Science 1206 Ch. 3 - Chemical names, formulas and equations

Transcription

1 Science 1206 Ch. 3 - Chemical names, formulas and equations Ionic and molecular compounds (pp ) Compounds A compound is a pure substance made of a combination of elements. The elements are joined together in defined proportions. In general, there are two classes of compounds: (1) Molecular (covalent) compounds (2) Ionic compounds

2 Elements Element - a pure substance made of one type of atom. Naming most elements is quite simple as they exist as individual atoms in nature. Therefore, to name them simply look up the symbol on the periodic table. Ex. iron - Fe Gold - Au Diatomic and polyatomic elements However, there are some elements which do not exist as individual atoms in nature. Rather they are units of two of the same atom (diatomic) or many atoms (polyatomic). List of diatomic elements to know: H 2, O 2, N 2, F 2, Cl 2, Br 2, I 2 List of polyatomic elements to know: P 4, S 8

3 Molecular compounds molecular compound - a compound formed of atoms of two or more elements that share electrons Molecule - two or more non-metal atoms joined by covalent bonds A molecule is the smallest independent unit of a molecular compound, and it consists of atoms of different elements bonded together. The bonds between the atoms are called covalent bonds and they are very strong. Covalent bonds form when atoms share electrons. They share electrons to fill their valence (outer) energy level. Covalent compounds Water is composed of molecules. Water molecules consist of two hydrogen atoms bonded to one oxygen atom.

4 Properties of Molecular compounds General properties of molecular compounds: - The forces in between molecules are relatively weak (as opposed to the covalent bonds within a molecule). Therefore molecular compounds can be solid, liquid or gas at room temperature and tend to have lower boiling and melting points. As well, they tend to be relatively soft. - They do not have free electrons available (all tied up in bonds) so they are poor conductors of electricity and heat. Ionic compounds ionic compound - a compound composed of oppositely charged ions of a metal and non-metal held together with ionic bonds Ion - an atom or group of atoms with a positive or negative charge Most elements can form ions, as their energy levels are not filled to capacity with electrons. Elements will either gain or lose electrons to obtain full energy levels. In general, metals will lose electrons forming positive ions (or cations)while non-metals gain electrons to form negative ions (or anions)

5 Ionic compounds - example For example, table salt, sodium chloride, forms when sodium atoms each transfer one electron to chlorine atoms. Each sodium atom becomes positively charged: a positive ion. Each chlorine atom becomes negatively charged: a negative ion. This is what happens when sodium metal reacts with chlorine gas to form sodium chloride. Structure of ionic compounds Unlike molecular compounds, ionic compounds do not consist of discrete units (molecules). Instead, they form large structures with regular and repeating patterns called lattices. The cubic structure of sodium chloride is an example of a crystal lattice structure.

6 Properties of ionic compounds - Ionic bonds are quite strong and hard to break. Therefore ionic compounds are solid at room temperature and tend to have high melting and boiling points. - They do not conduct heat or electricity well as solids, as the crystal lattice structure does not allow the movement of electrons. However, they can when dissolved in water or melted. Science 1206 Ch. 3 - Chemical names, formulas and equations Names and Formulas of Ionic and Molecular Compounds (pp )

7 IUPAC and systematic names The international system for naming chemicals is maintained by the International Union of Pure and Applied Chemistry (IUPAC). Founded in 1919, IUPAC has developed a systematic method to name chemicals according to their composition. Today, scientists all over the world use the IUPAC system. This system ensures that each pure substance has a unique name, called its systematic name. The name of a substance describes its composition. It also enables scientists to write its chemical formula and predict some of its properties Names and formulas of binary molecular compounds binary molecular compound - a compound composed of atoms of two different elements, usually two non-metals, that are joined together with covalent bonds Rules for naming binary covalent compounds: 1. Use the name of the name of the first element in full. 2. Shorten the name of the second element and add the suffix ide. 3. Use prefixes (see table 3.4, p. 120) to indicate the number of each element. The prefix mono- is used only for the second element in the name. When there is no prefix, mono- is implied, as in carbon monoxide. Also, when a prefix that ends with a vowel, such as mono or penta comes before -oxide, the vowel at the end of the prefix is dropped.

