Hess

Using the equations below:

C_(s) + O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s) + O_2(g) → MnO_2(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO_2(s) + C_(s) → Mn_(s) + CO_2(g)

• 910

• 130

• -130

• -910

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The enthalpies of combustion of C(s), H2(g) and C4H9OH(l) (in kJmol^-1) are as follows  
C(s) + O₂(g)   ->  CO₂(g)                                               ∆H=a
H₂(g) + ½O₂(g)   ->   H₂O(l)                                        ∆H=b
C₄H₉OH(l) + 6O₂(g)   ->   4CO₂(g) + 5H₂O(l)  ∆H=c
What is the enthalpy change for the reaction shown below?
  4C(g) + 5H₂(l) + ½O₂(g)   ->   C₄H₉OH(l)

• c – 4a – 5b

• 2a + 10b - c

• 4a + 5b - c

• 2a + 5b + c

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The enthalpy change for the reaction
C(s, graphite) + 1⁄2O₂(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction.
It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.
C(s, graphite) + O₂(g) --> CO₂    ΔH=-394 kJmol^–1
CO(g) + 1⁄2O₂(g) --> CO₂  ΔH=-283 kJmol^–1  
The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is 

• -677 kJmol^–1 

• +111 kJmol^–1 

• -111 kJmol^–1 

• +677 kJmol^–1 

Tags

NGSS.HS-PS1-4

Using Hess's Law, calculate the enthalpy change for the reaction: \( \ce{2H2(g) + O2(g) -> 2H2O(l)} \) given the following reactions: \( \ce{H2(g) + 1/2O2(g) -> H2O(l)} \) \( \Delta H = -286 \text{ kJ/mol} \)

• -572 kJ/mol

• -286 kJ/mol

• 0 kJ/mol

• 572 kJ/mol

Which of the following statements best describes Hess's Law?

• The enthalpy change of a reaction is independent of the pathway between the initial and final states.

• The enthalpy change of a reaction depends on the temperature and pressure.

• The enthalpy change of a reaction is always positive.

• The enthalpy change of a reaction is always negative.

Given the reactions: \( \ce{C(s) + O2(g) -> CO2(g)} \) \( \Delta H = -394 \text{ kJ/mol} \) and \( \ce{CO(g) + 1/2O2(g) -> CO2(g)} \) \( \Delta H = -283 \text{ kJ/mol} \), calculate \( \Delta H \) for the reaction: \( \ce{C(s) + 1/2O2(g) -> CO(g)} \).

• -677 kJ/mol

• +111 kJ/mol

• -111 kJ/mol

• +677 kJ/mol

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Hess's Law is used to calculate the enthalpy change for a reaction that cannot be measured directly. Which of the following is an example of such a reaction?

• Combustion of methane

• Formation of water from hydrogen and oxygen

• Conversion of diamond to graphite

• Sublimation of iodine

Using Hess's Law, determine the enthalpy change for the reaction: \( \ce{N2(g) + 3H2(g) -> 2NH3(g)} \) given the following reactions: \( \ce{N2(g) + O2(g) -> 2NO(g)} \) \( \Delta H = +180 \text{ kJ/mol} \) and \( \ce{2NO(g) + 3H2(g) -> 2NH3(g) + O2(g)} \) \( \Delta H = -905 \text{ kJ/mol} \).

• -725 kJ/mol

• -360 kJ/mol

• +725 kJ/mol

• +360 kJ/mol

Tags

NGSS.HS-PS3-1

Which of the following is a real-world application of Hess's Law?

• Predicting the weather

• Designing energy-efficient engines

• Calculating the energy content of food

• Determining the age of fossils

Tags

NGSS.HS-PS1-4

In an experiment, the enthalpy change for the reaction \( \ce{A -> B} \) is found to be \( -50 \text{ kJ/mol} \). If the reaction \( \ce{B -> C} \) has an enthalpy change of \( +30 \text{ kJ/mol} \), what is the enthalpy change for the reaction \( \ce{A -> C} \)?

• -80 kJ/mol

• -20 kJ/mol

• +20 kJ/mol

• +80 kJ/mol

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1