All Reactants All Products

Transcription

1 6.1: Describing Chemical Reactions page 225 Chemists use word equations to describe a wide variety of chemical reactions, simple and complicated, that occur everyday. In a chemical reaction, one or more substances change into different substances. One of the most familiar types of reactions is a combustion reaction. Chemists have developed a set of rules that enable people around the world to communicate and share information about chemical reactions. Describing Chemical Reactions with Equations Word Equations is one way of representing a chemical reaction: it tells what reacts and what is produced. Writing Word Equations Word equations follow a specific format. Reactants are found on the left side and products are found on the right side. An arrow ( ) is used to give direction, from reactants to products. All Reactants All Products Plus (+) signs are used to identify the reactants being combined and the products being produced. Reactant #1 + Reactant #2 Product #1 + Product #2

2 Word Equations for some Chemical Reactions Rusting iron is a slow process. The reactants are iron and oxygen, and the product is iron (III) oxide. Word Equations: iron + oxygen iron (III) oxide When copper is placed in a silver nitrate solution, the products would be silver and copper (II) nitrate. Word Equation: copper + silver nitrate silver + copper nitrate When zinc is added to hydrochloric acid, hydrogen gas and zinc chloride are produced. Word Equation: zinc + Hydrochloric acid hydrogen gas + zinc chloride Chemical reaction can either absorb or release energy. Word Equation: Iron + sulfur iron (III)sulfide + energy In this reaction, more energy is released than absorbed. The word energy is therefore written on the right side of the equation, with the products. If energy is absorbed then it is written on the left side, with the reactants.

3 Chemical Equations Chemical reactions can be communicated as a word equation or chemical equation. The chemical equation provides more detail: it gives the chemical formulas of the reactants and products, as well as their state. Word Equation: Table 1: common state Symbols in Chemical Equations energy + copper (II) carbonate carbon dioxide + copper (II) oxide Chemical Equation: Energy + CuCO 3 (s) CO 2 (g) + CuO (s) Check Your Learning: questions 1 9, pg. 227

4 Try This Activity: Completing Word Equations 1. aluminum + aluminum oxide 2. zinc + oxygen 3. aluminum + copper (II) chloride copper + 4. sodium sulfate + calcium chloride calcium sulfate +

5 6.3: Conserving Mass in Chemical Reactions page 230 Investigation 6.2: Is Mass Gained or Lost During a Chemical Reaction?, we see two solutions react to form a solid precipitate. The mass of the reactants is equal to the mass of the products. There was no mass gained or lost. Does this hold true for all reactions or are there reactions that may gain or lose their mass? During a combustion reaction, like the burning of wood, it appears the wood is changed to a pile of ash and mass is lost. Is mass truly lost or is there something else produced? During this reaction, gases are used as reactants and formed as products. To determine whether mass is lost, scientist must be able to measure the mass of these gases. After years of experimenting, scientists have determined that mass is neither gained nor lost in any chemical reaction. This is stated in the Law of Conservation of Mass. In any given chemical reaction, the total mass of the reactants equals the total mass of the products. Scientific Law - is a general statement that sums up the conclusions of many experiments, or a statement that summarizes an observed pattern in nature.

6 The Law of Conservation of Mass - states that,in a chemical reaction, the total mass of the reactants is always equal to the total mass of the products. Since the mass of the reactant is equal to the mass of the products, the number and type of atoms in the reactants must equal the number and types of atoms of the products. (Balanced Reaction) Experiments have shown that during a chemical reaction, the atoms in the reactant molecules are rearranged to form products. A chemical equation must follow the law of conservation of mass, it must show an equal number of each kind of atom on both sides of the equation. Figure 2: there are two (red) oxygen atoms and one (black) carbon atom on either side of the scale. Similarly, there are two oxygen atoms and one carbon atom on either side of the arrow in the chemical equation. i.e., carbon + oxygen carbon dioxide C (s) + O 2 (g) CO 2 (g) The same number and kind of atoms are found on both sides.

7 Hydrogen + chlorine hydrogen chloride (hydrochloric acid) H 2 (g) + Cl 2 (g) HCl (g) The equation does not accurately describe the reaction between hydrogen and chlorine. Count the number of atoms present on both sides of the equation. An equation in which the reactants and products are not balanced is sometimes called a skeleton equation. Atoms can not vanish; therefore it is assumed that 2 molecules of HCl are created. To show the both hydrogen chloride atoms in the product, a coefficient 2 is placed before HCl (g) in the chemical equation. H 2 (g) + Cl 2 (g) 2 HCl (g) The coefficient applies to all the atoms in the molecule, 2 molecules of HCl (g). The chemical equation now follows the Law of Conservation of Mass. Figure 3: In (a) there are two (white) hydrogen atoms and two (green) chlorine atoms on the left side of the scale, but only one hydrogen atoms and one chlorine atom on the right side. In (b), there are two hydrogen atoms and two chlorine atoms on each side of the scale. Reactants and products are balanced.

