Thermochemistry Equations & Symbols

Question 1

M

Answer 1

= molar mass (m/n)

Question 2

n

Answer 2

= number of moles (m/M)

Question 3

m

Answer 3

=mass (nxM)

Question 4

One mole

Answer 4

=6.022 × 10²³

Question 5

Calculating the number of neutrons

Answer 5

mass number - number or protons

Question 6

Equation for first ionisation energy (I1) of an element

Answer 6

Definition: The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions, under standard conditions.

Equation: X(g) → X⁺(g) + e⁻

Symbol: I₁ or ΔieH°

Units: kJ mol⁻¹

Question 7

Enthalpy of
fusion, Δ fusH°

Answer 7

Definition: The enthalpy change when 1 mole of a solid changes to a liquid (at melting point) under standard conditions.

Equation: X(s) → X(l)

Symbol: Δ fusH°

Units: kJ mol⁻¹

Question 8

Enthalpy of vaporisation,
Δ vapH°

Answer 8

Definition: The enthalpy change when 1 mole of a liquid changes to a gas (at boiling point) under standard conditions.

Equation: X(l) → X(g)

Symbol: Δ vapH°

Units: kJ mol⁻¹

Question 9

Enthalpy of
sublimation, Δ subH°

Answer 9

Definition: The enthalpy change when 1 mole of a solid changes directly into a gas under standard conditions.

Equation: X(s) → X(g)

Symbol: Δ subH°

Units: kJ mol⁻¹

Question 10

Enthalpy of
combustion, Δ cH°

Answer 10

Definition: The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

Equation: CₓHᵧ + O₂(g) → xCO₂(g) + y/2 H₂O(l)

Symbol: Δ cH°

Units: kJ mol⁻¹

Question 11

Enthalpy of formation, ΔfH°

Answer 11

Definition: The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Equation: A(s) + ½B₂(g) → AB(g)

Symbol: ΔfH°

Units: kJ mol⁻¹

Question 12

Calculating bond energy

Answer 12

ΔH= bond breaking - bond making

Question 13

Calculating RAM (relative atomic mass - showed by the periodic table) of elements

Answer 13

To find the RAM of a specific element:
1. Find all the isotopes of said element.
2. Find the weighted averages of each isotope individually: This is the isotope elements mass number (A) x its NA percentage.
4. Take this result, along with (if applicable) the result for any other isotopes, and add them together. (Add the weighted averages found from each isotope of the element together.
5. Equal RAM of that element

Example:
Find the RAM of Cl2 gas.
Cl2 gas is a mixture of two isotopes: Chlorine-35 & Chlorine-37

~ Cl-35
Mass number (A) = 35
NA = 75.77% (75%)

~ Cl-37
Mass number (A) = 37
NA = 24.23% (25%)

RAM for Cl =
(NA% x A) + (NA% + A) = !
(0.75 x 35) + (0.25 x 37) = 35.50

Relative atomic mass of Cl2
= 35.50

Note: Length of equation depends on number of isotopes, chlorine has two. Thus, two weighted averages are added together.

Question 14

Determining the effective nuclear charge (Zeff)

Answer 14

Zeff =
atomic number (z) - the no. of inner electrons.

• Effective nuclear charge increases across a period and stays the same down a group.