Chemistry Chapter 6.1 and 6.2 PDF

# 6.1 Evidence for a Chemical Reaction ## Objective To learn the signals that show a chemical reaction has occurred. ## How to Determine if a Chemical Reaction Has Occured - How do we know when a chemical reaction has occurred? - What are the clues that a chemical change has taken place? - A gl...

# 6.1 Evidence for a Chemical Reaction ## Objective To learn the signals that show a chemical reaction has occurred. ## How to Determine if a Chemical Reaction Has Occured - How do we know when a chemical reaction has occurred? - What are the clues that a chemical change has taken place? - A glance back at the processes in the introduction suggests that chemical reactions often give a visual signal. - Steel changes from a smooth, shiny material to a reddish-brown, flaky substance when it rusts. - Hair changes color when it is bleached. - Solid nylon is formed when two particular liquid solutions are brought into contact. - A blue flame appears when natural gas reacts with oxygen. - Chemical reactions, then, often give visual clues: - a color changes, - a solid forms, - bubbles are produced, - a flame occurs, and so on. - However, reactions are not always visible. - Sometimes the only signal that a reaction is occurring is a change in temperature as heat is produced or absorbed. ## Figure 6.1 Bubbles of hydrogen and oxygen gas form when an electric current is used to decompose water. ## Figure 6.2 - An injured girl using an ice pack to prevent swelling. - The pack is activated by breaking an ampule; this initiates a chemical reaction that absorbs heat rapidly, lowering the temperature of the area to which the pack is applied. - A hot pack used to warm hands and feet in winter. - When the package is opened, oxygen from the air penetrates a bag containing solid chemicals. - The resulting reaction produces heat for several hours. ## Table 6.1 Some Clues That a Chemical Reaction Has Occurred 1. The color changes. 2. A solid forms. 3. Bubbles form. 4. Heat and/or a flame is produced, or heat is absorbed. ## Figure 6.3 - When colorless hydrochloric acid is added to a red solution of cobalt (II) nitrate, the solution turns blue, a sign that a chemical reaction has taken place. - A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate. - Bubbles of hydrogen gas form when calcium metal reacts with water. - Methane gas reacts with oxygen to produce a flame in a Bunsen burner. # 6.2 Chemical Equations ## Objective To learn to identify the characteristics of a chemical reaction and the information given by a chemical equation. ## What are Chemical Equations? - Chemists have learned that a chemical change always involves a rearrangement of the ways in which the atoms are grouped. - For example, when the methane, CH4, in natural gas combines with oxygen, O2, in the air and burns, carbon dioxide, CO2, and water, H2O, are formed. - A chemical change such as this is called a chemical reaction. - We represent a chemical reaction by writing a chemical equation in which the chemicals present before the reaction (the reactants) are shown to the left of an arrow and the chemicals formed by the reaction (the products) are shown to the right of an arrow. - The arrow indicates the direction of the change and is read as "yields" or "produces". ## Reactants → Products For the reaction of methane with oxygen, we have: - Methane + Oxygen → Carbon dioxide + Water - CH4 + O2 → CO2 + H2O - Reactants → Products ## Important Note - Note from this equation that the products contain the same atoms as the reactants but that the atoms are associated in different ways. - That is, a chemical reaction involves changing the ways the atoms are grouped. ## Chemical Reactions Don't Create or Destroy Atoms - It is important to recognize that in a chemical reaction, atoms are neither created nor destroyed. - All atoms present in the reactants must be accounted for among the products. - In other words, there must be the same number of each type of atom on the product side as on the reactant side of the arrow. - Making sure that the equation for a reaction obeys this rule is called **balancing the chemical equation** for a reaction. ## Balancing the Chemical Equation - The equation that we have shown for the reaction between CH4 and O2 is not balanced. - We can see that it is not balanced by taking the reactants and products apart. ## Example: Balancing the Chemical Equation for Methane and Oxygen - CH4 + O2 → CO2 + H2O - 1C 4H 2O → 1C 2O 2H 1O - Totals: 1 C 4H 2O → 1C 2H 3O - The reaction cannot happen this way because, as it stands, this equation states that one oxygen atom is created and two hydrogen atoms are destroyed. - A reaction is only a rearrangement of the way the atoms are grouped; atoms are not created or destroyed. - The total number of each type of atom must be the same on both sides of the arrow. - We can fix the imbalance in this equation by involving one more O2 molecule on the left and by showing the