Set the system boundary
Exothermic processes transfer heat out of the system under common conditions; endothermic ones absorb it. At constant pressure, enthalpy change is the heat exchanged for a process when only pressure–volume work is relevant. A negative enthalpy change does not by itself guarantee a fast or spontaneous reaction.
Choose a system before assigning signs. If the reaction mixture is the system, heat flowing into the surrounding water is heat leaving the reaction. A measured temperature rise in the water therefore often corresponds to a negative heat change for the reaction, assuming the cup and outside air do not absorb much. If the reaction absorbs heat, the surroundings can cool. The sign belongs to a defined energy transfer, not to the word “hot.”
Enthalpy is convenient for constant-pressure chemistry because expanding gases can push on the surroundings. Under the usual introductory assumptions, the measured heat at constant pressure matches the system's enthalpy change. That does not make enthalpy identical to all forms of energy or make every experiment perfectly insulated. State changes, heat capacity of the apparatus, and unwanted heat exchange can all affect what a thermometer registers.
Follow energy through steps
Hess’s law works because enthalpy depends on initial and final states, not on the route. Reverse a reaction and its enthalpy sign reverses; multiply the equation and the energy change scales with it. Formation enthalpies let us estimate a reaction’s enthalpy by comparing products and reactants. Bond-energy calculations are approximate because quoted bond values are averages.
Hess's law is a consequence of state functions: if two routes begin and end with the same substances in the same physical states, their total enthalpy changes agree. You can reverse and add equations to build a target reaction. If you reverse an equation, reverse the sign of its enthalpy; if you double every coefficient, double its energy change. Canceling species must match in both formula and state.
The standard formation route is another way to organize this accounting. Products' formation enthalpies are combined and reactants' are subtracted according to coefficients. The answer applies to the specified conditions, often 1 bar and a stated temperature. Bond-energy estimates instead imagine breaking gaseous bonds and forming new ones; average bond energies can be helpful for direction or rough magnitude, but they miss the particular molecular environment.
What a calorimeter observes
A temperature change in water or a calorimeter is evidence of energy transferred. The surroundings gain heat when the reaction system loses it, assuming negligible leakage. Specific heat relates energy transfer to mass and temperature change, while phase changes require separate treatment.
A simple calorimetry calculation assumes the material's heat capacity stays approximately constant across the measured temperature interval. Multiply mass, specific heat, and temperature change to estimate heat gained by the solution. If the container itself warms, its heat capacity should be included for a better result. The reaction heat is the negative of the heat gained by surroundings only after all important energy paths are counted.
Heat is not temperature. A large amount of cool water may contain more internal energy than a small amount of hot water, and different substances can absorb different energy for the same temperature rise. During a phase change, energy changes the arrangement of particles while the temperature can stay nearly fixed. A smooth temperature graph does not automatically mean no chemical or physical change occurred.
Heating curves and phase changes
Sloped portions of a heating curve show temperature change within a phase; plateaus show energy being used to change phase at nearly constant temperature under fixed pressure. The energy for melting or vaporization depends on amount and the substance’s transition enthalpy. A coffee-cup calorimeter approximates constant pressure, while a sealed bomb calorimeter more directly measures energy change at constant volume.
On a heating curve, a plateau should be interpreted with pressure and purity in mind. For a pure substance at fixed pressure, melting or boiling can occur at nearly constant temperature. A mixture often changes phase over a range instead. Energy supplied during melting weakens the arrangement holding the solid together; during vaporization it separates particles much more extensively. Neither plateau means that all bonds within molecular particles have been broken.
Different calorimeters hold different conditions. A coffee-cup arrangement is open to atmospheric pressure and is often used for solution reactions; a rigid sealed bomb is closer to constant volume and commonly used for combustion. The observed temperature change must be converted with the appropriate total heat capacity. A distinction between enthalpy and internal energy can matter when gases are produced or consumed.
The reference matters
An enthalpy value belongs to a specified chemical equation and physical states. Liquid water and water vapor give different reaction enthalpies. Standard formation enthalpy is defined for making one mole of compound from elements in their standard states; an element in its standard state has a formation value of zero by definition. State the equation before interpreting the sign or number.
A reaction enthalpy without states can be ambiguous. Forming liquid water releases a different amount of heat from forming water vapor because condensation itself releases energy. Likewise, allotropes such as graphite and diamond are distinct reference forms of carbon. Writing states next to every species makes a numerical answer interpretable and prevents a hidden phase change from being overlooked.
Standard formation enthalpy sets the value of an element in its standard state to zero by convention. It does not mean that the element contains no energy. This reference system lets us compare transformations consistently. If the equation is multiplied by three, the enthalpy change for the equation also triples, but an intensive quantity such as a molar enthalpy must specify what amount or reaction event it refers to.