2 Reactions and Equations Objectives 1. Recognize evidence of chemical change.2. Represent chemical reactions with equations.3. Balance chemical equations.4. Classify chemical reactions.5. Identify the characteristics of different classesof chemical reactions.6. Describe aqueous solutions.7. Write complete ionic and net ionic equationsfor chemical reactions in aqueous solutions.8. Predict whether reactions in aqueous solutionswill produce a precipitate, water, or a gas.
3 Reactions and Equations chemical reaction: process by which one or more substances are rearranged to form different substances; also called a chemical change-reactants: the starting substances-products: resulting, new substancesThe law of conservation of mass states that the total mass of reactants must equal the total mass of the products.
4 Evidence of Chemical Reactions While some reactions are hard to detect, most provide evidence they have occurred.1. temperature change-exothermic-heat released-endothermic-heat absorbed2. color change-by itself it doesn’t necessarily indicate a chemicalchange since it is also an indication of a physicalchange
5 -light, sound, electricity production of a gas-may see bubbling-may also produce new odor4. formation of a precipitate, a solid formed as aresult of a chemical reaction in solution and thatseparates from the solutionenergy change-light, sound, electricity
6 -must always state the physical state of each reactant and product No matter the method of representation, there is a universal set of symbols all scientists use.-must always state the physical state of each reactant and product-arrow always points to the products; most often reactants are written on the left, products on the rightSymbolMeaning+plus; separates 2or more reactantsor products→produces, yields,or forms(s)solid(l)liquid(g)gas(aq)aqueous; dissolvedin water
7 Chemical Equationschemical equation: uses chemical formulas to show the identities and relative amounts of reactants and products in a chemical reaction.-must show mass being conserved# of reactant atoms = # of product atoms-accomplished by balancing equationsChemical equations are the representation chemists use to describe chemical reactions.
8 Balancing Chemical Equations coefficient: number written in front of a reactant or product that states the ratio of amounts for each substance-usually a whole number-number ‘1’ is assumed and not writtenSteps for balancing equations:1. Write the skeleton equation for the reaction2. Count the atoms of elements in the reactants/products3. Place coefficients in front of each substance; change until the equation is balanced4. Reduce coefficients to smallest possible ratio5. Check your work.
9 Note: Never change the subscript in the formula because that changes the identity of the substance. Example: Hydrogen chloride is formed during thereaction between hydrogen and chlorine.1. Write the skeleton equation for the reaction.hydrogen + chlorine hydrogen chlorideH2 (g) Cl2 (g) HCl (g)
10 2. Count the atoms of elements in the reactants/products ___H2 (g) + ___Cl2 (g) ___HCl (g)HCl3. Place coefficients in front of each substance; change until the equation is balanced4. Reduce coefficients to smallest possible ratioH2 (g) + Cl2 (g) 2HCl (g)5. Check your work.
12 Word EquationsWord equations use words to represent chemical reactions.Skeleton equation uses chemical formulas to identify the reactants and products.Chemical equations show conservation of mass by placing coefficients in front of each substance to balance it.“Iron and chlorine react to produce iron (III) chloride”word: iron (s) + chlorine (g) iron (III) chloride (s)skeleton: Fe (s) Cl2 (g) FeCl3 (s)chemical: 2Fe (s) Cl2 (g) 2FeCl3 (s)
13 Word Equations Know ~8 diatomics: H2, N2, O2, F2, Cl2, Br2, I2, At2 (red are gases, blue is a liquid and black is a solid)~nonmetal oxides tend to be gases (CO, SO2, …)~ionic compounds tend to be solids (watch for aq solution)~be careful with solubility rules in aq solution~pure metals are solids (except Hg)
14 Chemical Equations Practice 1 Write chemical equations and balance each:1. In water, iron (III) chloride reacts with sodium hydroxide, producing solid iron (III) hydroxide and sodium chloride.
15 Chemical Equations Practice 1 Write chemical equations and balance each:2. Liquid carbon disulfide reacts with oxygen, producing carbon dioxide and sulfur dioxide.
16 Chemical Equations Practice 1 Write chemical equations and balance each:3. Solid zinc and aqueous hydrogen sulfate react to produce hydrogen gas and aqueous zinc sulfate.
17 Classifying Chemical Reactions There are 4 basic types of reactions:1. synthesis reaction- 2 or more simple substances combine to form a new, more complex substance-examples: 2Na + Cl2 2NaCla. combustion reaction- a substance combines withoxygen, releasing a large amount of energy in the formof heat and light-ex: 2CH O2 2CO2 + 4H2O + energy
18 b. polymerization reaction: reaction in which monomer units (a small, simple organic molecule)are bonded together to form a polymer, a largemolecule consisting of many repeating structuralunits (monomers)-addition polymerization: all atoms present inmonomer are present in the polymer product~ex: ethene (ethylene) polyethyleneH2C=CH2 -CH2-CH2-CH2-CH2--condensation polymerization: when monomerscontaining at least 2 functional groups combinewith the loss of a small by-product, usually water
19 2. decomposition reaction - a complex substance breaks down into at least 2 or more simpler substances-opposite of synthesis reactions-many require energy to occur (endothermic)-examples:a) H2CO3 H2O + CO2b) electrolysis: decomposition by electric current
20 3. single-displacement reaction - an uncombined element replaces a similar element that is part of a compound-more reactive element displaces the less reactive one(use activity series; most metals replace H)-example: 2Na + 2H20 2NaOH + H2
21 4. double-displacement reaction- different atoms (usually ions) in 2 different compounds exchange places to form new compounds-one of the compounds formed is usually aprecipitate, gas, or liquid~use solubility rules-example: MgCO3 + 2HCl MgCl2 + H2CO3Classify each of the reactions (1-5) from Equations Practice 1.Using this information, you should be able to predict products when given reactants.
22 Reactions in Aqueous Solutions Some aqueous solutions contain molecules (such as sugar, ethanol).Others contain ionic compounds (or acids) that break apart, or dissociate, in water to form ions.-when two aqueous solutions that contain ions react,a double displacement reaction occurs-strong electrolytes ionize in water: strong acids,strong bases, soluble salts
23 Aqueous Solutions You need to memorize: -strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3-strong bases: all group IA hydroxides; group IIA hydroxides that include Ba, Sr, CaKnow:-gases, pure liquids, solids are non-electrolytes-H2CO3 decomposes into H2O & CO2-NH4OH deomposes into H2O & NH3Remember:-the products of a double displacement reaction is either a precipitate, liquid or gas.-Use your solubility rules to determine if a precipitate occurs.
24 Aqueous SolutionsChemists use ionic equations to show the driving force of a chemical reaction.-complete ionic equation: shows all particles in asolution-net ionic equation: shows only the particles thatparticipate in the reaction (forms a solid, liquid, gas)-spectator ions: ions that do not participate; theyjust ‘watch’
25 Net Ionic Equations Example: 2NaOH (aq) + CuCl2(aq) 2NaCl (aq) + Cu(OH)2 (s)2Na+(aq) + 2OH-(aq) + Cu+2(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH)2 (s)(notice the solid precipitate is not split up into its ions-this istrue for gases and liquids produced as well)2OH-(aq) + Cu+2(aq) Cu(OH)2 (s)Na+ and Cl- are the spectator ions since they are not part of the formation of Cu(OH)2 (s)
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