Topic 2 – Ionic compounds and analysis • • • • • • • • •
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IONIC BONDS An ion is an atom or groups of atoms with a positive or negative charge Ionic bonds form between positively and negatively charged ions (i.e between cations and anions – see below) Atoms are most stable with full outer shells However, atoms of most elements have incomplete (i.e not full) outer shellsàthey readily lose or gain electrons during chemical reactions to obtain full outer shells When electrons are gained or lost, the atoms become ions… Cations: Metal atoms readily lose their outermost electrons to form positively charged ions called cations For elements in groups 1 and 2, the number of outer electrons lost is the same as their group number…: o E.g sodium is in group 1àhas 1 electron in its outer shell (2.8.1). It can lose one electron to become a Na+ cation with a full outer shell (2.8) o E.g2 magnesium is in group 2àhas 2 electrons in its outer shell (2.8.2). It can lose two electrons to become a Mg2+ cation with a full outer shell (2.8) Anions: Non-metal atoms can gain electrons to form negatively charged ions called anions For elements in groups 6 and 7, the number of electrons they gain is 8 minus their group number…: o E.g oxygen is in group 6àhas 6 electrons in its outer shell (2.8.6). It can gain two electrons to become an O2- anion with a full outer shell (2.8.8) o E.g2 chlorine is in group 7àhas 7 electrons in its outer shell (2.8.7). It can gain one electron to become a Cl- anion with a full outer shell (2.8.8) Note: when non-metal atoms form ions, their name changes to –ide (e.g chlorine atoms are called chloride ions…oxygen atoms are called oxide ions) IONIC COMPOUNDS Ionic compounds form when a metal reacts with a non-metal: o Electrons lost by the metal are transferred to the non-metal o àboth positive and negative ions that form end up with stable, full outer shells o Oppositely charged ions attract each other stronglyàforming an ionic compound held together by ionic bonds E.g when sodium and chlorine react…: o Electrons are transferred from sodium atoms to chlorine atoms o This forms Na+ and Cl- ions (both have a stable, full outer shell) o These oppositely charged ions attract each other stronglyàforming the ionic compound NaCl (table salt) Working out a formula: Ionic compounds are electrically neutral because they contain equal numbers of positive and negative charges If you know the charges of the cations and anions you can work out the formula of the ionic compound… E.g for sodium chloride, one Na+ is needed for every Cl- ionàformula is NaCl E.g2 the ionic compound aluminium oxide: o Aluminium oxide contains Al3+ cations and O2- anions
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o For charges to be equal you must have…: § Two Al3+ cations ( + 6 charge) § Three O2- anions ( - 6 charge) o àformula of aluminium oxide is Al2O3 (neutral charge) Compound ions: Compound ions contain more than one element E.g the nitrate ion NO3- contains one nitrogen atom joined to three oxygen atoms plus an extra electron (ànegative charge) If two or more compound ions of the same type are needed in a formula, the ion must be written inside brackets (with the number on the outside)… E.g the ionic compound magnesium nitrate: o Magnesium nitrate contains Mg2+ and NO3- ions o For charges to be equal you need two NO3- ions for every Mg2+ ion o àformula of magnesium nitrate is Mg(NO3)2 Names of ionic compounds: Note: o Ionic compounds usually end in –ide (e.g NaCl - sodium chloride) o However, compounds that contain oxygen atoms end in –ate (e.g Mg(NO3)2 - magnesium nitrate) The structure of ionic compounds: The ions in an ionic compound are packed tightly together and arranged in a regular way, called a lattice structure The lattice structure is held together by strong electrostatic forces of attraction (i.e by ionic bonds) between oppositely charged ions PROPERTIES OF IONIC COMPOUNDS Conducting electricity: Ionic compounds don’t conduct electricity when solid However, they do conduct electricity…: o when molten (i.e when they’re heated until they turn into liquid) o when in aqueous solution (i.e dissolved in water) – this is why sea water (NaCl in aqueous solution) conducts electricity Explanation: o Two conditions must be met for a substance to conduct electricity… § Must contain charged particles § Charged particles must be free to move o Ionic compounds contain charged particles (ions) but these are only free to move when molten or in aqueous solution o In the solid form, ions can’t move (they’re strongly held together in a lattice structure by ionic bonds)àcan’t conduct electricity Melting points and boiling points: The melting point of a substance is the temperature at which it changes from a solid to a liquid The boiling point of a substance is the temperature at which it changes from a liquid to a gas (at its fastest possible rate) Ionic bonds holding ionic compounds together in a lattice are very strong: o This means lots of (heat) energy is needed to break the ionic bonds o àionic compounds have high melting and boiling points o àionic compounds are usually solids at room temperature SOLUBILITY
