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Bonding, structure, properties and energy changes Copy correctly Copying or scanning from ESA workbooks is subject to the NZ Copyright Act which limits copying to 3% of this workbook. Chemistry terms You should be able to use the following terms correctly in your answers. particle a very small piece of matter – may be used to describe an atom, ion, molecule, electron, etc. atom the smallest particle of a chemical element (shows the chemical properties of that element) e.g. an atom of sodium (symbol Na); an atom of oxygen (symbol O) electron a negatively charged particle found in the space around the nucleus of an atom nucleus the very dense, positively charge ‘core’ of the atom, made up of protons and neutrons electron configuration the arrangement of electrons in the energy levels around the nucleus molecule two or more atoms (the same or different atoms) chemically combined to form a single unit of a substance e.g. the oxygen molecule O2, the carbon dioxide molecule CO2 ion a charged particle formed from one or more atoms that has lost or gained one or more electrons e.g. the sodium ion Na+ ; the oxide ion O2–; the carbonate ion CO32– element a pure substance made up of only one type of atom e.g. sodium metal Na; oxygen gas O2 compound a pure substance made up of more than one type of atom e.g. carbon dioxide CO2; hydrochloric acid HCl molecular formula shows the actual number of each type of atom in the molecule ionic formula shows the simplest ratio of positive and negative ions present within the ionic substance © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 1 4/12/15 9:09 am 2 Achievement Standard 91164 (Chemistry 2.4) Atoms and the periodic table Atomic structure • The atom is thought to be made up of a positive nucleus (positive protons and neutrons) and negative electrons. • Electrons move around the nucleus, occupying certain regions of space, described as orbitals or electron ‘clouds’. • Electrons can only have certain fixed energies, and occupy discrete energy levels (‘shells’), which increase in energy as they get further from the nucleus. Each energy level can only hold up to a certain number of electrons: • The first energy level (K shell) holds up to 2 electrons. • The second energy level (L shell) holds up to 8 electrons. • The third energy level (M shell) holds up to 18 electrons. Planetary model of atom Electrons fill the lowest available energy level first. Electrons in the highest energy level are called valence (or outer shell) electrons. Proton Neutron Electron The periodic table The periodic table contains all known elements arranged in order of atomic number – a complete periodic table is located at the back of this book. Dmitri Mendeleev – creator of the first version of the periodic table of elements Structure of the periodic table The vertical columns are called groups. • Atoms of the elements in the same group have the same number of valence electrons (electrons in the outer energy level). • There are 18 groups in the full periodic table (only the eight groups pertaining to elements 1–20 are shown below). The horizontal rows are called periods. • From left to right across a period, the number of valence electrons in the outer energy level increases by one in successive atoms. • There are seven periods in the full periodic table (only four periods are shown below, with the fourth period incomplete). © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 2 4/12/15 9:09 am Demonstrate understanding of bonding, structure, properties and energy changes 3 The first twenty elements The symbols and atomic numbers of the first twenty elements follow. Group 1 Group 18 1H Group 2 Group 13 Group 14 Group 15 Group 16 Group 17 2He 3Li 4Be 5B 6C 7N 8O 9F 10Ne 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar 19K 20Ca The electron arrangements of the first twenty elements follow. H: 1 He: 2 Li: 2,1 Be: 2,2 B: 2,3 C: 2,4 N: 2,5 O: 2,6 F: 2,7 Ne: 2,8 Na: 