production of one more H2O molecule on the right. - CH4 + O2 + O2 → CO2 + H2O + H2O - 1C 4H 4O → 1C 2O 4H 2O - Totals: 1C 4H 4O → 1C 4H 4O - This balanced chemical equation shows the actual numbers of molecules involved in this reaction. ## Figure 6.4 - The reaction between methane and oxygen to give water and carbon dioxide. - Note that there are four oxygen atoms in the products and in the reactants; none has been gained or lost in the reaction. - Similarly, there are four hydrogen atoms and one carbon atom in the reactants and in the products. - The reaction simply changes the way the atoms are grouped. ## Writing Balanced Equations - When we write the balanced equation for a reaction, we group like molecules together. - Thus CH4 + O2 + O2 → CO2 + H2O + H2O is written CH4 + 2O2 → CO2 + 2H2O. ## The Chemical Equation Shows Two Important Things - The chemical equation for a reaction provides us with two important types of information: 1. The identities of the reactants and products 2. The relative numbers of each # Physical States ## Symbols for States - Besides specifying the compounds involved in the reaction, we often indicate in the equation the physical states of the reactants and products by using the following symbols: - (s) = solid - (l) = liquid - (g) = gas - (aq) = dissolved in water (in aqueous solution) ## Example: Potassium Reacting with Water - For example, when solid potassium reacts with liquid water, the products are hydrogen gas and potassium hydroxide; the latter remains dissolved in the water. - From this information about the reactants and products, we can write the equation for the reaction. - Solid potassium is represented by K(s); - liquid water is written as H2O(l); - hydrogen gas contains diatomic molecules and is represented as H2(g); - potassium hydroxide dissolved in water is written as KOH(aq). - So the unbalanced equation for the reaction is - Solid potassium + Water → Hydrogen gas + Potassium hydroxide dissolved in water - K(s) + H2O(l) → H2(g) + KOH(aq) ## Figure 6.5 - The reactant potassium metal (stored in mineral oil to prevent oxidation). - The reactant water. - The reaction of potassium with water. - The flame occurs because hydrogen gas, H2(g), produced by the reaction burns in air [reacts with O2(g)] at the high temperatures caused by the reaction. ## Second Reaction - The hydrogen gas produced in this reaction then reacts with the oxygen gas in the air, producing gaseous water and a flame. - The unbalanced equation for this second reaction is - H2 (g) + O2(g) → H2O(g) ## Example 6.1 Chemical Equations: Recognizing Reactants and Products - Write the unbalanced chemical equation for each of the following reactions. 1. Solid mercury(II) oxide decomposes to produce liquid mercury metal and gaseous oxygen. 2. Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide. 3. Solid zinc is added to an aqueous solution containing dissolved hydrogen chloride to produce gaseous hydrogen that bubbles out of the solution and zinc chloride that remains dissolved in the water. ## Solution 1. **Mercury(II) oxide decomposing:** In this case we have only one reactant, mercury(II) oxide. The name mercury(II) oxide means that the Hg2+ cation is present, so one O2- ion is required for a zero net charge. Thus the formula is HgO, which is written HgO(s) in this case because it is given as a solid. The products are liquid mercury, written Hg(l), and gaseous oxygen, written O2(g). (Remember that oxygen exists as a diatomic molecule under normal conditions.) The unbalanced equation is: HgO(s) → Hg(l) + O2 (g) 2. **Solid carbon reacting with gaseous oxygen:** In this case, solid carbon, written C(s), reacts with oxygen gas, O2(g), to form gaseous carbon dioxide, which is written CO2(g). The equation (happens to be balanced). C(s) + O2(g) → CO2 (g) 3. **Solid zinc reacting with hydrochloric acid:** In this reaction solid zinc, Zn(s), is added to an aqueous solution of hydrogen chloride, which is written HCl(aq) and called hydrochloric acid. These are the reactants. The products of the reaction are gaseous hydrogen, H2(g), and aqueous zinc chloride. The name zinc chloride means that the Zn2+ ion is present, so two Cl¯ ions are needed to achieve a zero net charge. Thus zinc chloride dissolved in water is written ZnCl2 (aq). The unbalanced equation for the reaction is: Zn(s) + HCl(aq) → H2(g) + ZnCl2 (aq) ## Self-Check: Exercise 6.1 - Identify the reactants and products and write the unbalanced equation (including symbols for states) for each of the following chemical reactions. 1. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. 2. Solid ammonium dichromate (review Table 5.4 if this compound is unfamiliar) decomposes to solid chromium(III) oxide, gaseous nitrogen, and gaseous water. 3. Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water. - See Problems 6.13, 6.14, 6.15, 6.16, 6.17, 6.18, 6.19, 6.20, 6.21, 6.22, 6.23, 6.24, 6.25, 6.26, 6.27, 6.28, 6.29, 6.30, 6.31, 6.32, 6.33, and 6.34.