8 Naming molecular compounds - examples CO 2 - carbon dioxide N 2 Cl 5 - dinitrogen pentachloride Br 4 S 7 - tetrabromine heptasulfide P 3 I 6 - triphosphorous hexaiodide Exceptions to the rules - Compounds that contain hydrogen (while technically molecular compounds) are not named according to those rules. No prefixes are used. Their naming also changes if they are dissolved in water. - Ex. HCl = hydrogen chloride - Ex. H 2 S = hydrogen sulfide - The naming of compounds containing carbon and hydrogen (and other elements) follows rules for organic chemistry.

9 Trivial Names Many chemicals have frequently used trivial names that do not follow the IUPAC rules, such as: H 2 O - water H 2 O 2 - hydrogen peroxide NH 3 - ammonia O 3 - ozone NaHCO 3 - baking soda Naming Ionic compounds Ionic compounds consist of positive ions and negative ions, and the formulas for them are written to express the lowest ratio where the net charge in the compound is zero. In order to get the coefficients for ionic formulas, you need to know the charge of each ion (found on the periodic table). Names for ionic compounds DO NOT require prefixes.

10 Categories of ionic compounds There are several categories of ionic compounds, with slight variations in naming conventions for each. However, the overall concept and goal is the same; formulas for ionic compounds are written to show the lowest whole number ratios for the ions in the compound. Types of ionic compounds covered this year: - Binary ionic - Multivalent ionic - Polyatomic ionic compounds Binary ionic compounds binary ionic compound - a compound composed of ions of two different elements: a positively charged metal ion and a negatively charged nonmetal ion Rules for naming binary ionic compounds: (1) Write the name of the first ion (the cation) in full (2) Shorten the ending of the second ion (the anion) and add the suffix ide

11 Naming binary ionic compounds - examples NaCl = sodium chloride MgO = magnesium oxide CaBr 2 = calcium bromide K 3 N = potassium nitride Al 2 O 3 = aluminum oxide Writing formulas for binary ionic compounds In order to write the correct subscripts for ionic compounds, the charges for each ion must be known, then one needs to determine the lowest ratio in order to get a net charge of zero. Ex. sodium bromide Sodium = Na 1+ Bromide = Br 1- Since the charges are equal, the ratio is 1:1 so the formula is NaBr

12 Binary Ionic compounds - example 2 - Aluminum sulfide Aluminum = Al 3+ Sulfide = S 2- In this case, you would need 2 Al 3+ for every 3 S 2- in order to get a compound with a net charge of zero. The formula would be Al 2 S 3 Multivalent Ionic compounds Some metals, in particular the transition metals, can form more than one ion. They are called multivalent metals. Ex. copper can form ions of 1+ and 2+ When naming compounds with these metals in them, it has to be identified which metal ion is present. This is done using either the Stock system (using Roman numerals, Table 3.2 p. 114) or the classical systems (with Latin names and endings). The Stock system is more commonly used today.

13 Naming Multivalent ionic compounds Steps: (1) Look up the name of the metal ion and write in full. Leave space for a Roman numeral (2) Look up the name of the non-metal. Shorten it and add ide (3) Use the prefixes given in the formula plus the charge of the non-metal to calculate the charge of the metal. Write the charge as a Roman numeral after the name of the metal. Naming multivalent compounds - example 1 Fe 2 O 3 Fe = iron (can be 2+ or 3+) O = oxygen, changes to oxide The charge on oxide is 2- and since there are 3 of them, total negative charge is 6-. Therefore total positive charge has to be 6+ (has to be equal). Since there are 2 iron ions, each one must be 3+ Name = iron (III) oxide

14 Naming multivalent compounds - example 2 PbS 2 Pb = lead (can be 2+ or 4+) S = sulfur, change to sulfide Sulfide has a charge of 2-, and since there are 2 of them total negatives = 4-. Total positives must be 4+ and since there is only one Pb ion, its charge must be 4+ Name = lead (IV) sulfide Formulas for Multivalent compounds - example 1 - Chromium (III) chloride Chrmomium (III) = Cr 3+ Chloride = Cl - You need 3 Cl for every 1 Cr, so the formula would be CrCl 3

