A: Na3PO4 + MnCl2 > Mn3(PO4)2 + NaCl
Answer:
The noble gases (Group 18) are located in the far right of the periodic table and were previously referred to as the "inert gases" due to the fact that their filled valence shells (octets) make them extremely nonreactive.
Dropping a 50 gram piece of metal with a temperature of 125° Celsius into 1000 grams of water at 20° Celsius would lead to a heat loss by the metal and an heat gain by the water. These loss and gain should be equal since energy cannot be created nor destroyed.
Answer:
- <u><em>It is positive when the bonds of the product store more energy than those of the reactants.</em></u> (the second statement)
Explanation:
ΔHf is the change of enthalpy during the reaction, which is equal to the sum of enthaply changes of the products less the sum of the enthalpy changes of the reactants.
- ΔHf = ∑ (ΔH products) - ∑ (ΔH reactants)
Also, ΔHrxn, per definition, is the potential chemical energy stored in the bonds of the products less the chemical potential energy stored in the bonds of the reactants.
Then, when the potential chemical energy stored in the bonds of the products is greater than the chemical potential energy stored in the bonds of the reactants ΔHrxn is positive.
Hence, you conclude that ΔHf is positive when the bonds of the product store more energy than those of the reactants (second statement from the choices).
Some brief comments about the other statements:
- The standard enthalpy of formation, ΔHf, is zero for an element in its standard state, not for a compound.
- For a compound the enthalpy of formation at 25ºC and 1 atm (the standard state) may be positive or negative.
- Also, note that the standard state for any element is not liquid: some are solids, some are gases, and some are liquids at 25ºC and 1 atm.
