Question

Part A

Watch the video on the explosive decomposition of nitrogen triiodide, NI3, and determine which of the following statements are correct.

Check all that apply.

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The enthalpy of NI3 is greater than the enthalpy of the decomposition products.

The decomposition of NI3 is an exothermic reaction.

In this reaction, iodine is produced as one of the products.

NI3 is a highly stable compound when it is dry.

In this reaction, heat is absorbed from the surroundings.

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Enthalpy—an extensive property

Enthalpy is an extensive property, and its value depends upon the amount of reactants undergoing the reaction. The enthalpy of the reaction when 5 molof NI3 reacts will be five times higher than that when 1 mol of NI3 reacts.

For example, consider a decomposition reaction of NI3:

2NI3(s)→N2(g)+3I2(g), ΔHrxn=−290.0 kJ

When 2 mol of NI3 decomposes, the enthalpy of the reaction is −290.0 kJ.

If 5 mol of NI3 decomposes, the enthalpy of the reaction will be (52×−290.0) = −725.0 kJ.

Part B

The decomposition of NI3 to form N2 and I2 releases −290.0 kJ of energy. The reaction can be represented as

2NI3(s)→N2(g)+3I2(g), ΔHrxn=−290.0 kJ

Find the change in enthaply when 12.0 g of NI3 decomposes.

Express your answer to three significant figures and include the appropriate units.

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Calculation of the heat of reaction based on the standard heat of formation

The enthalpy change for the formation of 1 mol of a substance in its standard state from its constituent elements in their standard states is called the standard heat of formation. It is denoted as ΔH∘f.

If the standard heat of formation of the reactants and the products is known, the standard heat of reaction can be calculated using the formula

ΔH∘rxn=∑npΔH∘f(products)−∑nrΔH∘f(reactants).

Here, np is the total number of moles of products, and nr is the total number of moles of reactants.

Part C

Consider the exothermic reaction

CH4(g)+2O2(g)→CO2(g)+2H2O(g)

Calculate the standard heat of reaction, or ΔH∘rxn, for this reaction using the given data. Also consider that the standard enthalpy of the formation of elements in their pure form is considered to be zero.

Reactant or product	ΔH∘f (kJ/mol)
CH4(g)	-201
CO2(g)	-393.5
H2O(g)	-241.8

Express your answer to four significant figures and include the appropriate units.

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Answer 1