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.
1) Never true - The joule is not the SI unit of force. The SI unit of force is the newton.
2) Never true - The standard heat of formation for a substance is determined at a reference temperature of 25 degrees Celsius or 298 Kelvin, not 100 degrees Celsius.
3) Sometimes true - Some free elements in their standard states do have a heat of formation of zero, but this is not true for all free elements. For example, oxygen (O2) gas has a heat of formation of zero in its standard state, but helium (He) gas has a non-zero heat of formation in its standard state.
To determine whether each statement is always true, sometimes true, or never true, we need to consider the definitions and concepts related to each statement.
1) The joule is the SI unit of force.
This statement is never true. The joule is the SI unit of energy, not force. The SI unit of force is the newton.
To get this answer, you can refer to any physics textbook or online resource that provides information about SI units and their definitions.
2) The standard heat of formation for a substance is determined at 100 degrees Celsius.
This statement is sometimes true. The standard heat of formation for a substance is actually determined at a reference temperature of 25 degrees Celsius (298 Kelvin). However, sometimes specific studies or experiments may use a different reference temperature like 100 degrees Celsius.
To get this answer, you can refer to thermodynamics textbooks or reputable online sources that provide information about the determination of standard heat of formation.
3) All free elements in their standard states have a heat of formation of zero.
This statement is always true. The heat of formation for an element in its standard state is defined as zero. This is because the heat of formation is the energy change that occurs when one mole of a compound is formed from its elements in their standard states. Since an element in its standard state is already in its most stable form, no energy change occurs when it is formed, and therefore, its heat of formation is zero.
To get this answer, you can refer to thermodynamics or chemistry textbooks that cover the concept of heat of formation and provide information about the heat of formation of free elements in their standard states.
Overall, the answers to these questions can be found by referring to trusted sources that provide information on the relevant concepts and definitions in physics and chemistry.