Physical Chemistry (HGPC 101) Lecture Notes PDF

Faculty of Allied Medical Sciences Physical Chemistry )HGPC 101( Significant Figures (digits) Atoms, molecules and ions Supervision Dr.Hesham Mostafa Kamel Dr. Ahmed Salah El-Kholany  Outcomes By the end of this lecture, the students will be able to...

Faculty of Allied Medical Sciences Physical Chemistry )HGPC 101( Significant Figures (digits) Atoms, molecules and ions Supervision Dr.Hesham Mostafa Kamel Dr. Ahmed Salah El-Kholany  Outcomes By the end of this lecture, the students will be able to Recognize the significant figures in-1 measurements To differentiate between International System-2 of Units To know Dalton’s atomic theory-3 Understand Atomic number, Mass number and-4 Isotopes Know the meaning of molecule and its types-5 learn to the meaning of ion and its types-6 …FOR EXAMPLE  Look at the ruler below  Each line is 0.1cm  You can read that the arrow is on 13.3 cm  However, using significant figures, you must estimate the next digit  That would give you 13.30 cm LET’S TRY THIS ONE  Look at the ruler below  What can you read before you estimate?  12.8 cm  Now estimate the next digit…  12.85 cm  THE SAME RULES APPLY WITH ALL INSTRUMENTS  The same rules apply  Read to the last digit that you know  Estimate the final digit LET’S TRY GRADUATED CYLINDERS  Look at the graduated cylinder below  What can you read with confidence?  56 ml  Now estimate the last digit  56.0 ml ONE MORE GRADUATED CYLINDER  Look at the cylinder below…  What is the measurement?  53.5 ml  3. Significant figures in measurements In any measurement, there is always a degree on uncertainty (accuracy of the value) Rules to determine the number of significant figures ►The numbers recorded in a measurement are called significant digits or significant numbers. 1- Digits other than a zero are always significant. 854 ► 3 sig figures and 3.6 ► 2 sig figures 2- Zeros between non-zero digits are Significant. ► 4 sig figures and 30.76 ► 4 sig figures 3- Zeros at the beginning of number are not significant. 012 ► 2 sig. fig. and 0.012 ► 2 sig. fig 4- Zeros at the end of number and after the decimal point are Significant. 19.60 ► 4 sig figures 5- Zeros at the end without decimal point May or may not be significant. (Use exponential notation). Example: 500 could have 3 sig figures ► 5.00 X 102 or have 2 sig figures ► 5.0 X 102 or have 1sig figure ► 5 X 102 Example Significant figures in calculations 1- In multiplication and division The result must have the same number of S.F. as the number having the least S. F. in the calculation. e.g. 7.5023  2.00 = 15.0046 wrong 5 S.F. 3 S.F  15.0 (only 3 S.F. Correct 34.103  0.0310 = 10.57193 wrong  10.6 (only 3 S.F.) correct 2- In addition and subtraction The result must have the same number of decimal digits as the number with the least decimal digits in calculation. e.g. 25.23 + 21.3207 = 46.5507 2 d.d. 4 d.d  46.55 (only 2 decimals) 10.31 + 0.002438 = 10.307562  10.31 (only 2 decimals) Rounding Look at the leftmost digit to be dropped, then 1- If this digit is greater than 5, add 1 to the last digit to be retained. Thus, rounding 1.2161 to 3 significant figures gives 1.22. 2- If digit is less than five, drop it, Thus, rounding 1.2141 to 3 significant figures gives 1.21. 3- If this digit is 5 : 15.55 = 15.6 odd 15.45 = 15.4 even xample: erform the following calculations and round the nswers to the correct number of significant figures: 37.76 + 3.907 + 226.4 Solution: The factor 5.8 has the fewest significant figures; therefore the answer should be reported to two significant figures. Round the answer to 3.6 The number with the least number of decimal places is 5.41. Therefore, round the answer to two decimal places, to 5.01 The answer is 0.37. Note how you have lost one significant figure in the subtraction. The answer is 268.1  nits of measurement Measurement always consists of two parts, a number and a unit. 1-Metric system ternational System of Units (SI system) system is commonly used in all countries. damental SI units Some unit conversations: 1 kilogram = 1  103 gram 1 megagram = 1  106 gram 60 second = 60  1015 femto second Femto second = 1  10–15 second Pico meter = 1  10–12 meter 1 meter = 1  1012 pico meter Derived SI units Units which are not fundamental SI units called derived.  Area = (Length)2  Volume = (Length)3  Liter = 103 cm3 = 103 mL  Velocity = m/s (length/time)  Density = g/cm3 (mass/volume)  Atoms, molecules and ions DALTON’S ATOMIC THEORY (1808) Elements are composed of extremely small particles -1.called atoms 2- All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 3- Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction. 