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UMass Amherst CHEM 110 - Chemistry Chapter 4

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Chapter 4Chemical Reactions in Aqueous SolutionsKey Idea Chemical reactions involve exchange of ions, atoms and/or electrons betweenreacting compounds● Solution: A homogeneous mixture of two or more substances.● Solvent: The substance in which the dissolution takes place.● Solute: The substance being dissolved.● Aqueous solution: When a compound is dissolved in water.● Example: Salt (NaCl) solution: NaCl is the solute and water is the solvent● Solvated: When constituent ions separate and become surrounded by solventmolecules.● Dissolution of NaCl:○ The constituent ions of ionic compounds separate when dissolved in a solventand become solvated, i.e. surrounded by solvent molecules.● Electrolytes (such as NaCl) form hydrated ions and increase the electrical conductivityof water. Breaks up into ions when dissolved in water.● Nonelectrolyte: A substance (such as C6H12O6) that does not break up into ions whendissolved in a solution. Do not conduct electricity.● Strong electrolytes: Compounds that dissociate 100% (completely) in water to formhydrated ions.● Weak electrolytes: Compounds that do not dissociate completely (less than 100%) inwater.● Solubility○ Not all ionic compounds are soluble in water○ Some ionic compounds do not dissolve in water and result in precipitates○ Solubility rules allow us to predict the solubility of ionic compounds in water○ Soluble compounds are defined as those that dissolve to the extent of 1 g ormore per 100 g water● Soluble: Compounds that dissolve in water or form solutions with hydrated ions.● Insoluble: Compounds that do not dissolve to an appreciable extent.● Double displacement or exchange (metathesis) reaction: A reaction where thereactants swap partners. Happens with ionic compounds. Happens with precipitate,acid-base, and gas forming reactions.○ AB + CD = CB + AD. A and C are Cations. B and D are anions.● Reactions like this are also called precipitation reactions If one or both of the products(CB and AD) are precipitates (insoluble).● Precipitate: A solid that is insoluble forms due to the mixing of 2 solutions.● Precipitation reaction: When 2 ionic compound solutions mix and a solid forms. Thesolid product is an insoluble ionic compound, while the other products and reactants aresoluble ionic compounds.● Complete Ionic Equation: Every single ion from the reactants and products are takeninto account.● Spectator ions: Ions that do not react and stay in the solution.● Net Ionic Equation: Shows only ions which participate in the reaction. When spectatorions are omitted from the equation, we are left with NIE.● Acid: A species which generates H+ ions (called protons) when dissolved in water.Donates H+ in solution, Taste sour, Sting your skin, Found in drinks● Strong Acid: Ionizes 100% in water/aqueous solutions, is a strong electrolyte.● Weak Acid: Ionizes less than 100% in water/aqueous solutions, is a weak electrolyte.● Monoprotic acid: Acid that only produces 1 mole H+ per mole of acid.● Diprotic acid: Acid that produces 2 mol H+ per mole of acid.● Base: A species which generates - OH ions (called hydroxide) when dissolved in water.Donates OH- in solution, react with H+ and are proton acceptors. Taste bitter, Feelslippery, Found in cleaners● Strong bases: ionize 100% in aqueous solutions, and is a strong electrolyte.● Weak bases: ionize less than 100% in aqueous solutions, and is a weak electrolyte.● Acid-Base (Neutralization) Reactions:○ Reactions between acids and bases are called neutralization reactions○ They always result in a salt (cation + anion) and water (H2O).● Mixing equimolar solutions of strong acid and strong base will always have NIE.● Oxidation-Reduction (Redox) Reactions○ Redox reactions involve the transfer of electrons (e- ) from one reactant toanother○ Oxidation is the loss of one or more electrons from a chemical species○ Reduction is the gain of one or more electrons by a chemical species○ LEO says GER■ Lose electron = oxidized■ Gain electrons = reduction○ In Redox reactions:■ Elements that get oxidized are called reducing agents or reductants.Reducing agents have low oxidation states.■ Elements that get reduced are called oxidizing agents or oxidants.Oxidizing agents have higher oxidation states.● Oxidation Numbers or Oxidation States○ Oxidation number/oxidation state of an atom, represents the number of electronsthat have been gained or lost○ If a neutral atom gains electrons, its oxidation number becomes more negative○ If a neutral atom loses electrons, its oxidation number becomes more positive○ Oxidation numbers are primarily used to identify the transfer of electrons in aRedox reaction and are not always representative of the actual charge carried byan atom in a compound● Rules to Assign Oxidation Numbers○ Rule 1:■ Each atom in a pure element has an oxidation number of zero.● Example: Fe in Fe(s) oxidation number = 0● Each O in O2(g) oxidation number = 0○ Rule 2: A monoatomic ion has an oxidation number equal to the ion charge.■ Example: Cl in Cl- oxidation number = -1■ Mg in Mg2+ oxidation number = +2○ Rule 3:■ In compounds, halogens (F, Cl, Br, I) have an oxidation number of −1.Exception: When halogens are combined with oxygen or fluorine, theoxidation number of Cl, Br, and I is not −1● Example: Each F in CF4 oxidation number = -1● Cl in ClF3 oxidation number = +3○ Rule 4:■ In compounds, oxygen has an oxidation number of −2. Exception: Incompounds containing the peroxide ion (O2 2−), oxygen has an oxidationnumber of −1● Example: Each O in CO2 oxidation number = -2● Each O in H2O2 oxidation number = -1○ Rule 5:■ When combined with nonmetals, hydrogen is assigned an oxidationnumber of +1. With metals, hydrogen has an oxidation number of −1● Example: Each H in CH4 oxidation number = +1● H in LiH oxidation number = -1○ Rule 6:■ For a neutral compound, the sum of all of the oxidation numbers is equalto zero. The sum of the oxidation numbers for all atoms in an ion is equalto the overall charge on the ion● Ex: CO2 (C oxidation number) + 2 × (O oxidation number) = 0○ C oxidation number = +4○ O oxidation number = -4● ClO4 - (Cl oxidation number) + 4 × (O oxidation number) = −1○ Cl oxidation number = +7○ O oxidation number = -8● Oxidation number Example1Atoms in their elemental state0Fe, H2, O22Monatomic ions= chargeF−, Fe3+● IN


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