Hess

The enthalpy change for the reaction, ∆H_r , is equal to

• ∆H₁ + ∆H₂

• -∆H₁ - ∆H₂

• ∆H₁ - ∆H₂

• -∆H₁ + ∆H₂

Tags

NGSS.HS-PS1-4

The standard enthalpy changes of combustion of carbon, hydrogen and methane are shown in the table.

Which one of the following expressions gives the correct value for the standard enthalpy change of formation of methane in kJ mol

^–1?

C(s) + 2H

₂(g) → CH₄(g)

• 394 + (2 × 286) – 891

• –394 – (2 × 286) + 891

• 394 + 286 – 891

• –394 – 286 + 891

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

• 4a + 5b - c

• 2a + 10b - c

• 2a + 5b + c

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide. C(s) +O2(g) → CO2(g) ΔH = –x kJ mol–1 CO(g) + O2(g) → CO2(g) ΔH = –y kJ mol–1 What is the enthalpy change, in kJ mol–1, for the oxidation of carbon to carbon monoxide? C(s) + O2(g) → CO(g)

• x + y

• -x - y

• y - x

• x - y

For Hess' Law to be used what must be the same for all of the reactions being studied?

• The initial conditions of pressure and temperature.

• The final conditions of pressure and temperature.

• The initial and the final conditions of pressure and temperature.

• The initial and the final conditions of pressure and temperature, and the number of moles of reactants.

In order to find the enthalpy of combustion of C₃H₈ how must the enthalpy changes be arranged?

• ΔH₃ = ΔH₁ + ΔH₂

• ΔH₂ = ΔH₃ - ΔH₁

• ΔH₁ = ΔH₂ - ΔH₃

• 0 = ΔH₁ + ΔH₂ + ΔH₃

Tags

NGSS.HS-PS1-4

Hess's Law is used to calculate the enthalpy change of a reaction. 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 pathway between the initial and final states.

• The enthalpy change of a reaction is always positive.

• The enthalpy change of a reaction is always negative.

Which of the following is a direct application of Hess's Law?

• Calculating the enthalpy change of a reaction using standard enthalpies of formation.

• Determining the rate of a chemical reaction.

• Measuring the temperature change in a calorimeter.

• Predicting the spontaneity of a reaction.

In a Hess's Law cycle, which of the following must be true for the enthalpy changes?

• The sum of the enthalpy changes around a closed cycle is zero.

• The enthalpy changes must all be positive.

• The enthalpy changes must all be negative.

• The enthalpy changes must be equal to the temperature change.

• +393.5 kJ/mol

• -393.5 kJ/mol

• 0 kJ/mol

• +787.0 kJ/mol