2,8,1 Mg: 2,8,2 Al: 2,8,3 Si: 2,8,4 P: 2,8,5 S: 2,8,6 Cl: 2,8,7 Ar: 2,8,8 K: 2,8,8,1 Ca: 2,8,8,2 Important groups of the periodic table These groups and their names are: • Group 1: the alkali metals – the most reactive metals • Group 2: the alkaline earth metals – moderately reactive metals • Group 17: the halogens – the most reactive nonmetals • Group 18: the noble gases – these elements are very unreactive. Elements in the same group (column) have the same number of valence electrons (similar electron arrangements). The number of valence electrons increases across a row (period). The number of occupied energy levels increases down a column. Halogens are used in some forms of lamp, such as those used for stage lighting. Atoms gain, lose or share electrons to produce full (stable) valence shells. • Metals are found on the left of the periodic table, in Groups 1, 2 and 13. They tend to lose electrons to form positive ions (cations), to gain full (stable) valence shells. • Non-metals are found to the right of the periodic table, in Groups 14, 15, 16 and 17. They tend to gain electrons to form negative ions (anions) or to share electrons, to gain full (stable) valence shells. © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 3 4/12/15 9:09 am 4 Achievement Standard 91164 (Chemistry 2.4) The periodic table Answers p. 75 1. For three elements with atomic numbers 2, 10 and 18: a. what is a common feature of the energy levels of the atoms of these elements? b. what is a common feature of the chemistry of these elements? 2. Write the symbol for the atom(s) and/or ion(s) that: a. have two electrons in its/their outer energy level b. gain or lose three electrons to form an ion; also write the symbol for the ion formed. 3. For each of the ions: a. name the ion Mg2+ Cl– Al3+ O2– b. write its electron arrangement. Mg2+ Cl– Al3+ O2– © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 4 4/12/15 9:09 am Demonstrate understanding of bonding, structure, properties and energy changes 5 Forming of ions Equations can be written for reactions involving the formation of ions. Examples Sodium atoms and sodium ions Sodium has an atomic number of 11 and has 11 protons and 11 electrons in its atom. Sodium ions form when sodium atoms lose an electron from the third energy level. Key: • = electron Energy level Sodium atom Sodium ion Na: 2,8,1 Na+: 2,8 3rd • 2nd • • • • • • • • • • • • • • • • 1st • • • • sodium atom sodium ion plus One electron + e– + • Sodium atom: Sodium ion: • Valence (outer) level is third energy level and has one electron. • Valence (outer) level is second energy level and has eight electrons. • Has no charge, because has 11 protons and 11 electrons. • Has a charge of +1, because has 11 protons but only 10 electrons. Chlorine atoms and chloride ions Chlorine atoms have 17 protons and 17 electrons. A chlorine atom can fill the third energy level by gaining one electron to form a chloride ion, Cl–. Energy level Chlorine atom plus One electron Cl: 2,8,7 + e– Cl–: 2,8,8 3rd • • • • • • • + • • • • • • • • • 2nd • • • • • • • • • • • • • • • • 1st • • • • chlorine atom chloride ion Chloride ion Chlorine atom: Chloride ion: • Valence level is third energy level and has seven electrons. • Valence level is third energy level and has eight electrons. • Has no charge, because it has 17 protons and 17 electrons. • Has a charge of –1, because it has 17 protons but 18 electrons. © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 5 4/12/15 9:09 am 6 Achievement Standard 91164 (Chemistry 2.4) Names and formulae of ionic compounds Ionic compounds are made up of positive ions (cations) and negative ions (anions). • A cation is usually a metal atom that has lost an electron or electrons. • An anion is formed when a non-metal atom gains an electron or electrons (monatomic) or when two or more atoms bond covalently with the addition of an electron or electrons (polyatomic). Common ions encountered in NCEA Level 2 Chemistry Cations Symbol Anions