If a substance dissolves well in a particular liquid, it is said to be ‘soluble’ If a substance doesn’t dissolve at all in a particular liquid, it is said to be ‘insoluble’ • A ‘salt’ (not NaCl which is ‘table salt’) is a substance that can be made by reacting an acid and an alkali • Solubility rules for salts in water: Soluble in water Insoluble in water All common sodium, potassium and ammonium salts All nitrates Most chlorides Silver/lead chlorides Most sulfates lead/barium/calcium sulfates sodium/potassium/ammonium carbonates Most carbonates sodium/potassium/ammonium hydroxides Most hydroxides • Precipitation reactions: • A reaction in which at least one insoluble solid (called the ‘precipitate’) is produced from two soluble substances is called a precipitation reaction: o E.g lead nitrate + potassium iodideàlead iodide + potassium nitrate o Balanced equation: Pb(NO3)2 (aq) + 2KI (aq) à PbI2 (s) + 2KNO3 (aq) o State symbols show that all substances are dissolved in water (aq aqueous) except for lead iodide, which is insoluble (so it’s shown as a solid – s)àforms a precipitate • In precipitation reactions, the precipitate can be separated from the unreacted ions by filtration. It is washed on filter paper and then dried in a warm oven • PRECIPITATES • Using the solubility rules, it’s possible to work out which precipitate will form when two solutions are mixed together • E.g what precipitate is formed when copper chloride and potassium hydroxide are mixed together? o When salts mix, the ions swap so that: copper chloride + potassium hydroxide à copper hydroxide + potassium chloride o All potassium salts are soluble, whereas most copper salts are insoluble o àcopper hydroxide forms a precipitate (a solid) o Balanced equation: CuCl2 (aq) + 2KOH (aq) à Cu(OH)2 (s) + 2KCl (aq) • E.g2 what would happen if copper chloride and potassium hydroxide were mixed together? o Sodium chloride + potassium carbonateàsodium carbonate + potassium chloride o All sodium and potassium salts are solubleàno precipitate is formed (both products of the reaction are soluble)àthis is not a precipitation reaction • Barium meals: • In the diagnosis of intestinal problems, patients are made to swallow a drink called a ‘barium meal’ - containing barium sulfate: o As the barium sulfate passes through the patient’s digestive system, x-ray photos are taken o Barium (like bone) absorbs x-raysàshows up as white on the photosàany problems with the patient’s digestive system can be seen • Most barium salts are toxic, however…: o barium sulphate is insolubleàcan’t enter the patient’s blood o This makes it safe to swallow • •
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ION TESTS Flame tests: Different metal ions produce different coloured flames when held over a Bunsen burner flame: o Sodium (Na+) – yellow o Potassium (K+) – lilac o Calcium (Ca2+) – red o Copper (II) (Cu2+) – green/blue Note: the most intense colours are obtained from solids, but flame tests also work when solids are dissolved in water (as aqueous solutions) Flame tests led to the discovery of new elements…: o Chemists in the 1800s did flame tests of different samples of mineral water and then used a prism to separate the colours of light given off - method is called ‘spectroscopy’ o They saw a grey-blue colour that hadn’t been seen before and realised they had discovered a new element – later called it caesium o A year later, using the same method, rubidium was discovered (gave off a dark red colour in a Bunsen flame) Precipitation tests: Some anions can be identified by precipitation tests…: o Chloride ions (Cl-): § Add dilute nitric acid and silver nitrate to the solution § If the sample contains chloride ions, a white precipitate of silver chloride will form o Sulfate ions (SO42-): § Add dilute hydrochloric acid and barium chloride to the solution § If solution contains sulfate ions, a white precipitate of barium sulfate will form Test for carbonate ions: Add a dilute acid to the solution If solution contains carbonate ions (CO3-), carbon dioxide gas will be given off, which when bubbled through limewater will turn the limewater milky