4- A chemical reaction involves only the separation, combination, or rearrangement of atoms X 16 + Y8 X2Y 8  SUBATOMIC PARTICLES Particle Symbol Charge Mass Electron e- - 9.11X10-28 g Proton p+ + 1.67X10-24 g Neutron n 0 1.67X10-24 g  Atomic number and Mass number An atom is neutral The net charge is zero Number of protons = Number of electrons Atomic number = Number of electrons Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons atomic number (Z) + number of neutrons = Mass Number A X Element Symbol Atomic Number Z  SUBATOMIC PARTICLES IN SOME ATOMS 16 31 65 O P Zn 8 15 30 8 p+ 15 p+ 30 p+ 8n 16 n 35 n 8 e- 15 e- 30 e- Isotopes: The isotopes of an element have the same atomic no. (same chemical properties) but differ in their mass no. (different physical properties) i.e they have same no. of protons but different no. of neutrons. Example: the isotopes of hydrogen Eg. 12 C, 13 C The 2 atoms have the same at.no. (equal no. of protons) But they differ in their mass no. (different no. of neutrons) C: at.no. = 6, mass no.=12, no. of neutrons=6 12 C: at. no. =6, mass no.=13, no. of neutrons=7 13 Eg. O atom 16 O, 17 O, O 18 8p, 8p, 8p 8n, 9n, 10n 8e, 8e, 8e  Molecules and Ions A molecule is an aggregate of two or more atoms held together by chemical forces H2 H2O NH3 CH4 The forces that hold atoms together in molecules.are called chemical bonds A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 Molecules can be represented in several different ways: Chemical formula Represent the type and actual number of atoms in ,a molecule for example, carbon dioxide CO2 (it has 1 carbon.atom and 2 oxygen atoms) Structral formula Formula by which the individual bonds are shown (may or may not show the actual shape of.molecule) Example LEARNING CHECK – COUNTING Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C 6 6 6 #p+ _______ _______ _______ #no _______ _______ _______ #e- _______ _______ _______  LEARNING CHECK An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 142) 16 3) 34 B. Its mass number is 1) 142) 16 3) 34 C. The element is 1) Si 2) Ca 3) Se D. Another isotope of this element is 1) 34X 2) 34X 3) 36X 16 14 14 IONS  IONS are atoms or groups of atoms with a positive or negative charge.  Taking away an electron from an atom gives a CATION with a positive charge  Adding an electron to an atom gives an ANION with a negative charge.  To tell the difference between an atom and an ion, look to see if there is a charge in the superscript! Examples: Na+ Ca+2 I- O-2 Na Ca I O FORMING CATIONS & ANIONS A CATION forms An ANION forms when an atom when an atom loses one or more gains one or.electrons more electrons -Mg --> Mg + 2 e 2+ F + e- --> F - PREDICTING ION CHARGES In general metals (Mg) lose electrons ---> cations nonmetals (F) gain electrons ---> anions LEARNING CHECK – COUNTING State the number of protons, neutrons, and electrons in each of these ions. 39 K+ O -2 16 Ca +2 41 19 8 20 #p+ ______ ______ _______ #no ______ ______ _______ #e- ______ ______ _______  CHARGES ON COMMON IONS 3- 2- 1- 1+ 2+ By losing or gaining e-, atom has same number of e-’s as nearest Group 8A atom. Ions An ion is an atom, or group of atoms, that has a net positive or negative charge. An ion may be a cation or an anion. Cation: ion with a positive charge. If a neutral atom loses one or more electrons it becomes a cation. Anion: ion with a negative charge. If a neutral atom gains one or more electrons it becomes an anion. Quiz time What is the number of protons and – electrons in S2 What is the number of protons and + electrons in aluminum ion, Al3 What is the number of protons, neutron and electrons in 16 O, 17 O, 18O How many Significant figures and decimal points in  Student Question Write the nuclear symbol form for the following atoms or ions: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 46 e- ___________ What the SI units of 1- time 2- length 3- temperature 4- amount of substance 5- mass Answer the following operation 18.5 + 31.3 = 3.2 + 5.09 + 3.007 = 36.36 ÷ 4.04 = 4.053 ÷ 4.00 = 27.00 x 2.00 = 0.0040 x 3001.37 =  How many Significant figures in 1) 32.01 2) 5.010 3) 32.00 4) 200 5) 8.1 Calculate the no. of proton, neutron and electrons for 35 59 +4 Cl Ni 17 , 28 Define and give example: Cation Anion Isotope Structural formula  :RECOMMENDED TEXTBOOKS 1- Ebbing & Gammon, General Chemistry, 11th ed, 2016 Houghton Mifflin Co. 2- Raymond Chang. Chemistry. 10thed. 2009 3- Zumdehl. International edition. 2009