Thermochemical Principles

Electron Configurations

Determined by measuring ionisation energies (ease with which they lose electrons when energy is absorbed) and studying their emission spectra (where electrons move from higher to lower energy levels with the release of energy). REMEMBER:The 4s level gains and loses electrons first!Chromium and copper are exceptions because they only have ONE electron in the 4s orbital

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Ionisation Energies

IS THE AMOUNT OF ENERGY (IN KILOJOULES) NEEDED TO REMOVE THE MOST LOOSELY HELD ELECTRON FROM ONE MOLE OF GASEOUS ATOMS TO PRODUCE ONE MOLE OF GASEOUS IONS.This is an endothermic process.The unit for ionisation energy is kilojoules per mol.The first ionisation energy of an element is the amount of energy needed to remove the first electron from one mole of gaseous atoms producing one mole of gaseous ions with one positive charge.e.g. Li(g) -------> Li+(g) + e- The second ionisation:Li+(g) --------> Li2+(g) + e-

Factors which influence ionisation energy: the size of the atom the charge of the nucleus the screening effect of inner energy levels (shielding effect) the type of electron involved e.g. due to shape of orbital, an s electron penetrates closer to the nucleus than a p or d electron and is more tightly held. Ionisation energy decreases down a group due to the increase in the amount of energy levels, meaning more electron shielding and so less attraction of electrons to the nucleus.Ionisation energy increases across a period due to the increase in nuclear charge (the number of protons in the nucleus) and so electrons are more strongly attracted.

Successive ionisation energies

If an atom containing several electrons is given sufficient energy, it will lose an electron.As each electron is stripped from its atom in this process, greater amounts of energy are required to remove successive electrons, since the remaining electrons are held more tightly to the nucleus because there is a greater net positive charge.

Periodic Trends - Atomic Radii

The size of an atom of an element decreases with increasing atomic number of an element across a period because as the number of protons in the nucleus increases, electrons are pulled in closer to the nucleus. This means the amount of positive charge will increase and the radius of the atom will decrease.

The size of an atom of an element increases with increasing atomic number of an element down a group. In spite of the attractive pull of the increased nuclear charge, the outer electrons are not only much further from the nucleus but they are also shielded by the electrons in inner energy levels.

Periodic Trends - Ionic Radii

Positive ions (cations) are always smaller than their parent atoms.Negative ions (anions) are always larger than their parent atoms.

Periodic Trends - Electronegativity

Is a measure of the relative tendency of an atom to attract a pair of bonding electrons.Differences in electronegativity between atoms can be used to determine the degree of polarity in chemical bonds. The larger the difference, the greater the polarity.

Electronegativity and Bond Polarity

Can be used to predict the degree of polarity or ionic character of chemical bonds using electronegativity difference between two atoms sharing electrons in a bond.If this electronegativity zero the bond is called a pure covalent bond.If the electronegativity difference is extremely large, one atom effectively takes over complete control of the shared electrons, which is called an ionic bond.

Lewis Structures and Bonding

A Lewis structure (or diagram) is a representation of either an individual or bonded atom, that shows all the electrons in the outer energy level.Covalent Bonding:Atoms share their outer energy level electrons in order to achieve a stable electron configuration.Ionic Bonding:One atom donates its outer energy level electron(s) to another atom in order to achieve a stable electron configuration.

Shapes of Molecules and Polyatomic Ions

The shape of a molecule or covalently bonded ion is determined by the number of regions of negative charge there are around an atom (count double or triple bonds as one region). These regions of negative charge can be either bonded pairs of electrons (bond pairs) or non-bonded electrons (lone pairs).Valence Shell Electron Pair Repulsion:Based on the principle that pairs of electrons will repel each other to the maximum extent (max. distance - focus answers on this).

Bond pairs of electrons will repel other bond pairs of electrons with equal intensity.Double bond pairs or triple bond pairs are more repelling than single bond pairs because of increase in number of electrons.

Shapes continued...

Linear: bond angle of 180°Trigonal Planar: bond angle of 120°Tetrahedral: bond angle of 109°Trigonal Bipyramidal: bond angles of 120° and 90°Octahedral: with bond angles of 90°

Polar Molecules

If a molecule contains only two atoms of unequal electronegativity then the molecule will have a permanent dipole, due to the bond being polar. This means that one part of the molecule will have more negative charge than another part of the molecule.

Intermolecular Forces

There are three types of intermolecular force of varying strength: temporary dipoles (the weakest) - every single molecule has this permanent dipoles (stronger) hydrogen bonding (strongest) - N.O.F are most electronegative These intermolecular forces govern melting point, boiling point, solubility or density.

Intermolecular Forces and Physical Properties

The covalent bonds which hold molecules together are very strong compared to the intermolecular bonds between molecules.Covalent bonds within molecules are not broken when a substance melts, boils or dissolves in a solvent like water.

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Caption: : Hess's Law

Entropy, Enthalpy and Spontaneous Reactions

The entropy of a substance is a measure of the disorder or randomness of a system and is dependent on temperature. An increase in entropy is favoured.Substances that are highly disordered have high entropies.Low entropy is associated with strongly ordered substances.Endothermic reactions remove heat energy.Exothermic reactions release heat energy.

If a reaction is exothermic and has a gain in entropy, then it will be spontaneous at any temperature.If a reaction is endothermic and has a drop in entropy, then it will be non-spontaneous at any temperature.If a reaction is exothermic and has a drop in entropy, then it might be spontaneous, if the value of enthalpy is enough to compensate for that drop in entropy.If a reaction is endothermic yet has a gain in entropy, then it might be spontaneous, if there is a significant gain in entropy to compensate.

Standard Heats of Reaction

Standard enthalpy (heat) of reaction Standard enthalpy (heat) of combustion: the enthalpy change when one mole of substance is burnt completely with all reactants and products in their standard states. Since combustion reactions are always exothermic, enthalpy of reaction values are always negative. (when balancing oxygen in equations, use lowest ratio possible) Standard enthalpy (heat) of formation: the enthalpy change when one mole of a substance is formed from its elements with all reactants and products in their standard states.

Measuring Enthalpy Changes

g ----> Jkg ------> KJ

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Types of bonds

Covalent - strongIonic - fairly strongMolecular - weakMetallic - fairly strong

Metals: maleable because pf delocalised electrons - electrons in the outer energy level of each atom, capable of flowing through the lattice.

Phase Changes

When heat energy is applied to a substance, the particles making up the substance absorb this energy and start to vibrate faster. If sufficient energy is applied, the particles can move further away from each other and a change of state occurs. The amount of heat energy required for this change of state depends on the strength of the inter-particle forces holding a substance together.The strongest forces that exist between particles are found in ionic compounds and giant covalent network solids. Large amounts of energy are required to break all the bonds in these lattices.

The melting point (or fusion point) is the temperature at which a solid changed to liquid. The molar enthalpy of fusion is the energy needed to change one mole of a substance from a solid to a liquid at its melting point. The boiling point (or vaporisation point) is the temperature at which a liquid changes to a gas. The molar enthalpy of vaporisation is the energy needed to change one mole of a substance from a liquid to a gas at its boiling point.

Shapes

From 6 bonding arrangement, remove pole electrons firstFrom 5 bonding arrangement, remove equator electrons firstA longer shape has a large surface area and so creates strong intermolecular bonds with other molecules compared with odd shapes of small surface area that create weak intermolecular bonds.