8 There is one molecule of methane and two molecules of oxygen gas as the reactants. The products are two molecules of water and one molecule of carbon dioxide, plus heat and light. CH 4 + 2O 2 2H 2 O + CO 2 + energy If you count the number of each type of atom present in the reactant portion of the equation and then compare that to the number of each type of atom found in the products. You can see that they are equal. Therefore the total mass of the reactants is equal to the total mass of the products, and the atoms of the reactants have been rearranged to form products. Check Your Learning, questions 1 7, pg. 232

9 6.4: Information in Chemical Equations page 233 Skeleton Equation: is a representation of a chemical reaction in which the formulas of the reactants are connected to the formulas of the products by an arrow. Word Equation: Methane + oxygen carbon dioxide + water Skeleton Equation: CH 4 + O 2 CO 2 + H 2 O ** The law of Conservation of Mass states that the mass of the reactants equals the mass of the products. There is an apparent imbalance between the numbers of atoms in the reactants and the number of atoms in the products. We can not change the types or formulas of the molecules. So how can we solve this imbalance? To do this, you need to change the number of molecules, not the formulas. Add one oxygen molecule to the reactants, and add a water molecule to the products. The equation becomes balanced. CH 4 + O 2 + O 2 CO 2 + H 2 O + H 2 O More appropriately, using coefficients, written as: (usual way to write balanced equations is to use coefficients, which indicate the number of atoms or molecules required)

10 CH 4 + 2O 2 CO 2 + 2H 2 O How to Balance an Equation Step 1. Write the word equation for the reaction. Iron + oxygen magnetic iron oxide Step 2. Write the skeleton equation by replacing each name with a correct formula. Step 3. Fe + O 2 Fe 3 O 4 Count the number of atoms of each type in the reactants and products. **use a table like this: Type of Atom Reactants Products Fe 1 3 O 2 4 Step 4. Multiply each of the formulas by the appropriate coefficient to balance the number of atoms.

11 Type of Atom Reactants Products Fe 3(1) 3 O 2(2) 4 The balanced chemical equation is written as this; 3Fe + 2O 2 Fe 3 O 4 The formulas are unchanged, and the numbers of atoms are balanced. i.e., 2: Magnesium metal reacts with Nitric acid. Step 1. Write the word equation for the reaction. magnesium + nitric acid hydrogen + magnesium nitrate Step 2. Write the skeleton equation by replacing each name with a correct formula. Mg + HNO 3 (aq) H 2 + Mg(NO 3 ) 2 Step 3. Count the numbers of atoms of each type in the reactants and products. This example is complicated by the polyatomic nitrate ion. Type of Atom Reactants Products Mg 1 1 H 1 2 N 1 2 O 3 6

12 Step 4: Multiply each of the formulas by the appropriate coefficient to balance the numbers of atoms. Type of Atom Reactants Products Mg 1 1 H 2(1) 2 N 2(1) 2 O 2(3) 6 The equation is now balanced. Mg + 2HNO 3 (aq) H 2 + Mg(NO 3 ) 2 Check Your Learning: questions 1 8, pg. 236

13 6.5: Types of Chemical Reactions: Synthesis and Decomposition page 237 Chemist will know which elements and compounds undergo particular types of chemical reactions. Elements are classified into different chemical families. All members of a chemical family will react a similar way. (valence electrons) Compounds are also classified as ionic or molecular, and have different patterns of chemical properties. Using these patterns, chemical changes can be classified into groups. These groups are; synthesis, decomposition, single and double displacement reactions. Knowing the types of reactions allows you to: a) You can better understand experimental observations of the behaviour of substances in chemical changes. b) You can predict the products of unknown reactions. Synthesis Reactions (combination reaction) Synthesis reactions involve the combination of smaller atoms and/or molecules into larger molecules. These reactions follow this general formula: A + B AB Figure 2: In some cases, the reactants are atoms (elements), while in others, they are molecules (elements or compounds)

14 These types of reactions can also involve combinations of small molecules that form one larger molecule. i.e., hydrogen chloride + ammonia ammonium chloride HCl + NH 3 NH 4 Cl water + carbon dioxide carbonic acid CO 2 + H 2 O H 2 CO 3 (aq) Decomposition Reactions Decomposition reactions involve the splitting of a large molecule into elements of smaller molecules. These reactions follow this general formula: AB A + B Figure 3: In a decomposition reaction, a complex molecule breaks down, or decomposes, into simpler products. The products can be elements or compounds. Check Your Learning: questions 1 4, pg. 239

