And click on the Time Machine icon. Click on âSelect Diskâ. Choose your external drive and click on âUse Backup Diskâ. Time Machine will copy everything (OS, Software, Files, and Preferences) to the external hard drive.
The standard enthalpy of formation of a pure element is in its reference form its standard enthalpy formation is zero. Carbon naturally exists as graphite and diamond. The enthalpy difference between graphite and diamond is too large for both to have a standard enthalpy of formation of zero. To determine which form is zero, the more stable form of carbon is chosen. This is also the form with the lowest enthalpy, so graphite has a standard enthalpy of formation equal to zero. Table 1 provides sample values of standard enthalpies of formation of various compounds.
Return to Matter Menu. All chemical substances are either solid, liquid or gas. To make comparisons easier, the chemistry community has agreed on a concept called the standard state.. The standard state of a chemical substance is its phase (solid, liquid, gas) at 25.0 Â°C and one atmosphere pressure. This temperature/pressure combo is often called room conditions.. Two elements are liquid in their standard state: mercury and bromine. Eleven elements are gas in their standard state.