8 Chemical Equations. Flames and sparks result when aluminum foil is dropped into liquid bromine.

Transcription

1 8 Chemical Equations Flames and sparks result when aluminum foil is dropped into liquid bromine.

2 Chapter Outline 8.1 The Chemical Equation 8.2 Writing and Balancing Chemical Equations 8.3 Types of Chemical Equations 8.4 Heat in Chemical Reactions 8.5 Global Warming: The Greenhouse Effects

3 Chemical Equations Chemical Equations: shorthand notation for chemical changes or reactions. Chemists use chemical equations to: a. Summarize a chemical reaction by displaying the substances reacting and forming. b. Indicate specific amounts of materials consumed or produced during the reaction. Reactants: substances consumed during the reaction. Products: substances formed during the reaction. a A + b B c C + d D Atom balance must be maintained in all chemical reactions. All atoms from reactants must appear as part of products.

4 Chemical Equations General Structure of Chemical Equations: 1. Reactants and products are separated by an arrow. Reactants are on the left side of the arrow, products are on the right. a A + b B Reactants c C + d D Products 2. Whole number coefficients are placed in front of substances to balance the atoms in the equation. The numbers indicate the units of the substance reacted or formed during the reaction. The coefficient 1 is not written in a balanced equation.

5 Chemical Equations General Structure of Chemical Equations: 3. Information about the reaction (temperature, time) may be placed above or below the reaction arrow. a A + b B c C + d D 4. The physical state is written in brackets after the formula of the substance. Δ Δ indicates heat (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous a A (s) + b B (l) c C (g) + d D (s)

6 Chemical Equations Symbol Summary Symbol Significance Produces (points towards products) (s) (l) (g) (aq) Δ Solid (written after substance) Liquid (written after substance) Gas (written after substance) Substance dissolved in an aqueous solution Heat is added (above or below reaction arrow)

7 Law of Conservation of Mass Law of Conservation of Mass: the total mass of substances in a chemical reaction must remain constant. water hydrogen + oxygen g 11.2 g 88.8 g reactants g total of products In any chemical reaction: Mass of reactants = Mass of products

8 Writing and Balancing Chemical Equations Balanced chemical equations: contain the same number of each kind of atom on both sides of the equation. General Method for Writing and Balancing Chemical Equations 1. Identify the Reaction. Write a word equation for the reaction. mercury(ii) oxide Δ mercury + oxygen 2. Write the unbalanced (skeleton) equation. Write the correct formula for each substance HgO Δ Hg + O2

9 Writing and Balancing Chemical Equations General Method for Writing and Balancing Chemical Equations 3. Balance the equation. a. Count the number of each atom on the reactants and products side and determine what requires balancing. HgO Hg: 1 O: 1 Δ Hg + O2 Hg: 1 O: 2 Oxygen atoms need balancing on the reactants side.

10 Writing and Balancing Chemical Equations General Method for Writing and Balancing Chemical Equations b. Balance each element sequentially, using whole numbers. It is often best to balance metals first. 2 HgO Hg: 2 O: 2 Δ Hg + O2 Hg: 1 O: 2 Now Hg atoms need balancing on the products side. c. Check after adding coefficients that all atoms still balance. Adjust as needed (a 2 is needed in front of Hg).

11 Writing and Balancing Chemical Equations General Method for Writing and Balancing Chemical Equations d. Do a final check to make sure all atoms now balance on both sides of the equation. Δ 2 HgO 2 Hg + O2 Hg: 2 O: 2 Hg: 2 O: 2 Note: always use the smallest whole numbers! Δ 4 HgO 4 Hg + 2 O2 Balanced but incorrect form!

12 Balancing Chemical Equations Practice Write the balanced chemical equation for when magnesium metal undergoes combustion to produce magnesium oxide. 1. Identify the Reaction. Write a word equation for the reaction. magnesium + oxygen magnesium oxide 2. Write the unbalanced (skeleton) equation. Write the correct formula for each substance Mg + O2 MgO

13 Balancing Chemical Equations Practice Write the balanced chemical equation for when magnesium metal undergoes combustion to produce magnesium oxide. 3. Balance the equation. a. Count the number of each atom on the reactants and products side and determine what requires balancing. Mg + O2 Mg: 1 O: 2 MgO Mg: 1 O: 1

14 Balancing Chemical Equations Practice Write the balanced chemical equation for the decomposition of potassium chlorate upon heating to give oxygen and potassium chloride. 1. Identify the Reaction. Write a word equation for the reaction. Potassium chlorate potassium chloride + oxygen 2. Write the unbalanced (skeleton) equation. Write the correct formula for each substance KClO3 Δ KCl + O2

15 Balancing Chemical Equations Practice Write the balanced chemical equation for the decomposition of potassium chlorate upon heating to give oxygen and potassium chloride. 3. Balance the equation. a. Count the number of each atom on the reactants and products side and determine what requires balancing. KClO3 K: 1 Cl: 1 O: 3 Δ KCl + O2 K: 1 Cl: 1 O: 2 Oxygen needs balancing on both sides of the equation.