Symbol sodium Na+ chloride Cl– potassium K+ bromide Br– silver Ag+ iodide I– ammonium NH4+ fluoride F– copper(I) Cu+ sulfide S2– hydrogen H+ oxide O2– lithium Li+ nitride N3– calcium Ca2+ phosphide P3– magnesium Mg2+ hydroxide OH– barium Ba2+ nitrate NO3– mercury Hg2+ nitrite NO2– lead Pb2+ hydrogencarbonate HCO3– tin Sn2+ hydrogensulfate HSO4– zinc Zn2+ hypochlorite OCl– copper(II) Cu2+ thiocyanate SCN– iron(II) Fe2+ permanganate MnO4– cobalt Co2+ carbonate CO32– aluminium Al3+ sulfate SO42– iron(III) Fe3+ sufite SO32– chromium Cr3+ chromate CrO42– dichromate Cr2O72– phosphate PO43– This salt lake contains two of the most commonly encountered ions – sodium and chloride. © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 6 4/12/15 9:09 am Demonstrate understanding of bonding, structure, properties and energy changes 43 Chemical equations and mole ratios A balanced chemical equation is a way of representing what happens in a chemical reaction. Chemical equations are written in the form: reactants products Reactants are the substances that react. Products are produced in a chemical reaction. A balanced chemical equation has: • the correct formulae for all reactants and products • the same number of each type of atom on both sides of the equation. A chemical reaction can be used to look at the relative amounts of reactants and products involved in a chemical reaction. Examples: Information from balanced equations 2 Mg(s) +O2(g) 2 mol Mg 2MgO(s) reacts with 1 mol O2 to produce 2 mol MgO or 20 mol Mg reacts with 10 mol O2 to produce 20 mol MgO or 48.6 g Mg reacts with 32.0 g O2 to produce 80.6 g MgO or 12.15 g Mg reacts with 8.0 g O2 to produce 20.15 g MgO [M(Mg) = 24.3 g mol–1; For the equation: 4Al + 3O2 M(O2) = 32.0 g mol–1] 2Al2O3 4 mol of Al reacts with 3 mol of O2 to produce 2 mol Al2O3 n(Al) n(O2) n(Al2O3) The ratio of amounts is: = = 2 3 4 or n(Al) : n(O2) : n(Al2O3) = 4 : 3 : 2 © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 43 4/12/15 9:09 am 44 Achievement Standard 91164 (Chemistry 2.4) Using equations to calculate masses of products or reactants Example 1.50 g of magnesium ribbon is burnt in air to produce magnesium oxide, MgO A series of four steps is used to calculate masses of products or reactants from chemical equations. What mass of MgO wll be produced? Step 1 2Mg + O2 Write a balanced equation for the reaction. 2MgO m(Mg)= 1.50 g m n(Mg)= M 1.50 g = 24.3 g mol–1 Step 2 Convert the mass of the known reactant / product to moles. = 0.0617 mol Step 3 From the equation, deduce the relationship between the moles of known and unknown. From the equation: n(MgO) n(Mg) = 2 2 n(MgO)= n(Mg) = 0.0617 mol n(MgO)= 0.0617 mol M(MgO)= 24.3 + 16.0 = 40.3 g mol–1 Step 4 Convert the moles of unknown to mass. m(MgO)= nM = 0.0617 mol × 40.3 g mol–1 = 2.49 g Burning magnesium ribbon © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 44 4/12/15 9:09 am Demonstrate understanding of bonding, structure, properties and energy changes 45 Reacting masses Answers p. 77 1. A chemical reaction was carried out to convert 1.27 g of copper oxide to copper metal using hydrogen gas. Calculate the mass of copper formed. The equation for the reaction is: CuO(s) + H2(g) Cu(s) + H2O(ℓ) 2. A student burned 1.68 g of magnesium in air. Calculate the mass of magnesium oxide formed. The equation for the reaction is: 2Mg(s) + O2(g) 2MgO(s) 3. Sodium hydrogen carbonate decomposes when heated. The equation can be represented as: 2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(ℓ) If 4.88 g of sodium carbonate is formed, calculate the mass of sodium hydrogen carbonate that was heated. 