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Law of conservation of mass Chapter 2 In a chemical reaction, matter is neither created nor destroyed, but changed from one form to another. The total mass of chemicals in your reaction mixture will not change as long as you do not add any more chemicals to the mixture, or allow any chemicals to escape. Atoms and Elements Dr. Peter Warburton [email protected] http://www.chem.mun.ca/zcourses/1010.php All media copyright of their respective owners Law of conservation of mass 2 Law of definite proportions All samples of a given compound, regardless of where they came from or how they were made, will have the same proportions of their constituent elements. The make-up of a compound is a fundamental aspect of it’s identity. No matter it’s source, a chemical must be made of the same pieces… All media copyright of their respective owners 3 All media copyright of their respective owners 4 1 Law of definite proportions Problem Two samples of carbon monoxide are decomposed into their constituent elements. One sample produces 17.2 g of oxygen and 12.9 g of carbon, while the other sample produces 10.5 g of oxygen and 7.88 g of carbon. Show these results are consistent with the law of definite proportions. Water ALWAYS contains 16.0 g of oxygen atom per 2.0 g of hydrogen. Water out of your bathroom tap, Quidi Vidi Lake, the Atlantic Ocean, or on Mars ALWAYS has an 8:1 ratio of O:H by mass All media copyright of their respective owners 5 Law of multiple proportions 6 Law of multiple proportions When two elements (A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small whole numbers. This points to something fundamental about how elements combine to form compounds, in that elements often combine in discrete integer combinations. All media copyright of their respective owners All media copyright of their respective owners Per gram of carbon: ܱ݉ܽܥ ݊݅ ܱ ݂ ݏݏଶ 2.67 ݃ 2 = = ܱ݉ܽܥ ݊݅ ܱ ݂ ݏݏ 1.33 ݃ 1 We can see in the formulas (and models shown) that CO2 has 2 oxygen atoms per carbon atom while CO has 1 oxygen atom per carbon atom. This is reflected in the mass ratio! 7 All media copyright of their respective owners 8 2 Dalton’s atomic theory (1808) 1. 2. 3. 4. Atomic structure Each element is made of tiny indestructible particles called atoms. All atoms of a given element have the same mass and physical/chemical properties that distinguish them from atoms of other elements. Atoms combine in simple whole number ratios to form compounds. Atoms of one element cannot change into atoms of another element. In chemistry atoms can only change how they are bound to other atoms. All media copyright of their respective owners What are these tiny, indestructible, unchangeable particles called atoms and what is there about them that makes them different from each other that we call the elements with different physical and chemical properties? It turns out atoms have several distinct pieces! All media copyright of their respective owners 9 The electron 10 The electron J.J. Thompson explored the properties of cathode rays in his cathode ray tube. All media copyright of their respective owners The cathode rays always travelled in straight lines from the negative end to the positive end, regardless of what the cathode was made of… 11 All media copyright of their respective owners 12 3 The electron The electron Through a delicate experiment Thompson showed cathode rays have about 1/2000th the mass of a hydrogen atom, the lightest element. Somehow the socalled indestructible atom is made of smaller pieces! He deduced that cathode rays must carry a negative electrical charge, and that all elements must contain them (since it didn’t matter what the cathode was made of)… All media copyright of their respective owners 13 The electron All media copyright of their respective owners 14 Charge of an electron Thompson realized cathode rays are small pieces of charged matter which we now call electrons. Negative charges attract positive charges and repel other negative charges All media copyright of their respective owners Millikan used charged oil droplets in an electric field to show the charge of one electron is the smallest possible charge you can have… -1.60 x 10-19 C 15 All media copyright of their respective owners 16 4 Charge of an electron Plum pudding model Thompson proposed that the negatively charged electrons were distributed throughout a positively charged sphere that held all the electrons together Just as importantly, he was able to deduce the mass of one electron… it’s an incredibly small 9.10 x 10-28 g All media copyright of their respective owners 17 Radioactivity 18 Rutherford’s gold foil experiment Henri Becquerel, Marie Curie and others studied the emission of small energetic particles from unstable atoms… we call this radioactivity. Alpha (α) particles are positively charged Beta (β) particles are negatively charged Gamma (γ) rays are very high energy All media copyright of their respective owners All media copyright of their respective owners Rutherford realized he could probe the “plum pudding” with the positively charged α particles. He “shot them” at a thin piece of gold foil to see how the negative electrons would deflect them 19 All media copyright of their respective owners 20 5 Rutherford’s gold foil experiment Rutherford’s gold foil experiment A few of the alpha particles were deflected through large angles. Rutherford found it “about as credible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” If the plum pudding model was correct, then the positively charged particle should pass through the evenly distributed charge of the atom. This didn’t happen! All media copyright of their respective owners The nuclear atom 22 The nucleus The positive charge of a nucleus actually comes from the positive charges of one or more protons. The charge of a proton is of the same size as the charge of an electron, but of opposite sign 1.60 x 10-19 C. However a proton has a mass 1836 times bigger than that of an electron! 