16 Balancing Chemical Equations Practice Write the balanced chemical equation for the decomposition of potassium chlorate upon heating to give oxygen and potassium chloride. b. Balance each element sequentially, using whole numbers. It is often best to balance metals first. Δ 2 KClO3 KCl + 3 O2 K: 2 Cl: 2 O: 6 K: 1 Cl: 1 O: 6 The lowest common multiple of 2 and 3 is 6. Add coefficients as appropriate to make 6 O atoms.

17 Balancing Chemical Equations Practice Write the balanced chemical equation for the decomposition of potassium chlorate upon heating to give oxygen and potassium chloride. c. Check after adding coefficients that all atoms still balance. Adjust as needed (a 2 is needed in front of KCl). Δ 2 KClO3 2 KCl + 3 O2 K: 2 Cl: 2 O: 6 K: 2 Cl: 2 O: 6 d. Do a final check to make sure all atoms now balance on both sides of the equation.

18 Balancing Chemical Equations Practice Write the balanced chemical equation for the reaction of silver nitrate with hydrogen sulfide to produce silver sulfide and nitric acid.

19 Balancing Chemical Equations Practice Write the balanced chemical equation for the combustion of butane (C4H10) gas, where carbon dioxide and water are the only products.

20 Information in a Chemical Equation

21 Information from a Chemical Equation From the chemical equation below, how many moles of HF can be produced from 2 moles of hydrogen gas? H2 (g) + F2 (g) 2 HF (g) For every 1 mol of H2, 2 mol of HF can be produced. Two moles of H2 could then produce 4 moles of HF.

22 Information from a Chemical Equation From the chemical equation below, how many moles of oxygen are needed to burn 2 molecules of propane (C3H8)? C3H8 + 5 O2 3 CO2 + 4 H2O a. 5 molecules of oxygen b. 6 molecules of oxygen c. 10 molecules of oxygen d. 15 molecules of oxygen For every 1 molecule of propane, 5 molecules of O2 are needed to fully react. Two molecules of propane would then require 2 x 5 = 10 molecules of oxygen.

23 Types of Chemical Equations Reactions are classified into subtypes to aide in predicting the products of chemical reactions. 1. Combination reactions 2. Decomposition reactions 3. Single displacement reactions 4. Double displacement reactions 5. Oxidation-reduction (redox) reactions (Chapter 17)

24 Combination Reactions Two reactants combine to give a single product. Representative Examples A + B AB where A or B are elements/compounds and AB is a compound. metal + nonmetal salt 2 Na (s) + Cl2 (g) 2 NaCl (s) metal oxide + water CaO (s) + H2O (l) metal hydroxide Ca(OH)2 (aq) Violent reaction occurs when Al and Br2 react to form AlBr3. 2 Al (s) + 3 Br2 (l) 2 AlBr3 (s)

25 Decomposition Reactions Representative Examples A single reactant breaks down (decomposes) into two or more products. AB A + B Metal oxides often decompose when heated. Δ 2 PbO2 (s) 2 PbO(s) + O2 (g) Carbonates form CO2 when heated. CaCO3 (s) CaO (s) + CO2 (g) H2O2 decomposes to steam (H2O (g)) and O2. 2 H2O2 (l) 2 H2O (g) + O2 (g)

26 Single Displacement Reactions One element (A) reacts with a compound (BC) to replace one element in the compound, giving a new element (B) and a different compound (AC). A + BC B + AC where A and B are metals and A is more reactive. A + BC or C + AB where A and C are halogens and A is more reactive. Example Zn reacts with HCl, resulting in H2 gas evolution. Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)

27 Activity Series Table listing in order of decreasing reactivity of various elements. More reactive elements replace less reactive ones. 2 Al (s) + 3 CuCl2 (s) 2 AlCl3 (aq) + 3 Cu (s) 2 Ag (s) + 3 CuSO4 (s) No Reaction

