thermochemical principles

What is the electron configuration for a Chromium ($Cr$) atom?

$$1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1$$

What is the electron configuration for a Copper ($Cu$) atom?

$$1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$$

Define 'First Ionization Energy'.

The energy required to remove one mole of electrons from one mole of gaseous atoms.

Explain the trend for atomic radius across a period.

Atomic radius decreases because the number of protons increases (increasing nuclear charge), pulling electrons closer while shielding from inner shells remains constant.

Define electronegativity.

The ability of an atom in a covalent bond to attract shared electrons towards itself.

Explain why a cation is smaller than its parent atom.

Cations have lost their outer shell of electrons and there is an increased proton-to-electron ratio, resulting in greater nuclear attraction on the remaining electrons.

Define standard enthalpy of formation ($\Delta_fH^∘$).

The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.

Define standard enthalpy of combustion ($\Delta_cH^∘$).

The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.

Define standard enthalpy of fusion ($\Delta_{fus}H^∘$).

The energy required to change one mole of a substance from a solid to a liquid at its melting point.

Define standard enthalpy of vaporization ($\Delta_{vap}H^∘$).

The energy required to change one mole of a substance from a liquid to a gas at its boiling point.

Write the formula relating the enthalpy of sublimation to the enthalpies of fusion and vaporization.

$$\Delta_{sub}H^∘ = \Delta_{fus}H^∘ + \Delta_{vap}H^∘$$

What is entropy ($S$)?

A measure of the degree of disorder or randomness in a chemical system.

What two main factors determine the spontaneity of a chemical reaction?

The enthalpy change ($\Delta H$) and the entropy change ($\Delta S$).

According to Hess's Law, what determines the enthalpy change of a reaction?

The enthalpy change is independent of the pathway taken, depending only on the initial reactants and final products.

What formula is used to calculate the energy absorbed by water in a calorimeter?

$$q = m \times c \times \Delta T$$

What value is typically used for the specific heat capacity of water in NCEA Level 3?

$$4.18\,J\,g^{-1}\,^∘ C^{-1}$$

What are temporary dipole-dipole forces?

Intermolecular forces caused by the instantaneous, uneven distribution of electrons creating a temporary dipole that induces dipoles in neighboring molecules.

Why do temporary dipole-dipole forces increase with molecular mass?

Larger molecules have more electrons and larger electron clouds, which are more easily distorted (more polarisable).

What are hydrogen bonds?

Strong intermolecular forces that occur when hydrogen is covalently bonded to highly electronegative elements: Nitrogen ($N$), Oxygen ($O$), or Fluorine ($F$).

What molecular shape results from 5 regions of electron density (all bonding)?

Trigonal bipyramidal

What molecular shape results from 6 regions of electron density (all bonding)?

Octahedral

What condition must be met for a molecule with polar bonds to be non-polar?

The molecule must be symmetrical so that the individual bond dipoles cancel each other out.

Why do melting and boiling points increase as the strength of intermolecular forces increases?

More energy is required to overcome the stronger attractive forces between the molecules.

Why does an increase in temperature lead to an increase in entropy?

Particles gain kinetic energy and move more randomly, increasing the disorder of the system.

State the unit for standard enthalpy change values.

$$kJ\,mol^{-1}$$