Chemistry
Adana
30

Which of these statements are true? Select all that apply. The Hrxn for the reaction C(s) + O2(g) CO2(g) is the same as the Hf for CO2(g). The Hrxn for the reaction Na(s) + Cl2(g) 2NaCl(s) is the same as the Hf for NaCl(s). The Hf for Br2(l) is 0 kJ/mol by definition. To determine the Hf for H2O(g), just reverse the sign of the Hrxn for the reaction 2H2O(l) 2H2(g) + O2(g). The Hf for N2(l) is 0 kJ/mol by definition. The Hrxn for the reaction 1.5H2(g) + 0.5N2(g) NH3(g) is the same as the Hf for NH3(g).

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(2) Answers
morrisellie

The heat of formation is the heat needed to form a compound from the constituent elements as they naturally exists in Earth. This will be equal to the heat of reaction when the reactants of the reaction are the elemental constituents. Also, the heat of formation of solids and fluids that are in their most natural state would be equal to zero. So the correct answers are: The Hrxn for the reaction C(s) + O2(g) = CO2(g) is the same as the Hf for CO2(g). The Hrxn for the reaction Na(s) + Cl2(g) 2NaCl(s) is the same as the Hf for NaCl(s). The Hf for Br2(l) is 0 kJ/mol by definition.  The Hf for N2(l) is 0 kJ/mol by definition. 

marcshell13

In order to find the two statements, we must first define what the enthalpy of formation and the enthalpy of reaction mean. Enthalpy of formation: The change in enthalpy when one mole of substance is formed from its constituent elemetns at standard state. Enthalpy of reaction: The change in enthalpy when a reaction occurs and the reactants and products are in their standard states. Now, we check the statements. The true ones are: The Hrxn for C(s) + O₂(g) → CO₂(g) is the same as Hf for CO₂ This is true because the formation of carbon dioxide requires carbon and oxygen in their standard states. The Hf for Br₂(l) is 0 kJ/mol by definition. Because the bromine is present in its standard state, the enthalpy of formation is 0. The Hrxn for the reaction 1.5H₂(g) + 0.5N₂(g) → NH₃(g) is the same as the Hf for NH₃(g) The reactants and products are present in their standard state, and the reaction is the same as the one occurring during the formation of ammonia.

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