Chemical Reactions - St. Johns County School District



Chemical ReactionsWhat You Should Already KnowHow to identify and name chemical compoundsHow to identify and balance chemical reactionsIf given percent composition, determine empirical and molecular formula for the compoundIf given molecular formula, determine the empirical and percent composition of the compoundBe able to convert between molecules to moles, moles to mass, and vice versaDetermine the mole ratios of a chemical reactionThe Nature of Chemical ReactionsChemical reactions change substancesFamiliar examples: grow, ripen, decay, burn, respiration, digestion, photosynthesis…Production of gas, color change, new products, and change in temperatureChemical reactions re-arrange atomsMatter is not destroyed, just rearrangedReactants make the products. Look for the →Isooctane (C8H18) + Oxygen (O2) → CO2 + H2OThe arrow points to the reactants (or new products)For a chemical reaction to occur, the atoms/molecules must come in contact with the other substanceThus, they collide with one another causing a chemical change and formation of new productsEnergy (heat, electricity, light, chemical, and mechanical) must be added to break the old bonds so that new bonds may be created. When the new bonds form, energy is released.Energy is conserved in chemical reactionsThe total energy before the reaction equals the total energy of the products and their surroundingsReactions may be exothermic or endothermicExothermic means releasing chemical energy - Things get/feel warmerEndothermic means absorbing - Things get/feel cooler Endergonic = endothermicExergonic = exothermicChemical Reactions4 Al(s) + 3 O2(g) → 2 Al2O3(s)Reactants Yield ProductsThe subscripted letters tell the phase of matter – (aq) is aqueous which a chemical dissolved in water isCan be depicted in two ways:Word EquationsDescribes chemical change using the names of the reactants and productsChemical EquationsNumerically balanced with symbols: 4 Al(s) + 3 O2(g) → 2 Al2O3(s)The numerical values in the reactions are stoichiometric coefficients These represent moles or molecules/atoms reacting (or liters (L) for all gas reactions)The Five Basic Types of ReactionsSynthesisCombine simpler substances to make more complex onesA + B → AB 2Na + Cl2 → 2NaCl 6CO2 + 6H2O → C6H2O6 + 6O2 DecompositionBreak complex substances into simpler onesAB → A + B 2H2O → 2H2 + O2CombustionUses oxygen (O2) as a reactant, normally with a hydrocarbon2CH4 + 4O2 → 2CO2 + 4H2OSingle-DisplacementAtoms of one element take the place of another element in a compoundXA + B → BA + X 3CuCl2 +2Al → 2AlCu3 + 3CuActivity SeriesA table that shows the natural reactivity of elements/atomsDouble-DisplacementThe exchange of ions between compounds - Atoms of one element in a compound switch places with an element in another compoundAX + BY → AY + BX Pb(NO3) + K2CrO4 → PbCrO4 + 2KNO3 Acid-Base or Neutralization Reactions are a specific type of Double DisplacementAX + BY → AY + BX A = Hydrogen HY = Hydroxide OHAY + BX one is H2O and the other is an ionic compoundHCl + NaOH → NaCl + H2O RedoxElectrons are transferred in redox (reduction/oxidation) reactions“Reduction” may be the loss of an oxygen or the gain of a hydrogen “Oxidation” may be the gain of an oxygen or the loss of a hydrogenThis will be discussed in the next semester for an entire unitBalancing Chemical ReactionsFollow the Law of Conservation of MassBalancing is done with coefficients, not subscripts. Changing subscripts will change the substances CH4 + 2O2 → CO2 + 2H2OSee Below for GuideHow Reactions OccurFor a chemical reaction to occur, the atoms/molecules must come in contact with the other substanceThus, they collide into one another causing a chemical change and formation of new productsCompounds are three-dimensional objects made of tiny microscopic atomsWhen they collide, they have to hit with the correct orientation to transfer electrons, atoms or even synthesis a larger productActivation EnergySome chemical reactions require energy to start the reaction, known as activation energy (Ea)This is the energy required for a collision to be hard enough for the compounds to reactantsActivation energy has a direct influence on reaction ratesHigh Ea means collisions will not have enough energy (momentum) to reactLow Ea means more collisions will cause reactions to occur between the reactantsThe Nature of ReactantsNatural Reactivity: Some chemicals are more