Chemical Equations. Intro to Stoichiometry. 2Na (s) + Cl 2(g) 2NaCl (s) Reactants Products. The Conservation Laws 4/6/09

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1 Intro to Stoichiometry Chemistry 11 Chemical Equations Various types of evidence suggests that a chemical reaction has occurred. These include: Change in temperature. A colour change may occur. New phases may be formed. Sodium and chlorine react to form sodium chloride. 2Na (s) + Cl 2(g) 2NaCl (s) Reactants Products Phases s = solid g = gas l = liquid Coefficients The number of molecules of each species. The Conservation Laws A couple of definitions first System: The part of the universe being studied in a particular situation. Open System: A system where matter and energy can enter or leave. Closed System: A system where matter and energy cannot enter and leave. 1

2 The Law of Conservation of Mass: The mass of a closed system will remain constant, regardless of the processes acting inside the system. Mass cannot be created or destroyed, although it may be rearranged in space, and changed into different types of particles. Implication to Chemistry: For any chemical process in a closed system, the mass of the reactants must equal the mass of the products. The Law of Conservation of Charge: The net charge of an isolated system remains constant. The total electrical charge in a closed system does not change during a chemical reaction. Implications to Chemistry: For any chemical process in a closed system, the total charge on the reactant side must equal the total charge on the product side. The Law of Conservation of Energy: The total amount of energy in an isolated system remains constant. The only thing that can happen with energy in an isolated system is that it can change form. Implications to Chemistry: For any chemical process in a closed system, energy cannot be created or destroyed. Balancing Chemical Equations!Here are a couple of tips " Balance the easy stuff first, the tough stuff will work itself out. Try to balance entire polyatomic groups (PO 4, SO 4, OH, etc.) if possible. Don t break them up! If an atom is part of a diatomic element, use fractions to balance them out, but, remember to get rid of the fraction in the final answer. Now, the examples Balance the following: (NH 4 ) 3 PO 4 + NaOH Na 3 PO 4 + NH 3 + H 2 O C 19 H 17 NO 3 + O 2 CO 2 + H 2 O + N 2 Cr 2 (SO 4 ) 3 + KI + KIO 3 + H 2 O Cr(OH) 3 + K 2 SO 4 + I 2 MoCl 3 + O 2 + AgCl MoCl 4 + Ag 2 O Using Chemical Word Equations Remember: (s) = solid (l) = liquid (g) = gas CaSi 2 + SbCl 3 Si + Sb + CaCl 2 2

3 When writing chemical equations from words or vice versa, remember the seven diatomic elements. The Hockey Stick and Puck! (also S 8, and P 4 ) NOTE: The words crystal, powder, or precipitate all mean the phase is a SOLID. A PRECIPITATE is used to refer to a solid formed when two aqueous solutions are mixed together to form a solid. Try Some: Solid Sodium reacts with Chlorine gas to produce solid Sodium chloride. Liquid water reacts with powdered Sodium oxide to produce aqueous Sodium hydroxide. Hydrochloric acid reacts with Calcium carbonate crystals, producing aqueous Calcium chloride, gaseous Carbon dioxide and, liquid water. Gaseous Xenon hexafluoride reacts violently with water to form solid Xenon trioxide and gaseous Hydrogen fluoride Do: W.S. 9-2 Balancing W.S. Homework: Types of Reactions There are 5 types of reactions you need to know in Chemistry Synthesis or Combination Involves the combination of two or more substances to form (or synthesize ) a compound. In general, a synthesis/combination reaction is represented by the equation: A + B AB Two elements react to form a compound. H 2(g) + F 2(g) 2 HF 4 Na (s) + O 2(g) 2 Na 2 O 16 Al (s) + 3 S 8 (s) 8 Al 2 S 3 3

