Chapter 9 part 2

Question 1

Sometimes ∆Hrxn cannot be determined directly but we can determine it indirectly by using other known information. Three ways to calculate ∆H:
Three mathematical models that help predict the energy absorbed or released during a chemical reaction.

Answer 1

1. Hess’s Law
2. Enthalpies of formation
3. Bond Energies

Question 2

The principle that the enthalpy of reaction for a process that is the sum of two or more reactions is equal to the sum of the ∆Hrxn values of the constituent reactions
Formula?

Answer 2

Hess’s Law

∆H total = ∆H1 + ∆H2 + ∆H3

Question 3

The enthalpy change associated with a reaction that takes place under standard conditions

Answer 3

Standard enthalpy of reaction (∆H°rxn)
Example:
CH4(g) + H2O(g) CO(g) + 3 H2(g) ∆H°= 206 kJ

Question 4

What are standard conditions?

Answer 4

Pressure of 1 atm and some specified temperature (25°C); for solutions, a concentration of 1 M

Question 5

What is standard state?

Answer 5

The most stable form of a substance under 1 atm pressure and some specified temperature (25°C)

Question 6

Most metals and metalloids are _____; mercury and bromine are ______; H2, N2, O2, F2, Cl2,and group 18 elements are _____

Answer 6

solid; liquid; gases

Question 7

A reaction in which one mole of substance is formed from its component elementsin their standard states

Answer 7

formation reaction

Question 8

The enthalpy change that takes place at a constant pressure (1 atm) when one mole of a substance is formed from its constituent elements in their standard states

Answer 8

Standard enthalpy of formation (∆H°f)

Question 9

Enthalpies of formation equation

Answer 9

∆H°rxn = [Σ n, products ∆H°f, products] - [Σ n, reactants ∆H°f, reactants]
the sum of the moles of the products times the enthalpy of formation of products - the sum of the moles of the reactants times the enthalpy formation of the reactants

Question 10

Which statement is correct?
A) ∆H is a measure of heat absorbed or released by the system for a process carried out at constant volume

B) According to sign convention, a positive q signifies that heat flows from the system to the surroundings

C) The standard enthalpy of formation of C (diamond) at 25°C and 1 atm is zero

D) The standard enthalpy of formation of CH4(g) is not the same as the standard enthalpy of combustion of CH4(g).

Answer 10

D) The standard enthalpy of formation of CH4(g) is not the same as the standard enthalpy of combustion of CH4(g).

Question 11

Bond energies equation

Answer 11

∆H°rxn = Σ ∆H bond breaking - Σ ∆H bond forming
(the sum of the bond energies times the enthalpy formation of the reactants - the sum of the bond energies times the enthalpy formation of the products