4. C alculate the mass of iron(III) chloride formed when 2.36 g of iron is burned in chlorine gas. The equation for the reaction can be represented as: 2Fe(s) + 3Cl2(g) 2FeCl3(s) 5. A mixture of 5.74 g iron and 2.28 g sulfur reacted completely to form iron(II) sulfide. The iron was in excess. The equation for the reaction can be represented as: Fe(s) + S(s) FeS(s) a. Calculate the mass of iron(II) sulfide that formed. b. Calculate the mass of iron left over at the end of the reaction. © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 45 4/12/15 9:09 am 46 Achievement Standard 91164 (Chemistry 2.4) Energy changes in chemical reactions Chemical reactions involve energy changes. During a reaction, energy changes from one form to another. Exothermic reactions If heat energy is released during a reaction, the temperature of the surroundings increases, and the reaction is described as being exothermic. Example: Dissolving sodium hydroxide When solid sodium hydroxide pellets, NaOH(s), are dissolved in a beaker of water, the water becomes warm. This is an exothermic reaction since heat energy has been produced and transferred to the water, increasing its temperature. Dissolved sodium hydroxide (Na+(aq) and OH–(aq) ions) contains less energy than the solid sodium hydroxide, NaOH(s). This energy difference can be shown using an energy diagram. Energy Energy diagram for sodium hydroxide dissolving NaOH(s) energy released as NaOH dissolves energy of solid NaOH energy of dissolved NaOH Na+(aq) + OH–(aq) Reaction proceeds Endothermic reactions If heat energy is absorbed during a reaction, the temperature of the surroundings decreases and the reaction is described as being endothermic. When methylated spirits (‘meths’) is spilt on the skin it quickly evaporates. This is an endothermic process – the meths absorbs heat energy from the skin as it evaporates (producing a ‘cold feeling’). Example: Dissolving ammonium nitrate When solid ammonium nitrate, NH4NO3(s), is dissolved in a beaker of water, the water becomes colder as ammonium nitrate absorbs energy from the water. This is an endothermic process. The dissolved ammonium nitrate (NH4+(aq) and NO3– (aq) ions) contains more energy than the solid ammonium nitrate. Energy Energy diagram for ammonium nitrate dissolving energy of dissolved NH4NO3 NH4NO3(s) energy absorbed as NH4NO3 dissolves energy of solid NH4NO3 NH4+(aq) + NO3–(aq) Reaction proceeds © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 46 4/12/15 9:09 am Answers The periodic table (page 4) 4.a. ionicb. 1.a. c. b. 2.a. b. Each element has a full outer (valence) energy level. e. ionicf. He, Li+ g.covalent Al3+, N Al N3–, P P3– magnesium ion 2,8 chloride ion 2,8,8 aluminium ion 2,8 oxide ion 2,8 Formulae of ionic compounds (page 8) zinc carbonate calcium hydrogencarbonate ammonium dichromate sodium hypochlorite iron(III) sulfate chromium chloride 2.a.Mg(OH)2 e.CuBr2 i.Ba3N2 m.CoBr2 q.KMnO4 b.Hg(NO3)2 f.LiHCO3 j.Ca3P2 n.Na2CrO4 r. SnO b. d. f. h. j. l. c. FeO g.(NH4)2CO3 k.K2SO3 o. CuCl s.Ag2S or F F F F 2. 3. O C O or O C O 4. 5. H C C H or H C C H 6. 7. 9. H N H H H C N 1. T he molecular shape is trigonal pyramid. Surrounding the central phosphorus atom are three bonding pairs of electrons and one nonbonding pair of electrons. The 4 pairs of electrons repel each other to a maximum, resulting in a tetrahedral distribution of electron pains. With only three chlorine atoms bonded, the shape is trigonal pyramid. S 2.a. V-shaped or bent H H b. H C N linear H c. d.PbI2 h.AgNO2 l.Al2(SO4)3 p.FeF3 t.NH4Cl or H N H 8. S C S or S C S b. f. molecule containing two atoms of oxygen, both joined by a A double polar covalent bond, to a central atom of carbon. There are small negative charges on the oxygen atoms and a small positive charge on the carbon atom. 3.a.A measure of how strongly the bonding electrons are attracted to the nucleus of an atom in a bond. b.i. Top right. ii. O C O f. linear S O trigonal planar O O g. Cl v-shaped or bent Cl H C O h. trigonal planar H 3.a.The region of negative charge, i.e. a bonding or a non-bonding pair of electrons. b. A pair of electrons forming a single covalent bond. c. A pair of electrons in the outer (valence) energy level of the central atom of a molecule not used in bond formation. 