CHM 2 Exam 1 (Thermodynamics laws and Energy/Enthalpy/Heat Capacity/Calorimetry)

First Law of Thermodynamics

• ΔE=q+w\

• Change in energy = heat added to system + work done on system

Signs: q and w

• q > 0 → greater than 0 ; heat absorbed

• w > 0 → greater than 0 ; work done on system

• q < 0 → less than 0 ; heat released

• w < 0 → less than 0 ; work done by system

Enthalpy (ΔH)

• Formula: ΔH = heat at constant pressure

ΔH > 0

• endothermic (absorbs heat) H greater than 0

ΔH < 0

• exothermic (releases heat) H less than 0

Heat Capacity

• q=mCΔTq = mC\Delta Tq=mCΔT

• Heat absorbed = mass × specific heat × temperature change

Calorimetry Principle

• Heat lost by one object = heat gained by another

Hess’s Law

Total ΔH for a reaction = sum of ΔH for individual steps. The total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken.

How to calculate ΔH°rxn from ΔHf°?

ΔH°rxn = ΣΔHf°(products) – ΣΔHf°(reactants)

calculates the standard enthalpy change of a reaction by subtracting the sum of the standard enthalpies of formation of the reactants from that of the products.

Standard enthalpy of formation (ΔHf°)?

Heat change to form 1 mole of compound from elements in standard state.