For each:
Always true /sometimes true/never true.
1) the joule is the SI unit of force
2)the standard heat of formation for a substance is determined at 100 degrees C
3)all free elements in their standard states have a heat of formation of zero.
sometimes
To determine whether each statement is always true, sometimes true, or never true, we can look at the definitions and principles behind each statement.
1) The joule is the SI unit of force.
This statement is never true. The SI unit of force is the newton, not the joule. The joule is the SI unit of energy.
2) The standard heat of formation for a substance is determined at 100 degrees C.
This statement is never true. The standard heat of formation for a substance is determined at a reference temperature of 25 degrees Celsius (298 Kelvin). This reference temperature allows for consistent comparisons between different substances.
3) All free elements in their standard states have a heat of formation of zero.
This statement is always true. The heat of formation is the enthalpy change when one mole of a substance is formed from its pure elements in their standard states at a specified temperature and pressure. For free elements in their standard states, since they are already in their most stable form, there is no net change in enthalpy when they are formed. Therefore, their heat of formation is zero.