Chemistry: Energy Change
13 Oct 09
Determining when to use q=mc∆t and q=n∆H
Graph1. Change in Temperature of H2O
a) When there is a change in temperature or kinetic energy of an element or compound, q=mc∆t is used to determine the enthalpy (or energy) change.
In the graph above, regions A, C, and E display a change in temperature. Since ∆t represents a change in temperature, tf - ti, q=mc∆t is used to determine the energy change within those regions.
b) When an element or compound undergoes a change in phase, represented by regions B and D, q=n∆H is used to determine the enthalpy change within those regions.
Regions B and D show no change in temperature (kinetic energy). In B, ice melts into water. In D, water vaporizes. When there is a change in phase, no temperature change is observed; therefore, q=n∆H must be used to determine the enthalpy change.