ATOMS, MOLECULES AND STOICHIOMETRY

Transcription

1 1 ATOMS, MOLECULES AN 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, and. We will also study how atoms combine in simple ratios, that is, the stoichiometry of reactions. Concept Map Atoms Subatomic particles Relative charge and relative mass Proton number and nucleon number Stable and unstable isotopes Relative atomic mass based on C Mass spectrometry Mole and Avogadro s constant Mole concept of gases and solutions ehaviour in electric field and magnetic field Mass spectrum

2 1 Atoms, Molecules and Stoichiometry 3 Solution 1 mol of C has a mass of g. 1 mol of C contains atoms. Mass of 1 atom of C = = g Effect of an Electric Field on Subatomic Particles 1 When a beam of protons, and is passed through an electric field, the observations are shown in Figure 1.1. beam of protons, and α β protons LINK TO STPM 2003/P2/Q1 Figure The positive protons are deflected towards the negative plate. The negative are deflected towards the positive plate while the, being electrically neutral, are undeflected. 3 The lighter are deflected more than the heavier protons, that is, <. Effect of a Magnetic Field on Subatomic Particles 1 When a beam of protons, and is passed through an electric field, the observations are shown in Figure 1.2. TAKE NOTE! beam of protons, and North pole of magnet protons The deflections of the protons and in a magnetic field can be determined by using Fleming s left hand rule. South pole of magnet Left Hand Rule Figure 1.2 direction of force magnetic field direction of current

3 8 ACE AHEA 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. 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. y 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. Checkpoint 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 EXAMPLE 1.4 Table 1.6 Half-life 8.14 days 5760 years years 1602 years Uranium-235 decays by emitting an alpha particle. etermine the proton number and the nucleon number of the isotope formed. Solution Proton number = 92 2 = 90 Nucleon number = = U Th 4 2He REFRESHER 1 The fundamental particles in an atom are protons, and. 2 Protons are positively charged, are negatively charged and are neutral. 3 Protons and are found in the nucleus while move around the nucleus in specific energy levels. 4 A neutral atom has an equal number of protons and. 5 A positive ion is formed when an atom or molecule loses. A negative ion is formed when an atom or molecule receives. 6 Isotopes are atoms of an element that have the same number of protons but different number of. 7 Unstable isotopes decay spontaneously. 1 etermine the number of subatomic particles of the following species. (a) O (b) 18 8O 2 (c) [ 14 7N 16 8O 3 ] 2 A beam of 1 H and 16 O ions is passed into an electric field as shown in the diagram below. beam of 1 H and 16 O ions x y path X path Y

4 24 ACE AHEA Chemistry First Term Focus on Exam 1 OJECTIVE QUESTIONS 1 Which combination shows the correct number of and of the respective species? Species Number of Number of A 14 6 C O C 24 11Na Cl An atom P has a nucleon number of 48. P forms P 2 ion which is isoelectronic with 40 20Ar atom. Which of the following shows the correct number of protons, and in an atom of P? Protons Electrons Neutrons A C Clone STPM 2004/P1/Q8 3 Which ion has the same number of protons and but has more than protons? A 2 H C [ 16 O 2 H] [ 2 H 3 16 O] [ 32 S 18 O 4 ] 2 4 Silver has two isotopes, Ag and Ag. If the relative atomic mass of silver is , what is the percentage abundance of the heavier isotope? A 35.7% C 56.5% 43.5% 59.2% 5 The symbol of a sulphur atom is 32 16S. Which statement is true regarding sulphur? A The relative atomic mass of sulphur is 32 g. A sulphur atom has 32. C 16 g of sulphur contain atoms. There are 16 protons and 16 in the nucleus of a sulphur atom. 6 The mass ratio of one atom of an isotope of Q to one atom of carbon- is What is the relative isotopic mass of the isotope of Q? A 8.0 C The function of the magnetic field in a mass spectrometer is to A vaporise a solid sample produce positive ions C separate ions with different masses separate ions from neutral molecules 8 Which species will be deflected the most in a mass spectrometer? A 2 1H C 6C 7 3Li 14 7N 2 9 The nitrogen dioxide molecule, NO 2, consists of 14 N, 16 O and 18 O isotopes. How many molecular peaks would there be in the mass spectrum of a sample of NO 2? A 1 C The mass spectrum of a compound showing some major peaks is shown below. relative intensity Which compound will give the above mass spectrum?

5 1 Atoms, Molecules and Stoichiometry 25 [Relative atomic mass: H, 1; C, ; N, 14; O, 16] A N 2 C CO C 2 H 4 NO 11 An element Y occurs naturally as 65 Y and 67 Y in the ratio of 2 : 1. ased on this information, which graph could represent the mass spectrum of Y 2? A C The relative atomic mass of 90 Sr based on the 16 O standard is ased on the C standard, the relative atomic mass of 16 O is What is the relative atomic mass of 90 Sr based on the C standard? A C The compound NCl 3 is formed from 14 N, 35 Cl and 37 Cl isotopes. The relative abundance of 35 Cl to 37 Cl is 3:1. Which statement about the mass spectrum of NCl 3 is true? A The base peak corresponds to N ion. The value for the last peak is 3. C The number of peaks for NCl 3 ion is 4. The relative abundance of N 35 Cl 3 ion to N 37 Cl 3 ion is 3:1. STPM 2010/P1/Q dm 3 of argon contains x atoms. How many atoms are there in 0.2 dm 3 of hydrogen gas under the same conditions? A x C 2x 2 x 4x 15 The relative molecular mass of carbon dioxide is 44. What is the mass, in g, of a carbon dioxide molecule? [Avogadro s constant = mol 1 ] A C The formula of a type of polystyrene is r 3 C 6 H 3 (C 8 H 8 ) n. One sample of the polystyrene contains 10.46% bromine. What is the value of n? [Relative atomic mass: H, 1; C, ; r, 80] A 3 C 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 cm 3 C 42.5 cm cm cm 3 A C 1 only 1 and 2 only 2 and 3 only 1, 2 and 3 18 Which statement(s) is/are true about the isotopes 16 8X and 18 8Y? 1 They are isotopes of the same element. 2 The 16 8X isotope has more per atom than the 18 8Y isotope. 3 They have the same density. 19 Which statement(s) is/are true of the nuclides of silicon, 14Si 28 and 14Si? 30 1 The mass spectrum shows two peaks. 2 14SiCl 28 4 and 14SiCl 30 4 molecules have different shapes. 3 Isotope 14Si 28 is more reactive than isotope 30 14Si. STPM 2010/P1/Q41 20 Which statement(s) is/are true about the isotope A ZX? 1 Its relative isotopic mass is A g. 2 Its relative isotopic mass is based on the C standard. 3 The isotope A ZX and its ion A ZX 2 have the same relative mass. 21 A sample of bromine gas contains 79 r and 81 r isotopes. Which mass/charge ratio(s)