4 2. Decomposition Involves breaking down a molecule/formula unit into simpler substances ( decomposing ). In general, a decomposition reaction can be thought of as the reverse of a combination reaction AB A + B A single compound on the reactant side. 2 H 2 O (l) 2 H 2 + O 2 2 HI (g) H 2 + I 2 MgCl 2(s) Mg + Cl 2 3. Single Replacement A single replacement involves replacing ONE atom in a compound by another atom. In general, a single replacement reaction is represented by the equation: A + BX B + AX An element reacts with a compound. CuI 2(aq) + Br 2(l) CuBr 2 + I 2 3 CuI 2(aq) + 2 Fe (s) 2 FeI Cu 4 Cr (s) + 3 SnCl 4(aq) 4 CrCl Sn 4. Double Replacement Involves an exchange of atoms or groups of atoms between two different compounds. A double replacement reaction is represented by the equation: AB + XY AY + XB Two compounds react. Cu(NO 3 ) 2(aq) + Na 2 S (aq) CuS + 2 NaNO 3 Na 2 CO 3(aq) + 2 HCl (aq) 2 NaCl + H 2 CO 3 K 2 CO 3(aq) + Ba(OH) 2(aq) 2 KOH + BaCO 3 4

5 A special type of double replacement called NEUTRALIZATION involves an acid reacting with a base to form a salt and water. Recall: Strong acids start with H-. When all the acid and base has neutralized each other, the resulting solution is said to be NEUTRAL (ph = 7). H 2 SO 4(aq) + 2 NaOH (aq) Na 2 SO H 2 O HCl, H 2 SO 4, HNO 3 Strong bases end in OH. H 3 PO 4(aq) + 3 KOH (aq) K 3 PO H 2 O NaOH, KOH, Ca(OH) 2 2 HNO 3(aq) + Sr(OH) 2(aq) Sr(NO 3 ) H 2 O 5. Combustion of Hydrocarbons C 6 H 12 O 6(g) + 6 O 2(g) 6 CO H 2 O Do: Homework: Types of Chemical Reactions W.S. C 3 H 6 OS 2(g) + 6 O 2(g) 3 CO H 2 O + 2 SO 2 2 C 6 H 4 (OH) 2(l) + 13 O 2(g) 12 CO H 2 O 5

6 Energy Changes in Chemical Reactions Molecules are held together by CHEMICAL BONDS. In order to break a bond, energy must be added to the bond. The reaction: 2 HCl (g) + energy H 2(g) + Cl 2(g) is shown graphically below: The separated atoms (H 2(g) and Cl 2(g) ) contain more energy than the original molecule (HCl (g) ). There are two types of reactions possible when taking energy into consideration. Either energy is absorbed, or released from the system. We always take the view of the system when considering energy in chemical reactions. Exothermic An EXOTHERMIC reaction GIVES OFF heat to its surroundings. (Heat EXits from the system) ENERGY! The energy flow is from the system to the surroundings. Since energy is released, the surroundings feel warmer (increase in temperature). Endothermic An ENDOTHERMIC reaction ABSORBS heat from its surroundings. (heat ENters the system) ENERGY! The energy flow is from the surroundings into the system. Since energy is absorbed, the surroundings feel cooler (decrease in temperature). 6

7 Enthalpy (H): The heat contained in a system. So: H = the change in enthalpy during a reaction. and... H = H PRODUCTS H REACTANTS Where: H PRODUCTS = the heat contained in the PRODUCTS H REACTANTS = the heat contained in the REACTANTS Exothermic and endothermic reactions can be written in alternate ways: ENDOTHERMIC: A + 50 KJ B A B; H = +50 KJ EXOTHERMIC: C D + 80 KJ C D; H = -80 KJ Do: Homework: Energy in Chemical Reactions W.S. What is on the exam? Chemical Equations Evidence of chemical reactions What a chemical equation is The Conservation Laws Know what they are How they apply to chemistry Balancing Chemical Equations Types of Chemical Reactions Identify 5 types Balance Predict products Energy Changes in Chemical Reactions Exothermic and endothermic Writing equations including energy term 7