Download CVB101 – Lecture 3 Chemical Bonding • Chemical bonding

CVB101 – Lecture 3
Chemical Bonding
 Chemical bonding describes the exchange and sharing of electrons
Chemical Formulae
 Chemical symbol – one or two letters which identify each element
 Chemical formula – expression of the composition of a compound in
terms of chemical symbols
 Chemical formulae indicate which elements are present and in what ratio
Molecules
 Molecule – an aggregate of at least two atoms in a definite arrangement
help together by covalent bond
 A molecule can contain atoms of the same element or atoms of two or
more elements which are in a fixed ratio  law of definite proportions
 Polyatomic molecules contain more than two atoms
Empirical formula
 Empirical formula – an expression with the smallest whole numbers
giving the correct ratios of the elements present
Structural formula
 A representation showing how atoms are bound in a compound
Electron arrangements within atoms
 Number of protons in the nucleus determines the chemical identity of the
atom
 Chemical properties, most importantly, chemical reactivity is determined
by the electrons, more precisely, electronic structure (number of eincluding their distribution around nucleus and their energies) –
explained by quantum theory
Electron subshells
 S subshell has 2 electrons
 P subshell has 6 electrons
 D subshell has 10 electrons
 F subshell has 14 electrons
 Valence electrons: the electrons in the outmost shell (valence shell) of an
atom
 Core electrons: the electrons in the inner most shells
Covalent bonds
 Atoms are held together by sharing electrons
 This type of bonding typically occurs between atoms of non-metals
Ionic bonds
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Ionic bonds are a result of electron transfer between atoms to form ions –
electrostatic attraction of positive and negative ions
This type of bonding occurs between ionic compounds
Ionic bonds are present in compounds of metals and non-metals
Metallic bonds
 Metallic bonds occur between metal atoms in metallic solids
 The atoms can be though of as held together in a “sea of electrons”
Ions
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An atom or group of atoms with a net positive or negative net charge
Electrons may be gained or lost
Anions – negative charged ion
Cations – positive charged ion
Ionic compound
 A combination of cations and anions with no overall net charge
MEMORIZE THESE
CVB101 – Lecture 4
Absolute and Relative uncertainty
 Absolute uncertainty – uncertainty in the measurement expressed in the
same unit
 Relative uncertainty – compares the size of the uncertainty with the
measurement
Stoichiometry
Blackman chapter 3
 Molecular mass – the mass of a compound in atomic mass units (u)
 Molecular mass = sum of atomic masses of each atom in its chemical
formula
 Avogadro’s number: 6.022x1023 mol-1
 Molar mass – the mass of 1 mole of something
 Molar mass is measured in grams per mole (g mol-1)
 How many molecules in 2 mol of water?
o Number of atoms = number of moles x avogadro’s number
o N=n x NA
CVB101 – Lecture 5
Percent composition
 Percent by mass of each element in a compound
 Mass % = mass of element in compound/mass of compound x 100
 We can calculate theoretical percentage composition from a chemical
formula
Chemical reactions
 A process in which a substance (or substances) is changed into one or
more new substances
 Involve reorganisation of the atoms in one or more substances
 Burning or combustion
 Atoms are neither created or destroyed in chemical reactions
o All atoms in the reactant must be accounted for in the products
o Chemical equations must be balanced
CVB101 – Lecture 6
Reaction Types
 Precipitation Reactions
 Acid base reactions
 Oxidation and reduction reactions
Precipitation Reactions
 Definition of precipitate – an insoluble solid that separates from a
solution
 Definition of precipitation reactions – reaction that result in a formation
of a precipitate
 Include state symbols in chemical equations!!
Solubility
 The maximum amount of solute that will dissolve in a given quantity of
solvent (at a specific temperature)
 Some compounds are very soluble e.g. NaCl
o It is possible to make very concentrated solutions on NaCl
 Other compounds are not very soluble e.g. AgCl
o If AgCl solid is placed in water, only very small amount will
dissolve. The rest stays as a solid
 Saturated solution – a solution in which no more solute will be dissolved
Ionic Equation
 An equation that shows dissolved species as free ions
 Net ionic equations – cancelling spectator ions from both sides gives the
net ion equation for the reaction
Acids and bases
 Acids: sour taste (acetic acid in vinegar, citric acid in lemons)
o Turn litmus from blue to red
o Acid solutions conduct electricity
 Bases: bitter tastes
o Turn litmus from red to blue
o Feels slippery (soaps are formed from bases plus fats)
o Base solutions conduct electricity
Bronsted acids and bases
 Arrhenius definition:
o Acid – a substance that ionize in water to produce H+ ions
o Base – a substance that ionize in water to produce OH- ions
 Bronsted definition
o Acid – a proton donor
o Base – a proton acceptor
o Acids and bases do not have to be in aqueous solution
Diprotic and Triprotic acids
 Diprotic acids can release two protons
 Triprotic acids can release three protons
Neutralisation Reactions
 Neutralisation reaction – a reaction between an acid and a base
 Salt – an ionic compound made up of cation(s) other than H+ and anion(s)
other than OHpH
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A measure of the acidity of a solution (i.e. hydrogen ion concentration)
Reduction and Oxidation (redox) reactions
 Definition of redox reaction – a reaction in which electrons are
transferred
 Half reactions are used to show electron transfer.
 Oxidation – loss of electrons
 Reduction – gain of electrons
Common redox reactions
 Combination reactions – a reaction in which 2 or more substances
combine to form a single product
 Decomposition reactions – the breakdown of a compound into 2 or more
products
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Combustion reactions – reactions of a substance with O2 usually with
release of heat and light
Oxidation numbers
 Oxidation number (oxidation state) – the number of charges an atom
would have in a molecule (or ionic compound) if electrons were
transferred completely
 Increased oxidation number, the atom has been oxidised
 Decrease oxidation number, the atom has been reduced
 The sum of the oxidation numbers for all atoms in a compound is always
equal to the total charge of that compound