How Can We Describe Chemical Reactions?. Chemistry Unit 9. Main Ideas. Chemical reactions are represented by balanced chemical equations. There are four main types of chemical reactions: synthesis, combustion, decomposition, and replacement reactions.
How Can We Describe Chemical Reactions? Chemistry Unit 9
Main Ideas • Chemical reactions are represented by balanced chemical equations. • There are four main types of chemical reactions: synthesis, combustion, decomposition, and replacement reactions. • Double-replacement reactions occur between substances in aqueous solutions and produce precipitates, water or gases.
Objectives • Recognize evidence of chemical change. • Represent chemical reactions with equations • Balance chemical equations.
Chemical Reactions The process by which one or more substances are rearranged to form different substances is called a chemical reaction. • Evidence of a chemical reaction is a chemical change: a process of changing one or more substances into a new substance. • Evidence: temperature changes, color changes, odor, gas bubbles or appearance of a precipitate
Representing Chemical Reactions Chemical equations – statements that show chemical reactions by the use of chemical formulas and conserved matter with the relative amounts of substances in the reaction. Parts of an equation reaction: • Reactantsare the starting substances. • Productsare the substances formed in the reaction.
Representing Reactions Word Equations: use of words for reactants and products • aluminum(s) + bromine(l) → aluminum bromide(s) Skeleton Equations: chemical formulas used for reactants and products but not balanced • Al(s) + Br(l) → AlBr3(s) • Skeleton equations lack information about how many atoms are involved in the reaction.
Representing Reactions Chemical Equation: Is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction. • 2Al(s) +3Br22AlBr3(s)
Balancing Chemical Equations A coefficientin a chemical equation is the number written in front of a reactant or product, describing the lowest whole-number ratio of the amounts of all the reactants and products. • The most fundamental law is the law of conservation of mass; a balanced equation shows this law.
Balancing Chemical Equations • Write the skeleton equation: • Make sure chemical formulas are correct. • Put in symbols and physical states. liquid sodium carbonate + aqueous calcium chloride yields solid calcium carbonate + aqueous sodium chloride
Balancing Chemical Equations • Count the atoms of the elements in the reactants • Group intact polyatomic ions as a single substance. Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations • Count the atoms of the elements in the products • Group intact polyatomic ions as a single substance. Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations • Change the coefficients to make the number of atoms of each element equatl on both sides of the equation • Never change subscripts Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations • Write the coefficients in their lowest possible ratios Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations • Go back and check math. Na2CO3(l) + CaCl2(aq) CaCO3(s) + 2 NaCl(aq)
Example aqueous sodium hydroxide + aqueous calcium bromide yields solid calcium hydroxide and aqueous sodium bromide 2112
Question 1 Which of the following is NOT a chemical reaction? A.a piece of wood burning B.a car rusting C.an ice cube melting into water D.red litmus paper turning blue
Question 1 What is the coefficient of bromine in the equation 2Al(s) + 3Br2(l) → 2AlBr3(s)? A.1 B.2 C.3 D.6
Practice Problems • Page 287 #4-6; page 288 #7-13
Objectives • Classify chemical reactions. • Identify the characteristics of different classes of chemical reactions.
Types of Chemical Reactions Chemists classify reactions in order to organize the many types. • Synthesis • Combustion • Decomposition • Single Replacement • Double Replacement (Metathesis)
Synthesis A synthesis reaction is a reaction in which two or more substances react to produce a single product. • When two elements react, the reaction is always a synthesis reaction.
Synthesis A synthesis reaction is a reaction in which two or more substances react to produce a single product. • When two compounds react: • AB + CD ABCD • AB + BC ABC
Combustion In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. • Example: Heated hydrogen reacts with oxygen to produce heat and water in a combustion reaction. This is also a synthesis reaction.
Combustion In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. • Element and oxygen react: A + O2 AO • Compound and oxygen react: AB + O2 AO + B
Decomposition A decomposition reactionis one in which a single compound breaks down into two or more elements or new compounds. • Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur.
Decomposition A decomposition reactionis one in which a single compound breaks down into two or more elements or new compounds. • Compound breaks down into two elements: AB A + B • Compound breaks down to form new compounds: ABCD AC + BD
Replacement/Displacement A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction. • A + BX → AX + B
Activity Series • A metal will not always replace a metal in a compound dissolved in water because of differing reactivities. • An activity series can be used to predict if reactions will occur.
Activity Series • Halogens frequently replace other halogens in replacement reactions. • Halogens also have different reactivities and do not always replace each other.
Activity Series • Metals/Halogens are listed in order of reactivity . A less reactive metal/halogen will not replace a more reactive metal/halogen
Practice Problems • Page 291 #14-17; page 292 #18-20 • Page 295 #21-24
Double Replacement Double replacement reactions (also called metathesis) occur when ions exchange between two compounds.
Double Replacement • Metathesis reactions often form one of three products: • The solid product produced during a chemical reaction in a solution is called a precipitate. • water – is usually formed with the combination of an acid and a base. A metal salt is also formed. • gas – formed when a gas is not a reactant.
Question 1 Which of the following is NOT one of the four types of reactions? A.deconstructive B.synthesis C.single replacement D.double replacement
Question 1 The following equation is what type of reaction? KCN(aq) + HBr(aq) → KBr(aq) + HCN(g) A.deconstructive B.synthesis C.single replacement D.double replacement
Practice Problems • Page 297 #25-28; Page 298 #29-34
Objectives • Identify new possible ionic compounds in a reaction • Define the terms soluble and insoluble • Predict solids based on solubility rules.
Ionic Compounds in Solutions Ionic compounds in aqueous solutions mix and exchange partners (double replacement). • example: Na2SO4(aq) + CaCl2(aq) • Some of these products are solids and some of these products remain aqueous.
Solubility Solubility rules are used to determine the state of matter of products in an aqueous solution. • Soluble means that the compound dissolves in water. • Insoluble means that the compound remains intact in the solid state in water.
Solubility Rules • Most nitrate (NO3-) salts are soluble • Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are soluble. • Most chloride, bromide, and iodide salts are soluble Exceptions: Ag+, Pb2+, Hg22+
Solubility Rules • Most sulfate salts are soluble . • Exceptions: Bas+, Pb2+, Hg22+, and Ca2+ • Most hydroxides are only slightly soluble (treat as insoluble). • Exceptions: Na+, K+ • Most sulfide (S2-), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) salts are only slightly soluble (treat as insoluble). • Exceptions: any containing Alkali metals and ammonium.
Solubility Summary Soluble Insoluble no exceptions no exceptions Ag+, Pb2+, Hg22+ Bas+, Pb2+, Hg22+, Ca2+ Hydroxides (OH-) S2-, CO32-, CrO42- PO43- • Nitrates (NO3-) • Group 1, NH4+ • Halogens • Sulfates (SO42-)