15 Formulas for multivalent compounds - example 2 - Vanadium (v) phosphide Vanadium (v) = V 5+ Phosphide = P 3- In this case, the lowest ration of V to P would be 3:5 so the formula will be V 3 P 5 Polyatomic Ions Polyatomic ions - ions that are made up of more than one element. Polyatomic ions tend to end it ate and ite (a few end in ide ), and the vast majority of them are anions (only a couple of cations such as ammonium, NH 4 + ) When writing names involving polyatomic ions, it is not necessary to change the ending of the name. If there is more than one of the polyatomic ion in the formula, the polyatomic ion is written in parenthesis with the number outside

16 Naming with polyatomic ions - example 1 Na 2 SO 4 Na + = sodium 2- SO 4 = sulfate Name = sodium sulfate Naming with polyatomic ions - example 2 Cu 3 (PO 4 ) 2 Cu = copper (can be 2+ or 3+) PO 4 3- = phosphate Given the charge and numbers of each ion, copper must be 2+, hence the name is copper (II) phosphate

17 Formulas with Polyatomic ions - example 1 Aluminum hydroxide Aluminum = Al 3+ Hydroxide = OH - You need 1 Al 3+ for every 3 OH -, hence the formula is Al(OH) 3 Formulas with Polyatomic ions - example 2 Iron (II) sulfite Iron (II) = Fe Sulfite = SO 3 You need 1 Fe 2+ for every 1 SO 2-3, so the formula will be FeSO 3

18 Naming and writing formulas - general tips and rules See Table 3.5, p. 124 for naming and formulas for molecular and ionic compounds For all ionic compounds, remember that the net charge of the formula is zero and the formula is the lowest possible ratio for the two ions For molecular compounds, prefixes are needed to denote the numbers of each element Science 1206 Ch. 3 - Chemical names, formulas and equations Chemical Equations and the Law of Conservation of Mass (pp )

19 Chemical reactions chemical reaction - a process in which pure substances undergo a change, forming different pure substances reactant - an element or compound that undergoes a chemical reaction (the starting materials) product - an element or compound that is formed in a chemical reaction (what gets made) In chemical equations, reactants are written on the left and products are written on the right. Word equations A word equation is a representation of a chemical reaction using the names of the chemicals plus symbols to denote what is being reacted and what is being produced + Means reacted with or and means to produce yields In word equations and chemical equations, reactants are written on the left and products are written on the right.

20 Word equations - examples Water decomposes into its elements, hydrogen and oxygen Water hydrogen + oxygen Sodium chloride reacts with calcium nitrate to produce sodium nitrate and calcium chloride Sodium chloride + calcium nitrate sodium nitrate + calcium chloride Conservation of mass in chemical reactions The Law of Conservation of Mass In a chemical reaction, the total mass of the products is always the same as the total mass of the reactants. This idea was first shown by Antoine Lavoisier in the late 1700s when he demonstrated that in a closed system (one where substances can t escape) the mass of the starting substances is the same as the ending substances.

21 Conservation of mass and conservation of atoms Lavoisier s work allowed John Dalton to re-introduce the idea of atoms to the world in the early 1800s. Since atoms make up each reactant and product, Dalton s atomic theory stated that each atom in the reactants is also present in the products. Since atoms are neither created nor destroyed in chemical reactions, the mass does not change, either. Chemical equations and the law of conservation of mass chemical equation - a representation of a chemical reaction that uses chemical formulas and symbols Chemical equations are the symbol translations of word equations, in a sense. A skeleton equation takes the names of the chemicals in the word equation and replaces them with the correct chemical names Ex. Water hydrogen and oxygen H 2 O H 2 + O 2

22 Balanced chemical equations The skeleton equation, however, is not complete. The number of atoms on each side are not equal. To remedy this, numbers called coefficients are written in front of the chemical formulas to balance the atoms on each side of the equation. As well, states of matter have to be included. Skeleton equation H 2 O H 2 + O 2 Balanced equation 2 H 2 O (l) 2 H 2 (g) + O 2 (g) (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water) Tips for balancing chemical equations - Make sure the chemical formulas for all substances in the equations are correct - Use coefficients in front of formulas to balance. DO NOT CHANGE THE FORMULAS IN THE EQUATION TO BALANCE. - Treat polyatomic ions as units when balancing (ie do not split them up).