Chemical Reactions Chapter 24

Clues that a Chemical Reaction is Occurring Evolution of heat or light Formation of a gas Formation of a precipitate Color change Some reactions have more than one clue!! Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Chemical Reactions Defined: a change in which one or more substances are converted into new substances Reactants= substances that react Products= new substances formed Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Chemical Equations are written in the following format: A + B --> C + D (reactants) (products) Free powerpoint template: www.brainybetty.com

Characteristics of Chemical Equations 1)The equation must contain the correct formulas for both the reactants and products!! Remember that some elements exist as diatomic molecules: HOFBrINCl Free powerpoint template: www.brainybetty.com

Characteristics of Chemical Equations Other exceptions- phosphorus is P4 and sulfur is S8. All other elements have no subscripts because they do not exist as molecular compounds in nature. Free powerpoint template: www.brainybetty.com

Characteristics of Chemical Equations 2) Law of conservation of mass Antoine Lavoisier, a French chemist, developed the law of conservation of mass in his work with mercury. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Law of conservation of mass states that the mass of the reactants equals the mass of the products. Lavoisier determined this by noting the increase in mass of a metal after being heated in the presence of oxygen. Free powerpoint template: www.brainybetty.com

Significance of the Law of Conservation of Mass The same number of atoms must be on each side of a balanced and correct chemical equation. To equalize or balance equations, coefficients are placed in front of a formula to specify the relative number of moles of that substance. Free powerpoint template: www.brainybetty.com

Writing Chemical Equations A chemical equation is a way to describe a chemical rxn using chemical formulas and other symbols. This shorthand method makes it easier for chemists to communicate. Free powerpoint template: www.brainybetty.com

Symbols Used in Writing Chemical Equations --> means produces + plus (s) solid (l) liquid (g) gas (aq) aqueous Heat- reactants are heated Light- reactants are exposed to light Double arrow means reversible rxn Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Unit Managers Numbers to the left of the chemical formulas are called coefficients, which represent the number of moles of each substance that are taking part in the reaction. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Balancing Equations A balanced equation has the same number of atoms on each side of the equation. Treat polyatomic ions as one unit. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Let’s do an Example HgO(s) heat--> Hg(l)+ O2 (g) How can we balance the equation? 2 Free powerpoint template: www.brainybetty.com

Choosing Coefficients Mostly a trial-and-error process, but it gets easier with practice. Let’s try another example: Iron metal plus oxygen produces iron(II) oxide Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Fe(s) + O2(g) --> FeO(s) This is the equation that you should have before it is balanced with coefficients. 2 Fe(s) + O2(g) --> 2 FeO(s) This is the equation that you should have once it is balanced. Free powerpoint template: www.brainybetty.com

Classifying Chemical Reactions 5 main categories: Synthesis Rxns Combustion Rxns Decomposition Rxns Single Replacement Rxns Double Replacement Rxns Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Synthesis Reactions Aka: composition reactions Two or more substances combine to form a new substance. Example: hydrogen burns in oxygen to form water. Free powerpoint template: www.brainybetty.com

General scheme of a synthesis reaction A + X --> AX There are a few basic types of synthesis reactions: 1) reaction of metals with oxygen to produce oxides 2 Mg(s) +O2(g) --> 2MgO(s) Free powerpoint template: www.brainybetty.com

Types of synthesis reactions 2) reactions of metals with sulfur to produce sulfides 16 Rb(s) + S8(s) --> 8Rb2S(s) Non-metals can also combine with oxygen and sulfides to form compounds. Free powerpoint template: www.brainybetty.com

Types of Synthesis Reactions 3) Reaction of metals with halogens (group 17 nonmetals) Sr(s) + Br2(l) --> SrBr2(s) 4) Synthesis reactions with oxides These oxides react with water to form metal hydroxides. CaO(s) + H2O(l) --> Ca(OH)2(s) Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Combustion Reactions Type of synthesis reaction Characterized by burning. Occurs when a substance reacts with oxygen to produce energy in the form of light and heat. Free powerpoint template: www.brainybetty.com

Example of combustion reaction Propane + oxygen --> carbon dioxide + water C3H8(g) + 5O2(g) --> 3CO2(g) + 4 H2O(g) Propane combustion!!!! Free powerpoint template: www.brainybetty.com

Decomposition Reactions One substance breaks down or decomposes into two or more substances. Many require the use of heat, light or electricity. Free powerpoint template: www.brainybetty.com

General scheme of Decomposition reactions AX --> A + X Examples: 1) Decomposition of binary compounds Electrolysis of water is an excellent example. Water is split into hydrogen and oxygen by adding an electric current. See if you can write the balanced equation for the electrolysis reaction described above! Free powerpoint template: www.brainybetty.com