28 General Types of Single Displacement Reactions Where A and B are metals and A is more reactive: Metal + acid hydrogen + salt 2 Al (s) + 3 H2SO4 (aq) Al2(SO4)3 (aq) + 3 H2 (g) Metal + water hydrogen + metal hydroxide/oxide 2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g) Metal (or halogen) + salt Fe (s) + CuSO4 (aq) Cl2 (g) + 2 KI (aq) metal (or halogen) + salt Cu (s) + FeSO4 (aq) I2 (s) + 2 KCl (aq)

29 Single Displacement Reactions Will a reaction occur between Ni and HCl? If so, write the balanced chemical equation. Based on the reactivity series, Ni is more reactive than hydrogen, so a reaction occurs. Ni (s) + HCl (aq) H2 (g) + NiCl2 (aq) Will a reaction occur between Sn and AlCl3? If so, write the balanced chemical equation. Based on the reactivity series, Sn is less reactive than Al, so no reaction occurs. Sn (s) + AlCl3 (aq) No Reaction

30 Single Displacement Reactions Write the reaction (if it occurs) between the following substances: Iron metal and a solution of magnesium chloride

31 Double Displacement Reactions Two compounds exchange partners with one another to yield two new compounds. AB + CD AD + CB Physical evidence for a double displacement reaction include: heat evolution, formation of a precipitate or gas production. PbI2 precipitates from the reaction of Pb(NO3)2 with KI. Pb(NO3)2 (aq) + 2 KI (aq) PbI2 (s) + 2KNO3 (aq)

32 General Types of Double Displacement Reactions Acid/Base Neutralization acid + base HCl (aq) + NaOH (aq) salt + water + heat NaCl (aq) + H2O (l) + heat Metal Oxide (base) + Acid Metal oxide + acid CuO (s) + 2 HNO3 (aq) salt + water + heat Cu(NO3)2 (aq) + H2O (l) + heat

33 General Types of Double Displacement Reactions Formation of a Precipitate Product solubilities are based on the table in Appendix V Insoluble products are indicated by s (solid) BaCl2 (aq) + 2 AgNO3 (aq) 2 AgCl (s) + 2 KNO3 (aq) Gas Formation H2SO4 (l) + NaCl (s) NaHSO4 (aq) + HCl (g) A gas can also form from a secondary reaction after displacement 2 HCl (aq) + Na2CO3 (aq) 2 NaCl (aq) + H2CO3 (aq) H2CO3 (aq) H2O (l) + CO2 (g)

34 Writing Reaction Equations Practice Write the reaction equation between aqueous solution of hydroiodic acid and sodium hydroxide.

35 Writing Reaction Equations Practice Write the balanced reaction equation between aqueous barium chloride and magnesium sulfate.

36 Writing Reaction Equations Practice Write the balanced reaction equation between aqueous potassium phosphate and barium chloride.

37 Writing Reaction Equations Practice Write the balanced reaction equation between aqueous ammonium chloride and sodium nitrate.

38 Heat in Chemical Reactions Terminology Energy transfer and changes accompany any chemical reaction. Heat of reaction: quantity of heat actually produced during a chemical reaction. Units: kilojoules (kj) or kilocalories (kcal) Exothermic reactions: release heat H2 (g) + Cl2 (g) 2 HCl (g) kj Heat can be treated as a product Endothermic reactions: absorb heat N2 (g) + O2 (g) kj 2 NO (g) Heat can be treated as a reactant

39 Heat in Chemical Reactions Practice In the reaction: H2 (g) + I2 (s) kj 2 HI (g) When 4 moles of HI are produced: a kj of energy is absorbed b kj of energy is absorbed c kj of energy is released d kj of energy is released 12.6 kj are absorbed to form 2 mol of HI. 2 x 12.6 kj are needed to produce 4 mol of HI.

40 Heat as an Energy Transfer Vehicle in Nature Fossil Fuels: petroleum, coal, and natural gas all release significant amounts of energy during combustion. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) kj Combustion does not occur at normal temperatures without a spark added energy. Activation energy: Amount of energy needed to initiate a chemical reaction.

41 Graphical Representations of Endothermic Reactions Reaction Coordinate Diagram Products are at a higher potential energy than reactants.

42 Graphical Representations of Exothermic Reactions Reaction Coordinate Diagram Products are at a lower potential energy than reactants.