1. Most of the atom’s mass and its positive charge are in a small core called the nucleus. 2. Most of an atom is actually empty space where the tiny negatively charged electrons are dispersed. All media copyright of their respective owners All media copyright of their respective owners 21 23 All media copyright of their respective owners 24 6 Neutrons The particles of an atom If a nucleus contains more than one proton, why don’t the positive charges repel each other and the nucleus falls apart? Nuclei also contain neutrons, which have no charge and are just slightly more massive than the protons. Neutrons act like “a glue” that hold protons together. All media copyright of their respective owners Relative charges ignore the magnitude of the charge and just focus on the charge. We can do this since there has never been seen a charge less than that of a proton or electron. The atomic mass unit (amu) is defined as 1/12th of the mass of a carbon atom with 6 protons and 6 neutrons. 25 Atomic number Z 26 How much glue do you need? We saw that atoms of an element have the same properties and mass as other atoms of that element, but they are different than the atoms of a different element. The fundamental difference between elements is measured by the atomic number Z. This number tells us how many protons (and also an equal number of electrons) an atom of that element contains. All media copyright of their respective owners All media copyright of their respective owners If neutrons are the “glue of the nucleus” then how many do you need to “do the job”? It turns out that the answer can vary, EVEN for atoms of the same element! 27 http://www.craftsy.com/blog/2014/05/how-to-glue-up-wood/ All media copyright of their respective owners 28 7 How much glue do you need? In other words, Dalton’s definition of the atom is partially wrong. Not all atoms of the same element have the same mass because they have different numbers of neutrons! Isotopes We call atoms of the same element that have differing numbers of neutrons the isotopes of that element. We can easily describe all the fundamental information about an element and it’s isotopes by using three pieces of information: the chemical symbol, the atomic number and the mass number. http://www.craftsy.com/blog/2014/05/how-to-glue-up-wood/ All media copyright of their respective owners All media copyright of their respective owners 29 Isotopes 30 Isotopes The chemical symbol X is a one- or two-letter symbol representing the elements name. For most elements the symbol connects easily to the English name: C for carbon, Ne for neon, Cs for cesium, etc For 10 of the elements the chemical symbol is actually based on the Latin name for the element: Na for sodium (natrium), K for potassium (kalium), Fe for iron (ferrum), Cu for copper (cuprum), Ag for silver (argentum), Sn for tin (stannum), Sb for antimony (stibium), Au for gold (aurum), Hg for mercury (hydragyrum) and Pb for lead (plumbum) One element has a symbol based on it’s Swedish name : W for tungsten (wolfram) we can know how many neutrons are in the nucleus of that given isotope. All media copyright of their respective owners All media copyright of their respective owners 31 As we saw before, the atomic number Z tells us how many protons (or electrons) are in ANY atom of the element. The mass number A tells us how many protons AND neutrons are in the SPECIFIC ISOTOPE of the element. Therefore if we calculate A–Z 32 8 Isotopes Isotopes of neon Since the atomic number Z and the chemical symbol ultimately tell us exactly the same thing (what element is this?) we often refer to an isotopes name just by the chemical symbol and it’s mass number. i.e. Ne-20, Ne-21 or Ne-22 All of them contain 10 protons (Z = 10 for neon). The first isotope has 10 neutrons, the second isotope has 11 neutrons and the third has 12 neutrons. All media copyright of their respective owners Neon has three naturally occurring isotopes. The natural abundance tells us the relative percentage of each type of isotope we would find in a sample of that element. For example, if I had a sample of 10000 neon atoms, I would find: 9048 of them are Ne-20 with 10 neutrons 27 of them are Ne-21 with 11 neutrons 925 of them are Ne-22 with 12 neutrons. All media copyright of their respective owners 33 Problem 34 Ions Sometimes atoms can gain or lose electrons to form charged ions as compared to the neutral atoms. Positively charged ions (loss of electrons) are called cations while negatively charged ions (gain of electrons) are called anions. What are the atomic number, mass number and symbol for the carbon isotope with seven neutrons? How many protons and neutrons are present in an atom of ଷଽ ଵଽ All media copyright of their respective owners Li → Li+ + 1 eF + 1 e- → F - 35 Mg → Mg2+ + 2 eO + 2 e- → O2- All media copyright of their respective owners 36 9 Atomic mass Atomic mass The concept of isotopes leads to an interesting conclusion. The chemical properties of an element are determined by the number of protons and electrons in that element, and NOT by the mass of the element. If mass were a factor, then the chemical