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1 ATOMS, MOLECULES AND STOICHIOMETRY Matter is anything that has mass and occupies space. Everything around us consists of matter. Matter is classified into two main groups, elements and compounds. Elements are substances that cannot be broken down into simpler substances. They contain only one type of atom. The particles in elements can be atoms or molecules. For example, sodium and magnesium contain atoms while oxygen and nitrogen contain molecules. Compounds are substances that contain more than one type of atoms combined chemically. The particles in compounds can be molecules or ions. For example, water and carbon dioxide contain molecules while sodium chloride and magnesium oxide contain ions. Subatomic particles were discovered during the 1800s. We will concentrate only on the fundamental subatomic particles, which are the protons, neutrons and electrons. We will also study how atoms combine in simple ratios, that is, the stoichiometry of reactions. Concept Map Atoms Subatomic particles Proton number and nucleon number Relative atomic mass based on 12C Relative charge and relative mass Stable and unstable isotopes Mass spectrometry • Mole and Avogadro’s constant • Mole concept of gases and solutions Mass spectrum Behaviour in electric field and magnetic field CH001.indd 1 3/17/2012 2:24:47 PM 1 Atoms, Molecules and Stoichiometry 3 Solution 1 mol of 12C has a mass of 12 g. 1 mol of 12C contains 6.02 × 1023 atoms. 12 = 1.99 × 10–23 g ⬖ Mass of 1 atom of 12C = 6.02 × 1023 Effect of an Electric Field on Subatomic Particles 1 When a beam of protons, neutrons and electrons is passed through an electric field, the observations are shown in Figure 1.1. − beam of protons, electrons and neutrons LINK TO STPM 2003/P2/Q1 protons α neutrons β Figure 1.1 + electrons 2 The positive protons are deflected towards the negative plate. The negative electrons are deflected towards the positive plate while the neutrons, being electrically neutral, are undeflected. 3 The lighter electrons are deflected more than the heavier protons, that is, ␣ < . Effect of a Magnetic Field on Subatomic Particles 1 When a beam of protons, neutrons and electrons is passed through an electric field, the observations are shown in Figure 1.2. TAKE NOTE! North pole of magnet The deflections of the protons and electrons in a magnetic field can be determined by using Fleming’s left hand rule. protons beam of protons, electrons and neutrons neutrons electrons South pole of magnet Figure 1.2 Left Hand Rule direction of force magnetic field direction of current CH001.indd 3 3/17/2012 2:24:50 PM 8 ACE AHEAD Chemistry First Term TAKE NOTE! Radioactive disintegration is a first order reaction. The half-life of a radioisotope is independent of the amount of the radioactive substance present and is not influenced by catalyst or changes in temperature and pressure. 14 The stability of radioisotopes is measured by its half-life. The shorter the half-life, the less stable the radioisotope. 15 The half-lives of some radioisotopes are given in Table 1.6. Radioisotope Iodine-131 Carbon-14 Uranium-238 Radium-226 Half-life 8.14 days 5760 years 4.51 × 109 years 1602 years Table 1.6 CHEM FILE Radium was discovered in 1898 by Marie S. Curie and her husband Pierre. Radium has 33 isotopes, with nucleon numbers 202 to 234 and all are unstable. 226 Ra has the longest half-life of 1602 years. CHEM FILE When a tree is alive, C-14 is continually disintegrating, and it is continually being replaced by photosynthesis. However, when the tree dies, photosynthesis does not take place. Only disintegration is occurring and the C-14 concentration in the tree begins to decrease. By measuring the C-14 level in the tree, it is possible to tell how long the tree has been dead. This is called carbon dating. EXAMPLE 1.4 Uranium-235 decays by emitting an alpha particle. Determine the proton number and the nucleon number of the isotope formed. Solution Proton number = 92 – 2 = 90 Nucleon number = 235 – 4 = 231 235 231 4 92U 90Th + 2He REFRESHER 1 The fundamental particles in an atom are protons, neutrons and electrons. 2 Protons are positively charged, electrons are negatively charged and neutrons are neutral. 3 Protons and neutrons are found in the nucleus while electrons move around the nucleus in specific energy levels. 4 A neutral atom has an equal number of protons and electrons. 5 A positive ion is formed when an atom or molecule loses electrons. A negative ion is formed when an atom or molecule receives electrons. 6 Isotopes are atoms of an element that have the same number of protons but different number of neutrons. 