reactive by nature due to their chemical propertiesPressure: Increasing gas pressure causes volume to decrease putting chemicals closer to collideConcentration: Increasing the quantity of the substance in space will likely have more collisionsSurface Area: Increasing surface area will allow for more contact surface for collisionsPulverizing or grinding, like in a mortar and pestle, is a good exampleTemperature: Kinetic-Molecular Theory: Increasing kinetic energy causes more collisionsCatalysts: Lower the Ea for reactions, thus increasing the rate (Enzymes are biological catalysts)Inhibitors are those that slow down reactionsReversible ReactionsSome reactions go to completion and are done while others are reversible (? or ?)The reaction goes one way, then reverses seeking an Equilibrium or Dynamic balanced stateAgain, this will be studied in the next semesterIdentification of Chemical ReactionsFirst, know the five major chemical reactions: synthesis, decomposition, combustion, single-displacement and double-displacement.Then, identify the reactants from the products by finding the yield sign ().If you see oxygen gas, O2, in the reactants and carbon dioxide (CO2) with water (H2O) in the products. This is a combustion reaction.Assuming it’s not combustion, next look for two smaller compounds forming larger compounds in the products (you may have a smaller leftover in the products) then its synthesis. Na + 2Cl NaCl2If it is the reverse of #4 (above) and one large compound is being split into smaller compounds after the reaction, then its decomposition. 2NaClO3 2NaCl + 3O2If it’s not one of these three, then it has to be a displacement reaction.To distinguish between single-displacement and double-displacement, look for a single element in the reactants and a different element in the products. They “switched” and therefore it is a single-displacement reaction. 2Al + Fe3N2 2AlN + 3FeIf there are two different compounds in the reactants and two similarly sized different compounds in the products, then its double-displacement. (The anions of the compounds have switched)Hint—Look for the chemical reaction that has the most written from left to right on the paper and its most likely going to be double-displacement.Balancing Chemical ReactionsConservation of Mass – All atoms have to be equal on reactants and products. Treat the yield sign as an equal sign for an equation (=).Before starting, remember to double check your work, after you think you is completed to be sure it’s correct.Remember that coefficients will be used to multiply the number of atoms. (3H2O = 6 hydrogen atoms and 3 oxygen atoms)If it is single-displacement or double-displacement, then look for parenthesis in the reaction. The number outside of the parenthesis will most likely be the coefficient on the other side of the reaction. LiBr + (NH4)2SO4 Li2SO4 + NH4BrThe subscripted two, after the ammonium, will cause a coefficient of two before the ammonium in the products2LiBr + (NH4)2SO4 Li2SO4 + 2NH4BrIf combustion, then it will have a 50:50 odds that will be a simple balancing reaction or one that will have to be doubled to balance out the fractions.Simple combustion: C6H12O6 + 6O2 6H2O + 6CO2The six on the carbon will most likely be the coefficient for the compoundsSynthesis/Decomposition: just do it normally.Sorry, no hints or tricks with these.Predicting Chemical ReactionsLook for oxygen gas (O2) in reactants. If it is there, it is probably combustion.If an organic compound (C containing) is with the oxygen gas, IT IS COMBUSTION!Therefore, it will have CO2 and H2O as products.If it is one reactant, then it’s decomposition and look for four smaller pieces within it (You will probably be able to use your polyatomic ion sheet to help figure the smaller products).Look for CO2, O2, H2O or H2 as productsIf there is a single element other than oxygen gas, then it is most likely single-displacement and all you need to do is swap it with the anion from the ionic compound.Double-displacement and synthesis are the last two types of reactions. Double-displacement must have two ionic compounds and synthesis does not (and most likely won’t). Typically, double-displacement will also form larger compounds than synthesis.REMEMBER TO BALANCE YOUR CHEMICAL REACTIONS FOR CONSERVATION OF MASS!USE YOUR KNOWLEDGE OF NAMING COMPOUNDS & OXIDATION TO CORRECTLY IDENTIFY PRODUCTS ................
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