properties of different isotopes of an element would be different for each isotope. They’re not! All media copyright of their respective owners Because of different isotopes having different masses and natural abundances, it makes sense to assign an atomic mass to each element that includes all isotopes. The atomic mass is a weighted average. Atomic mass = ݂݊ ݁ݐݏ݅ ݂ ݏݏܽ݉ ݊ ݁ݐݏ݅ ݂ ݊݅ݐܿܽݎ 37 Atomic mass All media copyright of their respective owners 38 I need some volunteers… Chlorine has two isotopes: Cl-35 and Cl-37 Cl-35 makes up 75.77% of all chlorine atoms Cl-37 makes up 24.23% of all chlorine atoms You only need to stand up in front of the class and be willing to say how tall you are. 0.7577 34.97 ܽ݉ ݑ+ 0.2423 36.97 ܽ݉ݑ = 35.45 amu Any takers? All media copyright of their respective owners 39 All media copyright of their respective owners 40 10 Problem Measuring isotope masses Titanium has five naturally occurring isotopes. Using the info in the table below, calculate the atomic mass of titanium: Isotope Natural abundance (%) Isotope mass (amu) Ti-46 8.25 Ti-47 7.44 46.9518 Ti-48 73.72 47.9479 Ti-49 5.41 48.9479 Ti-50 5.18 49.9448 All media copyright of their respective owners 45.9526 All media copyright of their respective owners 41 Moles and molar mass 42 Mole If the mass of an isotope does not affect it’s chemical properties, then we are likely more interested in how many atoms of that element we have. The problem is how do we count atoms? It turns out we can do that by mass (since weighing is easy to do). But we do need to know the connection between mass and number of atoms! We saw earlier that the SI unit of amount of substance is the mole (mol), which we characterized as the “chemist’s dozen” All media copyright of their respective owners All media copyright of their respective owners 43 1 mol = 6.02214 x 1023 particles This value is Avagadro’s number. Notice the use of the word “particles”, since we can talk about a mole of atoms, electrons, ions, or molecules. 44 11 Mole Problems Since the mole is a SI unit it actually has a VERY specific definition: How many atoms are there in 3.92 mol of phosphorous? “The value of the mole is equal to the number of atoms in exactly 12 grams of carbon-12” Notice this connects a number of atoms to the total mass they have! If I have 3.69 x 1018 fluoride ions (F-), then how many moles of fluoride ions do I have? All media copyright of their respective owners All media copyright of their respective owners 45 Molar mass 46 Molar mass We’ve seen the definition of the mole is connected to a specific mass of carbon-12. If we want to convert from a mass of a substance to the number of moles of that substance (or vice versa) then we would need equivalent mass/moles relationships for other substances. We call this the The molar mass of any element, in grams per mol (g mol-1 or g/mol), is numerically equal to the atomic mass of the element in atomic mass units. Earlier we saw that the atomic mass of chlorine was 35.45 amu. This means the molar mass of chlorine is molar mass (M) 35.45 g mol-1 All media copyright of their respective owners 47 All media copyright of their respective owners 48 12 Molar mass Problems How many carbon atoms are there in a 1.3-carat diamond (which is pure carbon) if 1 carat = 0.20 g)? Calculate the mass of 2.25 x 1022 tungsten atoms. All media copyright of their respective owners 49 The periodic table All media copyright of their respective owners 50 Mendeleev’s periodic table Dmitiri Mendeleev organized the known elements (in 1869) according to his periodic law that the elements showed repeating patterns in terms of what proportions they reacted with each other and trends in their physical properties. Ordered by increasing mass Grouped by formulas of hydrides and oxides Blanks left for elements yet to be discovered All media copyright of their respective owners 51 All media copyright of their respective owners 52 13 Modern periodic table Additional detail in periodic table Elements are classified as Metals Metalloids Nonmetals Now grouped by atomic number Missing elements are here! All media copyright of their respective owners 53 Modern periodic table All media copyright of their respective owners 54 Group 18 – Noble gases The elements of the last column (Group 18) are the noble gases. These elements are remarkably unreactive (thus the name). He Ne Down the periods (columns) the elements of the group show similarity of chemistry and trends in properties http://en.wikipedia.org/wiki/Neon#/media/File:NeTube.jpg All media copyright of their respective owners 55 All media copyright of their respective owners 56 14 Group 1 – Alkali metals Group 17 - Halogens The first column of the periodic table (Group 1) are the very reactive alkali metals which react vigorously with water, for example. All media copyright of their respective owners Next to the noble gases are the Group 17 halogens which are very reactive non-metals. They will react quite vigorously with alkali metals, for example. 57 Ions and the periodic table 58 Ions and the periodic table These tendencies explain why the alkali metals and halogens are very reactive, especially with each other! Main group metals tend to lose electrons to form cations with the same number of electrons as the noble gas in the previous row. Main group non-metals tend to gain electrons to form anions with the same number of electrons as the noble gas in the same row. All media copyright of their respective owners All media copyright of their respective owners 59 All media copyright of their respective owners 60 15