enthalpies of formation
- The standard enthalpy change ( H°) gives the enthalpy change for a reaction performed at standard state and a given temperature.
- The standard molar enthalpy of formation describes the standard enthalpy change for the formation of one mole of a compound from its elements.
- Hess’s law can be used to calculate the standard enthalpy change for a reaction from the standard molar enthalpies of formation of the reactants and products.
note
- The standard enthalpy change ( H°) gives the enthalpy
change for a reaction performed at standard state and is
reported for 25 °C, since it depends on temperature. - At standard state substances are in their most stable form at 1 atmosphere of pressure at the given temperature.
- When substances have different allotropes such as carbon (e.g. graphite and diamond), the most stable form is used (graphite).
- The standard molar enthalpy of formation describes the
standard enthalpy change for the formation of one mole of a compound from its elements. - Standard formation reactions begin with reactants that are elements in their standard states and produce one mole of product.
- Because they form only one mole of product, standard
formation reactions often contain fractional coefficients. - The standard molar enthalpy of formation for an element in standard state is zero.
Which of the following statements is not true about allotropes?
Allotropes have different forms at room temperature, but the same enthalpy.
Which of the following best describes the standard enthalpy change?
The enthalpy change with all the reactants and products in their standard state
Which of the following is true about a standard formation reaction?
A standard formation reaction results in exactly 1 mole of product.
Which of the following statements is not true?
The standard enthalpy of formation for a pure element in its standard state is +1
Which of the following is(are) true about the standard enthalpy change of formation?
- Standard enthalpy change of formation also referred to as the standard heat of formation.
- Standard enthalpy change of formation the standard enthalpy change to prepare 1 mole of a compound from the elements in their standard state.
- Standard enthalpy change of formation is written ΔH°f .
Which of the following statements about a standard enthalpy change is true?
An enthalpy change with all the reactants and products in their standard state
What is the temperature and pressure most often used to determine standard states?
25°C, 1 atm
Which of the following is not a standard enthalpy of formation reaction?
CaO(s) + CO2(g) -> CaCO2(s)
Which of the following is not a standard enthalpy of formation reaction?
2H2(g) + O2(g) → 2H2O(l)