Part A

Watch the video on the explosive decomposition of nitrogentriiodide, NI3, and determine which of the following statements arecorrect.

Check all that apply.

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Check all that apply.

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

The decomposition of NI3 is an exothermic reaction.

In this reaction, iodine is produced as one of theproducts.

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 valuedepends upon the amount of reactants undergoing the reaction. Theenthalpy of the reaction when 5 molof NI3 reacts will be five timeshigher 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 ofenergy. 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 theappropriate units.

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

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

If the standard heat of formation of the reactants and theproducts is known, the standard heat of reaction can be calculatedusing the formula

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

Here, np is the total number of moles of products, andnr 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, forthis reaction using the given data. Also consider that the standardenthalpy of the formation of elements in their pure form isconsidered 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 theappropriate units.

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