Standard Enthalpy of Formation Quiz

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Questions and Answers

What formula represents the relationship between the enthalpy changes during a combustion reaction?

ΔH1 = ΔHf + ΔH2 (correct)

ΔHf = ΔH1 - ΔH2

ΔHf + ΔH2 = ΔH1

ΔH1 = ΔHf - ΔH2

If ΔHf for NaHCO3(s) is -950.8 kJ/mol, what is the overall enthalpy change, ΔHr, for the reaction involving Na2CO3(s), CO2(g), and H2O(l)?

-104.2 kJ/mol

84.6 kJ/mol

91.6 kJ/mol (correct)

100.3 kJ/mol

What does Hess's Law state regarding enthalpy changes in a chemical reaction?

The total enthalpy change is always zero.

The enthalpy change can differ based on the path taken.

The enthalpy change is dependent solely on the products formed.

The total enthalpy change is the same regardless of the route taken. (correct)

What is the first step in applying Hess's Law for enthalpy change calculations?

List all reactants and products of the reaction. Signup and view all the answers

Which of the following statements about standard enthalpy of formation is correct?

It refers to the enthalpy change when one mole of a compound forms from its elements in their standard states. Signup and view all the answers

How can one indirectly determine the standard enthalpy of formation from combustion products?

By calculating using the enthalpy changes of reactants to products. Signup and view all the answers

What does ΔH2 represent in the equation ΔH1 = ΔHf + ΔH2?

The enthalpy change of combustion reactions. Signup and view all the answers

When performing enthalpy change calculations, what must be considered about the elements involved?

They must be in their standard states. Signup and view all the answers

Which statement best describes Hess’s Law?

The total enthalpy change is independent of the reaction route taken. Signup and view all the answers

What is a typical application of Hess's Law?

Determining enthalpy changes that cannot be measured directly. Signup and view all the answers

In an energy cycle using Hess's Law, how are ΔHr and ΔHf related?

ΔHr is equal to the sum of ΔHf values of the products minus the reactants. Signup and view all the answers

What does the symbol ΔHf indicate?

The standard enthalpy of formation of a substance. Signup and view all the answers

Which of the following statements is true regarding combustion enthalpy?

Combustion enthalpy can vary depending on the conditions of the reaction. Signup and view all the answers

Which reaction pathway does Hess's Law allow you to compare?

Direct and indirect pathways between reactants and products. Signup and view all the answers

In a given reaction represented by 3C (s) + 4H2 (g) → C3H8(g), why can ΔHf (propane) not be found experimentally?

Hydrogen and carbon do not react under standard conditions. Signup and view all the answers

What is the significance of the initial and final conditions in Hess's Law?

They must be the same for Hess’s Law to apply. Signup and view all the answers

What is the enthalpy change of combustion for carbon (graphite) when reacting with oxygen?

-393.5 kJ mol-1 Signup and view all the answers

Using Hess's Law, how is the enthalpy of formation (ΔHf) for ethane calculated?

ΔHf = ΔH1 - ΔH2 Signup and view all the answers

What is the enthalpy of formation of ethane (C2H6), calculated from the provided values?

-84.7 kJ mol-1 Signup and view all the answers

What is the definition of the enthalpy change of atomisation (ΔHθat)?

Enthalpy change when one mole of gaseous atoms is formed from elements Signup and view all the answers

When calculating average bond energies using Hess's cycles, what is essential?

Utilising enthalpy changes of atomisation and combustion Signup and view all the answers

In which reaction is water produced as a product?

H2 (g) + ½O2 (g) → H2O (l) Signup and view all the answers

What do the values of ΔH1, ΔH2, and ΔH3 represent in the enthalpy formation calculation using Hess's Law?

ΔH1: Formation of products, ΔH2: Combustion of reactants Signup and view all the answers

What is the overall process used for calculating the enthalpy change of formation of a compound?

Using standard enthalpy values and Hess's cycles Signup and view all the answers

Study Notes

Enthalpy of Formation and Hess's Law

Standard enthalpy of formation (ΔHf) refers to the change in enthalpy when one mole of a compound is formed from its elements in their standard states.
Hess’s Law states that the total enthalpy change for a reaction is the same, regardless of the paths taken, as long as the initial and final conditions are identical.

Applying Hess's Law

Formula to apply Hess's Law: ΔHr = ΔH2 – ΔH1
Example calculation for sodium carbonate:
- ΔHr = (ΔHf [Na2CO3 (s)] + ΔHf [CO2 (g)] + ΔHf [H2O (l)]) – 2ΔHf [NaHCO3 (s)]
- Result: ΔHr = +91.6 kJ mol⁻¹

Calculating ΔHf from ΔHc

Combustion products can be derived directly from elements (ΔH1) or indirectly through intermediates (ΔHf + ΔH2).
Relationship: ΔHf = ΔH1 – ΔH2
Example for ethane (C2H6):
- Combustion reactions with associated ΔHc values are needed.
- Calculation yields ΔHf = –84.7 kJ mol⁻¹ for ethane.

Average Bond Energies

Average bond energies are determined using Hess’s law and cannot be measured directly.
Enthalpy changes of atomisation and combustion or formation are used to calculate average bond energies.

Summary of Hess's Law Applications

Hess's Law allows for the calculation of enthalpy changes that are difficult to obtain experimentally.
The method helps predict the enthalpy change in reactions where direct measurement is not feasible, such as the reaction of hydrogen and carbon under standard conditions.

Key Points

Direct routes for reactions yield the same enthalpy change as indirect routes.
Understanding and applying Hess's Law is crucial for thermodynamic calculations in chemistry.

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Description

Test your knowledge on the standard enthalpy of formation and Hess's Law. This quiz includes questions that will help you understand how to apply thermodynamic principles effectively. Whether you're preparing for an exam or brushing up on your chemistry knowledge, this quiz is for you.