Notes on Chemical Equations and Reactions (Chap. 8) Describing Chemical Reactions Chemical reactions involve the rearrangement of atoms (although the atoms are often incorporated within molecules of ions). Chemical equations summarize chemical reactions, using chemical formulas to represent the reactants and products, rather than full names. All the atoms that appear on one side of a balanced equation must also appear on the other side. Chemical reactions, in other words, adhere to the Law of Conservation of Mass (matter). Real chemical reactions are more complex than the neat summaries depicted in equations. Some reactants usually remain unreacted, some side reactions generate additional minor products, and most reactions proceed through a variety of chemical intermediates which are usually not included in the equations. The main factors influencing reaction rates are temperature, concentration, and the presence of catalysts. Chemical equations can be used to calculate the specific quantities of reactants and products involved in the reactions, either as masses or as moles. The general term for the process of performing such calculations is stoichiometry. Indications of a Chemical Reaction 1. Evolution of heat and light (energy). 2. Production of a gas. 3. Formation of a precipitate. 4. Change in an intensive property, such as color or odor, not caused by mixing. Characteristics of Chemical Equations 1. The equation must represent known facts. 2. The equation must contain the correct formulas for the reactants and products. 3. The law of conservation of mass must be satisfied (equation must be balanced). Vocabulary of Equations (also known as reactions) REACTANTS - the substances that exist before a chemical change (or reaction) takes place. PRODUCTS - the new substance(s) that are formed during the chemical changes. WORD EQUATION - describes a chemical change using the names of the reactants and products. FORMULA EQUATION - indicates the reactants and products of a reaction using formulas rather than names. CHEMICAL EQUATION - indicates the formulas and amounts of reactants and products of a reaction. PRECIPITATE - a solid substance formed by a physical or chemical change in a liquid (or even gaseous) medium. (Abbreviated: ppt) Note that a precipitate must be a product. COEFFICIENTS: the numbers in front of the formulas of substances in chemical equations which tell the relative number of molecules or formula units taking part in a chemical reaction. Sample Problem 1: Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. Answer: methane + oxygen → carbon dioxide + water Sample Problem 2: Write the following chemical equation as a word equation: 2 NaI + Cl2 2 NaCl + I2 Answer: sodium iodide + chlorine → sodium chloride + iodine Balancing Chemical Equations BALANCED EQUATION - one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product. All correctly written chemical equations are balanced. To balance a reaction (turning a formula equation into a chemical equation), coefficients are added (or changed) but compound subscripts are NOT changed. Sample Problem 3: Determine whether the following equation is balanced. All the formulas are written correctly. 2 Na + H2O 2 NaOH + H2 Answer: This equation is NOT balanced – on the reactant side of the equation, there are 2 H atoms but on the product side of the equation, there are 4 H atoms (2 in the NaOH and 2 in the H2). Steps for Balancing Chemical Equations Step 1: Write a word equation for the reaction Step 2: Write the correct formulas for all reactants and products. Step 3: Determine coefficients that make the equation balance. Sample Problem 4: Write a chemical (balanced) equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. Answer: Cl2 + 2 NaBr → Br2 + 2 NaCl Sample Problem 5: Write the balanced equation for the reaction between solutions of aluminum sulfate and calcium chloride and a white precipitate of calcium sulfate. Answer: Al2(SO4)3 (aq) + 3 CaCl2 (aq) → 3 CaSO4 (s) + 2 AlCl3 (aq) Symbols and Showing Phases in Chemical Equations SOLID PHASE (s) - pure substance is relatively rigid and has a definite volume and shape. NaCl (s) LIQUID PHASE (ℓ) - pure substance has a definite volume, but is able to change shape by flowing. H2O (ℓ) GASEOUS PHASE (g) - pure substance has no definite volume or shape, and it shows little response to gravity. Cl2 (g) AQUEOUS SOLUTION (aq) - homogeneous mixture of the substance dissolved in water. AlCl3 (aq) Additional symbols used in chemical equations are given in your textbook in Table 8-2 on pg 246. Catalyst - a substance that speeds up a chemical reaction without itself being permanently altered. Therefore calalysts are NOT written as part of the equation, but are written above the reaction arrow. A catalyst functions by lowering the activation energy, EA, of the reaction. TYPES OF CHEMICAL REACTIONS One of the several schemes used to classify reactions by the type of substances that react and are produced. The scheme that follows is based on whether these substances are compounds or elements. 1. SYNTHESIS (also called direct combination or composition) Two or more substances combine to produce a single, more-complex substance. A + B ----------> AB element or cmpd element or cmpd. compound (cmpd) 2. DECOMPOSITION (also called analysis) A single substance is broken down into two or more simpler substances. AB ----------> A + B compound element or cmpd. element or cmpd. 3. SINGLE REPLACEMENT (also called single displacement) A single replacement reaction is one in which a free element becomes an ion in a compound and an ion in a compunds becomes a neutral element. A + BC ----------> B + AC element compound element compound 4. DOUBLE REPLACEMENT (also called double displacement or Partner Exchange of Ions) In a double replacement reaction, the cation of one compound replaces the cation in the other compound to produce two new compounds. For a double replacement reaction to actually occur, one of the products must be "removed" from the reaction. Generally this "removal" occurs when the product is in a different state from the reactants. The most common double replacement reactions take place between two ionic compounds that are dissolved in water, forming a precipitate. AB + CD ----------> AD + CB compound compound compound compound 5. COMBUSTION C xH y + O2 (g) ----------> CO2 (g) + H2O (g) hydrocarbon cmpd oxygen gas carbon dioxide water vapor 6. OXIDATION-REDUCTION (also called REDOX) Unlike the above reaction types, redox reactions cannot readily be identified by the type of substances involved in the reaction. Instead, they are identified by the fact that some of the elements have a different oxidation number on the reactant side of the equation from the oxidation number for the same element on the product side of the equation. This change in the oxidation number is caused by a transfer of electrons that occurs during the reaction. Redox reactions often are also synthesis or single-replacement reactions. All redox reactions are comosed of two half-reactions: the oxidation half-reaction and the reduction half- reaction. +2 - OXIDATION: element's oxidation number is increased due to a loss of electrons e.g. Fe → Fe + 2e - - REDUCTION: element's oxidation number is decreased due to a gain of electrons e.g. Cl2 + 2 e → 2 Cl An example of a redox reaction is Fe + Cl2 → FeCl2. This reaction can also be classified as a synthesis reaction. An acronym that is commonly used to help remember the 2 half-reactions is LEO the lion says GER. LEO stands for Loss of Electron is Oxidation; GER stands for Gain of Electrons is Reduction. ACTIVITY SERIES OF THE ELEMENTS The Activity Series of the elements, especially metals, is an invaluable aid to predicting the products of single replacement reactions. It also can be used as an aid in predicting products of some other reactions. Each element on the list replaces from a compound any of the elements below it. The larger the interval between elements, the more vigorous the reaction.
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