Answer to Question #97593 in General Chemistry for Brittany Wallace

Question

1. The standard enthalpy of formation of SiH4(g) is +34.3 kJ. The standard enthalpy of formation reaction is below.
Si(s) + 2 H2(g) SiH4(g)
What is the enthalpy change for the following reaction?
3 SiH4(g) 3 Si(s) + 6 H2(g)

2. The heat of vaporization of carbon tetrachloride is 30.0 kJ/mol at its boiling point of 76.6 °C. What is the change in enthalpy when 10.5 g CCl4 evaporates at 76.6 °C?

Answer 1

For a standard enthalpy change everything has to be present in its standard state. That is the physical and chemical state that you would expect to find it in under standard conditions.

That means that the standard state for water, for example, is liquid water, H₂O(l) - not steam or water vapour or ice.

Oxygen's standard state is the gas, O₂(g) - not liquid oxygen or oxygen atoms.

For elements which have allotropes (two different forms of the element in the same physical state), the standard state is the most energetically stable of the allotropes.

For example, carbon exists in the solid state as both diamond and graphite. Graphite is energetically slightly more stable than diamond, and so graphite is taken as the standard state of carbon.

Similarly, under standard conditions, oxygen can exist as O₂ (simply called oxygen) or as O₃ (called ozone - but it is just an allotrope of oxygen). The O₂ form is far more energetically stable than O₃, so the standard state for oxygen is the common O₂(g).