15 6.6: Types of Chemical Reactions: Single and Double Displacement page 240 What happens when an element and a compound combine in a chemical reaction? What happens when two ionic compounds react with each other? In each type of displacement reaction, atoms are recombined to form new compounds. How do we decide which element is displaced? Your knowledge of the periodic table and naming compounds is helpful. Metals found on the left and centre of the periodic table. Nonmetals are found on the right side of the table. The general rule is metal replaces metal and nonmetal replace nonmetal. Single Displacement Reactions Single displacement reactions are chemical changes that involve an element and a compound as reactants. One element displaces or replaces another element from a compound. Single displacement reactions have the following general formula: Z + + AB ZB + A + or Y - + AB AY + B - This reaction can be represented as the following word, skeleton, and chemical equations. magnesium + silver nitrate silver + magnesium nitrate Mg + AgNO 3 Ag + Mg(NO 3 ) 2 Mg + 2AgNO 3 2Ag + Mg(NO 3 ) 2

16 Double Displacement Reactions Double displacement reactions occur when elements in different compounds displace each other or exchange places. Double displacement reactions have the following general formula: AB + ZY AY + ZB A and X are metallic elements and B and Y are nonmetallic elements. During the reaction, B and Y (or A and X) exchange places. This reaction can be represented in the following word, skeleton, and chemical equations. lead (II) nitrate + potassium iodide lead (II) iodide + potassium nitrate Pb(NO 3 ) 2 (aq) + KI (aq) PbI 2 (s) + KNO 3 (aq) Pb(NO 3 ) 2 (aq) + 2KI (aq) PbI 2 (s) + 2KNO 3 (aq) PbI 2 (s) is a yellow precipitate. Precipitate: a solid formed from the reaction of two solutions. Through experimentation, it was determined that some ionic compounds do not dissolve in water. If insoluble compounds are formed during the reaction and become visible, it is called a precipitate. Not all double displacement reaction result in the formation of a precipitate, but most do. Check Your Learning: questions 1 8, pg. 243

17 6.9: Combustion page 248 There are different categories of chemical reactions; combustion, synthesis, decomposition, single and double displacement. Combustion is a rapid reaction of a substance with oxygen, which produces compounds called oxides, and releases energy in the form of light and heat. Complete Combustion If the oxygen available is plentiful, hydrocarbons burn completely to release the energy they contain. The only products of complete combustion are carbon dioxide and water. i.e., fuel + oxygen oxides and energy The fuels in which we commonly burn are Hydrocarbons. (gasoline, natural gas, kerosene and candles) Hydrocarbon a compound composed of hydrogen and carbon. The products from a combustion reaction are carbon dioxide and water. Millions of tons of carbon dioxide and water are released directly into the atmosphere. i.e., hydrocarbon + oxygen carbon dioxide + water

18 Word Equation: butane + oxygen carbon dioxide + water + energy Balanced Chemical Equation: 2C 4 H O 2 8CO H 2 O + energy Incomplete Combustion Incomplete combustion occurs when there is not enough oxygen available. Instead of producing carbon dioxide and water, the products released are carbon monoxide, carbon, carbon dioxide and water. Word Equation: butane + oxygen carbon monoxide + carbon + carbon + water + energy dioxide Balanced Chemical Equation: C 4 H O 2 CO (g) + C (s) + 2CO 2 (g) + 5H 2 O (g) + energy Carbon monoxide is a colourless and odourless gas and it is a very poisonous gas. Your cells die, because they are unable to obtain oxygen. Hemoglobin which normally combines to oxygen, to deliver it to your cells, prefers to combine with carbon monoxide gas instead. Check Your Learning: questions 1 7, pg. 251

19 6.10: Corrosion page 252 Corrosion: the breakdown of a metal resulting from reactions with chemicals in its environment. Beneficial Corrosion Sometimes corrosion of some metals forms a tough protective layer that prevents further corrosion. For example, 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) + energy Causes of Rust Rust Rusting is the corrosion or iron and steel that leads to the degradation and damage of the metal. Rusting is caused or accelerated by exposure to oxygen exposure to water explosure to electrolytes Preventing Corrosion Corrosion is the breakdown of a metal as it reacts with chemicals in the environment. Methods of preventing corrosion include protective coatings (paint, plastic coatings, chrome) corrosion-resistant materials (stainless steel, plastics) galvanizing Check Your Learning: questions 1 7, pg. 254 Chapter 6 Review: questions 1 18, pg