Types of Decomposition reactions 1) Decomp of metal carbonates produces metal oxide and carbon dioxide gas CaCO3(s) --> CaO(s) +CO2(g) 2) Decomp of metal hydroxides (except group 1 metals) yield metal oxides and water Ca(OH)2(s) --> CaO(s) + H2O(g) Free powerpoint template: www.brainybetty.com

Types of Decomposition Reactions 3) Decomp of metal chlorates to produce a metal chloride and oxygen 4) Decomp of some acids into nonmetal oxides and water Free powerpoint template: www.brainybetty.com

Single Displacement Reactions When one element replaces another in a compound. Many of these reactions take place in solution. General scheme: A + BC --> AC + B or Y + BC --> BY + C Silver nitrate and copper metal form copper(II) nitrate and silver metal. Free powerpoint template: www.brainybetty.com

Tips for single replacement reactions When predicting products, remember that metals will always be first in a chemical formula or they will be by themselves. Also remember that nonmetals or polyatomic ions will be second in a chemical formula, NEVER first! Free powerpoint template: www.brainybetty.com

Types of single replacement rxns 1) Replacement of one metal by another 2) Replacement of hydrogen in water by a metal to produce metal hydroxides and hydrogen gas Free powerpoint template: www.brainybetty.com

Types of single replacement rxns 3) Replacement of hydrogen in an acid by a metal to produce a salt and hydrogen gas 4) Replacement of halogens Free powerpoint template: www.brainybetty.com

Double Displacement Reactions The positive ion of one compound replaces the positive ion from the other compound, forming two new compounds. Free powerpoint template: www.brainybetty.com

General Scheme of Double replacement rxns AX + BY --> AY + BX Types of Double Replacement Rxns: 1) Formation of a precipitate Free powerpoint template: www.brainybetty.com

More double replacement rxns 2) Formation of a gas 3) Formation of water Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Acids These were recognized as a distinct class of chemicals due to the following properties: 1) Aqueous solutions have a sour taste. 2) Acids change the color of acid-base indicators, like litmus paper or pH paper. 3) Some acids react with metals to form hydrogen gas. 4) Acids react with bases to produce salts and water. 5) Some acids conduct electric current. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Bases These are the properties of bases: 1) Aqueous solutions of bases taste bitter. 2) Bases change the color of acid-base indicators. 3) Dilute aqueous solutions of bases feel slippery. 4) Bases react with acids to produce salts and water. 5) Bases conduct electric current. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Acid-Base Reactions When strong acids and bases react, the products are ALWAYS a salt and water! An example of this is hydrochloric acid and sodium hydroxide (from double replacement reactions example). Acids and bases will be a primary focus in SPAN/AP chemistry. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Acid and Base Strength The strength of an acid or base depends on how well the ions separate in aqueous solution. The more they separate, the stronger they are. The pH scale indicates how strong the acid or base is by indicating how much hydrogen ion is in the solution. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com The pH scale This scale usually runs from 0-14. A ranking of 0-6 is acidic, 7 is neutral and 8-14 is basic. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Oxidation Review of rules for assigning oxidation numbers. 1) The oxidation number of any uncombined element is zero. ie: Zn 2) The oxidation number of a monatomic ion equals the charge on the ion… Group 1=+1 and so on… 3) Oxidation number of H is +1 in most cases, unless it is combined with a metal…then it is -1. 4) The sum of the oxidation numbers of all atoms in a neutral compound is zero. Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Oxidation Reactions Defined as reactions in which ions or atoms of an element experience an increase in oxidation state. Example: In oxidation reactions, electrons are lost so the oxidation state becomes greater (more positive). Free powerpoint template: www.brainybetty.com

Factors that Affect the Rates of Chemical Reactions 1) Temperature- an increase in temperature may cause the rate of the reaction to speed up and a decrease in temperature may cause a reaction to slow down Why? Increasing the temperature causes the kinetic energy of the particles to increase, which increases the collisions of the particles Free powerpoint template: www.brainybetty.com

Factors that Affect the Rates of Chemical Reactions 2) Surface area- the rate of reaction depends on the area of contact for each substance An increase in the surface area will increase the rate of heterogeneous reactions- which are reactions that involve reactants in two different phases. ie: solids and liquids Free powerpoint template: www.brainybetty.com

Free powerpoint template: www.brainybetty.com Reduction Reactions Defined as reactions in which the ions or atoms of an element experience a decrease in oxidation state. Example: In reduction reactions, electrons are gained so the oxidation number decreases (becomes more negative). Free powerpoint template: www.brainybetty.com