Thermochemistry Review (Ch.1-7)

Question-and-answer flashcards covering key concepts from the thermochemistry lecture notes, including state functions, standard state, enthalpy of formation, reaction enthalpies, calorimetry, and heat capacities.

What is a state function in thermochemistry?

A property that depends only on the initial and final state, not on the path taken (e.g., ΔH).

How is the standard enthalpy change of a reaction (ΔHrxn) calculated from standard enthalpies of formation?

ΔHrxn = Σ ΔHf°(products) − Σ ΔHf°(reactants).

What does the term 'standard state' mean for substances in thermochemistry?

Defined reference conditions: gases at 1 atm, solids/liquids at 1 atm, solutions at 1 M; typically T = 25°C unless stated.

What is the standard enthalpy of formation ΔHf°?

The enthalpy change to form 1 mole of a substance from its elements in their standard states.

Why is ΔHf° defined per mole of substance formed?

Because formation enthalpy is the energy change associated with forming one mole of the substance.

What is ΔHf° for an element in its standard state?

Zero.

Which elements have standard-state forms as liquids at room temperature?

Mercury (Hg) and bromine (Br2).

What is a formation reaction?

A reaction that forms 1 mole of a product from its elements in their standard states.

How do coefficients affect ΔHrxn when using formation enthalpies?

ΔHrxn = Σ(n × ΔHf°)products − Σ(m × ΔHf°)reactants; multiply ΔHf° by each coefficient.

What characterizes an exothermic reaction in terms of ΔH?

Heat is released; ΔHrxn is negative.

What characterizes an endothermic reaction in terms of ΔH?

Heat is absorbed; ΔHrxn is positive.

What is a thermal chemical equation?

A chemical equation that includes the enthalpy change (ΔH) for the reaction.

What can you do with chemical equations in thermochemistry (manipulation)?

Multiply both sides by a factor, or reverse (flip) the equation; ΔH scales proportionally or changes sign; phase changes matter.

What is the purpose of a calorimeter?

To measure the heat exchanged with the surroundings during a process.

What is the basic calorimetry equation to calculate heat transfer?

q = c × m × ΔT, where c is the specific heat capacity, m is mass, and ΔT is the temperature change.

What is the difference between specific heat capacity and molar heat capacity?

Specific heat capacity (c) is per gram; molar heat capacity (C_m) is per mole.

How are specific heat capacity and molar mass related?

Cm = c × M; c = Cm / M, where M is molar mass.

What is a typical sign convention for q when the surroundings gain heat?

qsurroundings > 0; qsystem = −qsurroundings; exothermic system has qsystem < 0.

What is the equation to calculate heat for a calorimeter reading given a temperature change?

q = c × m × ΔT; this gives energy transferred to the surroundings, which equals −ΔH for the system if reaction occurs at constant pressure.

How do you perform energy stoichiometry to find energy released for a given mass of reactant?

Convert mass to moles using molar mass, multiply by ΔH per mole (from ΔHf° or ΔHrxn), and account for stoichiometric coefficients.