7 Unstable isotopes decay spontaneously. Checkpoint 1.1 1 Determine the number of subatomic particles of the following species. (a) 21D2168O (b) 188O2− (c) [147N168O3]− 2 A beam of 1H+ and 16O– ions is passed into an electric field as shown in the diagram below. + beam of 1 + H and 16 − O ions path X x y − path Y CH001.indd 8 3/17/2012 2:24:51 PM 24 ACE AHEAD Chemistry First Term Focus on Exam 1 OBJECTIVE QUESTIONS Species Number of electrons Number of neutrons C− 6 8 A 14 6 B 18 8 O2− 10 10 C 24 11 Na+ 10 11 D 37 17 Cl2+ 19 20 2 An atom P has a nucleon number of 48. P forms P2+ ion which is isoelectronic with 40 20Ar atom. Which of the following shows the correct number of protons, electrons and neutrons in an atom of P? A B C D Protons Electrons Neutrons 20 22 22 22 20 20 22 22 28 26 26 22 Clone STPM 2004/P1/Q8 3 Which ion has the same number of protons and neutrons but has more electrons than protons? A 2H+ C [16O2H]– 2 + 16 B [ H3 O] D [32S18O4]2– 109 4 Silver has two isotopes, 107 47Ag and 47 Ag. If the relative atomic mass of silver is 107.87, what is the percentage abundance of the heavier isotope? A 35.7% C 56.5% B 43.5% D 59.2% A B C D The relative atomic mass of sulphur is 32 g. A sulphur atom has 32 electrons. 16 g of sulphur contain 6.02 × 1023 atoms. There are 16 protons and 16 neutrons in the nucleus of a sulphur atom. 6 The mass ratio of one atom of an isotope of Q to one atom of carbon-12 is 1.50. What is the relative isotopic mass of the isotope of Q? A 8.0 C 15.0 B 10.0 D 18.0 7 The function of the magnetic field in a mass spectrometer is to A vaporise a solid sample B produce positive ions C separate ions with different masses D separate ions from neutral molecules 8 Which species will be deflected the most in a mass spectrometer? A 21H+ C 126 C+ B 73Li+ D 147N2+ 9 The nitrogen dioxide molecule, NO2, consists of 14N, 16O and 18O isotopes. How many molecular peaks would there be in the mass spectrum of a sample of NO2? A1 C3 B2 D4 10 The mass spectrum of a compound showing some major peaks is shown below. relative intensity 1 Which combination shows the correct number of electrons and neutrons of the respective species? 12 32 16 5 The symbol of a sulphur atom is S. Which statement is true regarding sulphur? CH001.indd 24 14 28 m/e Which compound will give the above mass spectrum? 3/17/2012 2:24:53 PM 1 Atoms, Molecules and Stoichiometry [Relative atomic mass: H, 1; C, 12; N, 14; O, 16] A N2 C CO D NO B C2H4 % abundance % abundance 11 An element Y occurs naturally as 65Y and 67 Y in the ratio of 2 : 1. Based on this information, which graph could represent the mass spectrum of Y2? A C m/e 65 m/e 67 130 134 % abundance D % abundance B 132 m/e 130 132 134 [Avogadro’s constant = 6.02 × 1023 mol–1] 44 A 44 × 6.02 × 1023 C 6.02 × 1023 23 1 D B 6.02 × 10 44 × 6.02 × 1023 44 16 The formula of a type of polystyrene is Br3C6H3(C8H8)n. One sample of the polystyrene contains 10.46% bromine. What is the value of n? [Relative atomic mass: H, 1; C, 12; Br, 80] A3 C 19 B 12 D 22 17 0.85 g of magnesium powder is reacted with 2.0 mol dm–3 hydrochloric acid. What is the volume of hydrochloric acid required for complete reaction? [Relative atomic mass: Mg, 24.3] A 17.5 cm3 C 42.5 cm3 3 B 35.0 cm D 70.0 cm3 m/e 130 132 134 90 12 The relative atomic mass of Sr based on the 16O standard is 89.936. Based on the 12C standard, the relative atomic mass of 16O is 15.995. What is the relative atomic mass of 90 Sr based on the 12C standard? A 89.908 C 89.964 B 89.931 D 90.000 13 The compound NCl3 is formed from 14N, 35 Cl and 37Cl isotopes. The relative abundance of 35Cl to 37Cl is 3:1. Which statement about the mass spectrum of NCl3 is true? A The base peak corresponds to N+ ion. B The m/e value for the last peak is 123. C The number of peaks for NCl3+ ion is 4. D The relative abundance of N35Cl3+ ion to N37Cl3+ ion is 3:1. STPM 2010/P1/Q1 14 0.1 dm3 of argon contains x atoms. How many atoms are there in 0.2 dm3 of hydrogen gas under the same conditions? Ax C 2x 2 Bx D 4x 15 The relative molecular mass of carbon dioxide is 44. What is the mass, in g, of a carbon dioxide molecule? CH001.indd 25 25 A B C D 1 only 1 and 2 only 2 and 3 only 1, 2 and 3 18 Which statement(s) is/are true about the isotopes 168 X and 188Y ? 1 They are isotopes of the same element. 2 The 168 X isotope has more neutrons per atom than the 188Y isotope. 3 They have the same density. 19 Which statement(s) is/are true of the 28 30 nuclides of silicon, 14Si and 14Si? 1 The mass spectrum shows two peaks. 28 30 2 14SiCl4 and 14SiCl4 molecules have different shapes. 28 3 Isotope 14Si is more reactive than isotope 30 14Si. STPM 2010/P1/Q41 20 Which statement(s) is/are true about the isotope ZAX? 1 Its relative isotopic mass is A g. 2 Its relative isotopic mass is based on the 12 C standard. 3 The isotope ZAX and its ion ZAX 2+ have the same relative mass. 21 A sample of bromine gas contains 79Br and 81 Br isotopes. Which mass/charge ratio(s) 3/17/2012 2:24:53 PM