Form 6 Chemistry Notes Section 1 1/7 Section 1 Atoms, Molecules and Stoichiometry

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1 Section 1 1/7 Section 1 Atoms, Molecules and Stoichiometry 1.1 Atomic structure Pre-knowledge 1. Explain each of the following terms (a) element (b) atom (c) ion (d) relative atomic mass (e) mole (f) the Avogadro constant, L 2. Describe, with the aid of a diagram, a simple model of the atom, using the terms : proton, neutron, electron, nucleus, shell (or orbit). 3. Complete the following table Fundamental particle Relative mass Relative charge Proton Neutron Electron Relative sizes The radius of a atom is of the order m and that of a nucleus is of the order m. As an analogy, if the entire football stadium represents the atom, then the nucleus would be situated in the middle where a football match is kicked off.

2 Section 1 2/7 1.2 Radioactivity Properties of α, β and γ emissions α, β particles are charged particles but γ-radiation is a short wavelength of electromagnetic radiation. All of them are emitted from unstable nuclei. Emission Nature Charge Mass Velocity Penetration Power (Identity) (a.m.u.) Alpha, α Beta, β Gamma, γ Nuclear reactions Whereas chemical reactions leave the nucleus unchanged and involve only the electrons, nuclear reactions rearrange the particles within the nucleus. The basic nuclear reactions are α- and β- particle emission, often called α- and β- decay. Example : When thorium-228 decays, each atom emits one α- particle. Write a balanced nuclear equation to show the process. Write similar balanced nuclear equations to show the (a) α- decay of (c) β- decay of Po Po (b) α- decay of (d) β- decay of Rn Na Uses of isotopes (1) Leak detection Underground leaks in water pipes can be detected by introducing a short-lived radioisotope into the pipe. The level of radioactivity on the surface will increase drastically should the pipe be broken. (2) Radiotherapy Cancerous tissue is destroyed by radioactivity in preference to healthy tissue. A carefully dosed cobalt-60 source can be used to irradiate cancer patients.

3 Section 1 3/7 (3) Nuclear power Most of the nuclear power generated comes from nuclear fission of Uranium-235. As heat is generated in the nuclear reactor, it is transferred to a stream of gas which circulates around the reactor. The hot gas is used to boil water; the steam produced is used to drive a turbine and generate electricity. (4) Tracers Radioisotopes are used to track the path of an element through the body. Radioactive iodine (iodine-131) is administered to patients with defective thyroids to enable doctors to follow the path of iodine through the body. 1.3 Relative isotopic, atomic and molecular masses The mass spectrometer The mass spectrometer is an instrument used to accurately measure the relative atomic mass of an element. An important use of the mass spectrometer is to determine the relative atomic mass of elements. The sample is put in the vaporization chamber and the output of the mass spectrometer is often in the form of a mass spectrum. A mass spectrum of an element with different isotopes will show both the relative abundance (the Y-axis) and the occurrence of the particular isotopes (the X-axis).

4 Section 1 4/7 The following mass spectrum is obtained on a sample of naturally occurring magnesium isotopes. Determine the relative atomic mass of magnesium (to 3 d.p.). Workings: 1.4 The Mole Concept Ideal Gas Equation The ideal gas equation follows from the combined gas law and Avogadro s theory. pv 1 1 T 1 = pv 2 2 T 2 For one mole of gas, the volume is written as V m, while the constant is written as R, and is known as the gas constant : i.e. pv m = R T Avogadro s theory states that molar volume, V m, is the same for all gases under the same conditions. It follows that R is the same for all gases. Hence pv m = RT (for one mole of gas) For n mole of gas pv = nrt

5 Section 1 5/7 Applying the formula pv = nrt to find the relative molecular mass of a volatile liquid 1. The hypodemic syringe containing the volatile liquid was weighed before and after the injection. 2. After the injection the volatile liquid was allowed to vaporise. 3. The volume occupied was measured using a graduated gas syringe. The following results were obtained : g of a liquid was injected into a gas syringe. The volume of vapour formed was 84.0 cm 3 at 17 C and 1.01 x 10 5 Nm -2. Calculate the molar mass of the liquid. (Given R = J K -1 mol -1 ) Dalton s Law of Partial Pressure In a mixture of gases, the total pressure is the sum of the pressures that each of the gases would exert if it alone occupied the same volume as the mixture. The contribution that each gas makes to the total pressure is called the partial pressure. Example ; 2.0 dm 3 of carbon dioxide at a pressure of 200 kpa and 1.0 dm 3 of nitrogen at a pressure of 300 kpa are introduced into a 4.0 dm 3 vessel. What is the total pressure in the vessel?

6 Section 1 6/7 1.5 The Faraday and the mole The Faraday constant F is defined as the quantity of electric charge Q on one mole of electrons. most calculations F = C mol -1. F =Q/n and Q = It Q = quantity of electricity I = current n = amount of electrons in moles t = time For Example 1 Calculate the mass of copper deposited at the cathode when a current of 0.30 A is passed through a copper(ii) sulphate solution for 20 minutes. (F = C mol -1, Cu=63.5) Example 2 A metal of relative atomic mass 27 is deposited by electrolysis. If g of the metal is deposited on the cathode when 0.15 A flows for 3.5 hours, what is the charge on the cations of this metal? (F = C mol -1 ) 1.6 Empirical and Molecular Formulae The empirical formula of a compound is the simplest form of the ratio of the atoms of different elements in it. The molecular formula tells the actual number of each find of atom in a molecule of the substance. Example 1 An 18.3 g of sample of a hydrated compound contained 4.0 g of calcium, 7.1 g of chlorine and 7.2 g of water only. Calculate its empirical formula. (Ca=40, Cl=35.5, H=1, O=16)

7 Section 1 7/7 Example g of an organic compound containing carbon, hydrogen and oxygen gave an analysis g of CO 2 and g of H 2 O. Find the empirical formula of the substance. If the molecular mass of the compound is 32, suggest a structural formula for it. (C=12, H=1, O=16) 1.7 Chemical equations and stoichiometry Stoichiometry in its broadest sense includes all the quantitative relationships in chemical reactions. It has to do with how much of one substance will react with another. N 2 (g) + 3H 2 (g) 2NH 3 (g) The numbers 1, 3 and 2 are called stoichiometric coefficients of the equation. Example 1 What mass of phosphorus(v) oxide, P 2 O 5, would be formed by complete oxidation of 4.00 g of phosphorus? (P=31, O=16) 4P(s) + 5O 2 (g) 2P 2 O 5 (s) Example 2 A mixture containing 2.80 g of iron and 2.00 g of sulphur is heated together. What mass of iron(ii) sulphide, FeS, is produced? (Fe=56, S=12) Fe(s) + S(s) FeS(s) Example 3 When 0.27 g of aluminium is added to excess copper(ii) sulphate solution, 0.96 g of copper is precipitated. Deduce the equation for the reaction which takes place.

8 Section 1 8/7 (Al=27, Cu=63.5, S=32, O=16)

Isotopes and Radioactive Decay

NAME PERIOD DATE CHAPTER 4 NOTES: ISOTOPES Isotopes and Radioactive Decay ISOTOPES: Atoms that contain the same number of protons but a different number of neutrons. Isotopes containing more neutrons have