1.a. 2.a.A molecule containing two atoms, joined by a single polar covalent bond. The fluorine atom is more electronegative than the chlorine atom; there is a small negative charge on the fluorine atom and an equal small positive charge on the chlorine atom. b. trigonal pyramid Cl Polarities of molecules (page 22) c. Cl O Polar and non-polar bonds (page 16) 1.a. N Cl or H C N d. tetrahedral H H d. or H O H H Si H H H H Si H or H Si H H H H H H H C C C C or H H H H H O H polar covalent Shapes of molecules (page 20) e. Lewis structures (page 12) 1. potassium phosphate copper(I) oxide potassium thiocyanate cobalt nitrate lead hydroxide sodium permanganate polar covalent d.ionic All three elements are chemically inert. 3. 1.a. c. e. g. i. k. polar covalent H–H ii. The bond is not polar. b. i.O O iii. The molecule is not polar. ii. c. i.H Clii. iii. The molecule is polar. d.i.Cl F ii. The bond is not polar. d+ d- d+ d- The bond is polar as shown: H Cl The bond is polar as shown: Cl F iii. The molecule is polar. 2.a.i. O Cl Bottom left. c.The bond must be formed either from two identical atoms, or from two atoms of different elements that have the same electronegativity. i. iii. The molecule is not polar. The shape of the molecule is bent (V-shaped) Cl because there are four regions of electron density surrounding the oxygen atom (two bond pairs and two non-bonding pairs). ii.Both bonds will be polar with the slightly negative charge d+ d- (d–) on the oxygen atom, i.e. Cl O © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 75 4/12/15 9:09 am Index allotrope 32 anion 3, 5 atom 1, 39 Avogadro’s number 38 balancing equations 43 boiling point 29, 32, 33, 34 bond energy (enthalphy) 54 bonding electrons 9 calculations 38, 39, 44, 50–1, 54 cation 3, 5 changes of state 48, 51 chemical equations 43–4 compound 1 conductivity 31 covalent bonding 9–11, 15 covalent network solids 31–2 delocalised electrons 31, 34 diamond 32 dissolving 26, 46 double bond 40 ductile 34 electrical conductivity 31, 32, 33, 34 electron 1 electron arrangement 2, 3, 10 electron configuration 1 electron density 17 electronegativity (EN) 14–15, 25 electrostatic attraction 9, 14, 15 element 1 EN (electronegativity) 14–15, 25 endothermic reaction 46–7, 48, 54 energy diagrams 46, 47 energy levels 2, 5 enthalpy and enthalpy changes 47–8, 50–1, 54–5 exothermic reactions 46, 47, 48, 54 formation of ions 5 formation of molecules 9 gas 28 graphite 31 groups (periodic table) 2, 3 hardness 31, 32, 33, 34 heat of reaction 47, 51 hydrated ions 26 insoluble 26 intermolecular forces 25, 29, 30, 31 intramolecular forces 29 ion 1, 33 ion formation 5 ionic bonding 9, 15, 26, 33 ionic compounds 6, 33 ionic formula 1, 7 ionic solids 33 ionic substance 39 kilojoule 48 lattice 33 lattice energy 26 Lewis structures 10–11, 17–19 linear 17 liquid 28 malleable 34 matter 28 melting point 29, 32, 33, 34 metallic bond 34 metallic solid 34 metals (on periodic table) 3 molar mass 38 mole (mol) 38 mole calculations 38, 41 mole ratios 43 molecular compound 39 molecular formula 1, 9 molecular substances 29–31 molecule 1, 39 molecules, formation 9 multiple covalent bonds 10 neutral (ionic compounds) 7 non-bonding pairs of electrons 17, 19 non-metals (on periodic table) 3 non-polar bonds 13, 15 non-polar solvent 33, 34 nucleus 1 orbitals 2 particle 1 periodic table 2–3, 84 periods (periodic table) 2 photosynthesis 49 polar bonds 13–15, 21, 25 polar solvent 33, 34 polarity of molecules 13–15, 21, 25, 26, 30 polyethene 31 product 43 properties of solids 28–34 quartz (silica) 32 reactant 43 respiration 49 shapes of molecules 17–19, 25 silica (quartz) 32 solids (types) 28–34 solubility 26, 30, 32, 33, 34 states of matter 28, 51 sublimaton 48 tetrahedral 17 thermochemical equations 50 trigonal planar 17 trigonal pyramid 17 triple bond 10 V-shaped (molecules) 17 valence electron 2, 3, 10, 11 valence shell 3, 5 © ESA Publications (NZ) Ltd – ISBN 978-0-908340-11-8 – Copying or scanning from ESA workbooks is limited to 3% under the NZ Copyright Act. L2 Chem 2.4 LWB.indd 83 4/12/15 9:09 am