = (100,000 J)/(250 J/mol·K) = 400 K