NCEA Level 3 Chemistry - Thermochemical properties

Define Covalent bonding

the strong attraction between two positive nuclei and one or more shared pairs of electrons

What is a polar substance

substance with distinct regions of positive and negative charge

What are the exceptions to the Octet rule

Be - only 2 bonds wo/ lone pairs

B, Al - only 3 bonds wo/ lone pairs

H - only 1 bond wo/ lone pairs

180° bond angle name

linear

120° bond angle name

trigonal planar

109.5° bond angle name

tetrahedral

What bond angle does molecules with 2 electron areas have

180°

What bond angle does molecules with 3 electron areas have

120°

What bond angle does molecules with 4 electron areas have

109.5°

2 bonding areas 0 non-bonding

linear shape

3 bonding areas 0 non-bonding

trigonal planar

2 bonding areas 1 non-bonding

bent (120°)

4 bonding areas 0 non-bonding

tetrahedral

3 bonding areas 1 non-bonding

trigonal pyramid

2 bonding areas 2 non-bonding

bent (109.5°)

What does bond polarity require

the atoms in the bond

Define electronegativity

the ability of an atom to attract the bonding pair of electrons in a covalent bond. - closer to Fluorine=more electronegativity

What determines an asymmetrical molecule

lone pairs around the central atom

Define enthalpy

a thermodynamic quantity equivalent to the total heat content of a system

Enthalpy of breaking bonds

Endothermic-requires energy

Enthalpy of forming bonds

Exothermic-releases energy

Define H (kJ)

Amount of energy/heat released or absorbed by the system

Define ∆ᵣH⁰ (kJmol⁻¹)

Change in the enthalpy of a reaction system

What happens in an endothermic reaction

1. temp of surroundings decreases

2. bonds are broken

3. ∆ᵣH⁰ is POSITIVE

4. The products have more enthalpy than reactants

What happens in an exothermic reaction

1. temp of surroundings increases

2. bonds are formed

3. ∆ᵣH⁰ is NEGATIVE

4. The products have less enthalpy than reactants

VSEPR theory

• angles are controlled by the total # of electron areas around the central atom

• shape is controlled by the number of bonding and non-bonding electron areas around the central atom

Shape code

The shape and angle are controlled by minimising the repulsion of the electron areas around the central ______ atom

The electron arrangement that minimises repulsion for __electron areas is ____ which results in an angle of____ Because ____electron areas are bonding and___ non-bonding, it results in a ____ shape with a bond angle of _____.

Polarity Code

Molecular polarity is controlled by the distribution of charge about the central ____atom.

The __-__ bonds are polar because__are more electronegative than __.

This results in uneven sharing of electrons which forms bond dipoles.

This molecule has a____shape, which is symmetric/asymmetric, as the bond dipoles are all the same/different they will cancel/not cancel and the molecule will be non-polar/polar.

∆ᵣH⁰=

bonds broken - bonds formed

5 bonding areas 0 non-bonding

Trigonal bipyramid

4 bonding 1 non-bonding

seesaw

3 bonding 2 non-bonding area

T shape

2 bonding 3 non-bonding area

Linear

6 bonding 0 non-bonding areas

octohedral

5 bonding 1 non-bonding area

square based pyramid

4 bonding 2 non bonding areas

square planar

what are the 3 factors affecting electrostatic attraction between nucleus and valence electron

1. Nuclear charge (number of proton) greater the charge=greater attraction to valence electron

2. Number of energy levels/distance of electrons

   • Higher energy levels are further away from the nucleus

   • valence electrons are shielded more from the nucleus

3. More or less valence electron-electron repulsion

Trends down a group paragraph

Going down a group. the valence electrons are in energy levels further from the nucleus with increase shielding from the inner shells. Although the number of protons increase down a group , this attraction is offset by the increase in distance between the nucleus and the valence elections so, the electrostatic attraction between the positive nucleus and its vanlece electrons decrease.

Trends across a period paragraph

Across a period, the valence electrons are added to the same energy levels with the same shielding from the inner levels. The number of protons increase from period to period, This results in an increased electrostatic attraction between the positive nucleus and the valence electrons across a period.

Atomic radius trend

• decreases across a period

• increases doen a group

Ionisation energy meaning

The energy required to remove one mole of electrons froom one mole of gaseous positive ions

e.g Na₍₉₎→Na⁺₍₉₎+e⁻ remeber state symbol

Ionisation energy trend

• Increases across a period

• Decreases down a group

Electronegativity trend

• Increases across a period

• decreases down a group

Explain why anions are larger than the atom

When an anion forms, the atom gains electrons in the valence shell, while the nuclear charge remains constant, This results in greater valence electron - electron repulsion, resulting in the anion having greater radius than atoms

Explain why cations are smaller than the atom

When an atom becomes a cation, an electron/s are removed from the valence shell. This reduces electron - electron repulsion or removes an entire energy level.

Types of intermolecular forces

1. Temporary dipole - induced dipole

2. Permanent dipole - dipole forces

3. Hydrogen bonding

Temporary dipole - induced dipole

• Weakest of all intermolecular forces

• Temporary charge imbalances caused by random movement of electrons inducing a charge in an adjacent molecule

• As #e-/molar mass increase b.p increases

• The greater the S.A, the greater the td-id forces

Permanent dipole - dipole forces

• Occurs between polar molecules

• acts in addition to td-id

Hydrogen bonding

• strongest intermolecular force

• requires H bonded to a highly electronegative element (N, O, F)

• AND a non bonding electron pair on (N, O, F) of an adjacent molecule

What is enthalpy of combustion ∆_cH°

Enthalpy change when one mole of the substance is completely burnt under standard conditions e.g ∆_cH°(H₂_(g)) is H₂_(g)+1/2 O₂_(g)→H₂O_(l)

What is enthalpy for formation ∆_fH°

enthalpy change to form 1 mole from its elements, all in their standard state e.g.

∆_fH° (CO₂_(g)) is C_(s)+O₂_(g)→CO₂_(g)

What is enthalpy of vaporisation ∆_vapH°

enthalpy change when one mole of the substance changes from a liquid to gas eg

∆_vapH°(H₂0_(l)) H₂0_(l)→H₂0_(g)

what is enthalpy of fusion ∆_fusH°

enthalpy change when one mole of the substance changes from a solid to a liquid

∆_fusH°(C₅H₁₀_(s)) C₅H₁₀_(s)→C₅H₁₀_(l)

what is enthalpy of sublimation ∆_subH°

enthalpy change when one mole of substance changes from a solid to a gas

∆_subH°(CO₂_(s)) CO₂_(s)→CO₂_(g)