Thermochemistry — Flashcards & Quiz

Q1: What is enthalpy change (ΔH) and how does it relate to exothermic and endothermic reactions?

Enthalpy change (ΔH) is the heat energy absorbed or released during a reaction at constant pressure. Exothermic reactions release heat to the surroundings: ΔH is negative (products have less energy than reactants). Endothermic reactions absorb heat from the surroundings: ΔH is positive (products have more energy than reactants). ΔH = H(products) - H(reactants).

Q2: What is standard enthalpy of formation (ΔH°f)?

The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states (most stable form at 25°C, 100 kPa). By definition, ΔH°f for elements in their standard states is zero. ΔH°rxn = Σ ΔH°f(products) - Σ ΔH°f(reactants). This provides another method to calculate reaction enthalpies using tabulated data.

Q3: What is activation energy and how does it relate to enthalpy?

Activation energy (Eₐ) is the minimum energy required for reactant molecules to collide and react. It represents the energy barrier between reactants and products. Eₐ is independent of ΔH — a reaction can be exothermic but have a high activation energy (slow without catalyst) or endothermic with low Eₐ. Catalysts lower Eₐ by providing an alternative reaction pathway but do NOT change ΔH.

Q4: What is the difference between enthalpy of combustion and enthalpy of formation?

Enthalpy of combustion (ΔH°c): heat released when one mole of a substance is completely burned in excess oxygen under standard conditions. Always negative (exothermic). Enthalpy of formation (ΔH°f): heat change when one mole of a compound is formed from its elements in their standard states. Can be positive or negative. Both are standard molar enthalpies measured at 25°C and 100 kPa.

Q5: What is Hess's law cycle using combustion data?

When direct ΔH°f data is unavailable, combustion data can be used via Hess's law: ΔH°rxn = Σ ΔH°c(reactants) - Σ ΔH°c(products). Note the subtraction is reversed compared to using formation data. This works because combustion reactions convert all substances to the same products (CO₂ and H₂O), creating a common reference point.

Q1: An exothermic reaction has a positive ΔH value.

Answer: FALSE

Exothermic reactions RELEASE heat, so ΔH is NEGATIVE (products have lower enthalpy than reactants). Endothermic reactions absorb heat and have positive ΔH.

Q2: The enthalpy of formation of an element in its standard state is defined as zero.

Answer: TRUE

By definition, ΔH°f = 0 for all elements in their most stable form at standard conditions (25°C, 100 kPa). This provides a reference point for calculating formation enthalpies of compounds.

Q3: A catalyst changes the enthalpy change (ΔH) of a reaction.

Answer: FALSE

A catalyst lowers the ACTIVATION ENERGY (Eₐ) by providing an alternative pathway, but does NOT change ΔH. The enthalpy difference between reactants and products remains the same — the catalyst only affects the rate, not the thermodynamics.