Chapter 8 Chemical Equations and Reactions

Transcription

1 Chapter 8 Chemical Equations and Reactions

2 8-1 Describing Chemical Reactions

3 Chemical reactions Matter undergoes 2 types of changes: 1.Physical changes no new substance produced 2.Chemical changes new/different substance produced Also called chemical reactions represented by chemical equations

4 Basic Anatomy of a Chemical Equation (NH 4 ) 2 Cr 2 O 7 (s) à N 2 (g) + Cr 2 O 3 (s) + 4H 2 O (g) Reactants original substances that will react Located to the the left of the arrow Products Substances that are produced Located to the right of the arrow Arrow = means yields or produces

5 How do you know that a reaction has happened? 1. Change in energy Energy is released or absorbed 2. Color change 3. Production of a gas Bubbles are seen 4. Formation of a precipitate A solid is produced from mixing two liquids

6 Rules for Chemical Equations 1. The equation must represent known facts. 2. The equation must contain the correct formulas. Write correct formulas for compounds Monatomic elements Fe, C, Li Diatomic elements Brother Honclif Br, H, O, Cl, I, F Exceptions: P 4 and S 8 3. The equation must follow the Law of Conservation of Mass. Number of atoms in reactants = number of atoms in products

7 What if the Law of Conservation of Mass isn t followed? Balance the equation by adding coefficients! H 2 + O 2 à H 2 0

8 Word Equations vs. Formula Equations Word equation: way of describing a reaction in words Arrow can be described as: react to yield, yields, produces or form Formula equation: explains a reaction in terms of symbols and formulas can also provide info about states of matter (g) gas (l) liquid (s) solid (aq) aqueous, dissolved in water

9 practice Solid sodium combines with chlorine gas to produce solid sodium chloride.

10 When solid copper reacts with aqueous silver nitrate, the products are aqueous copper(ii) nitrate and solid silver.

11 In a blast furnace, the reaction between solid iron(iii) oxide and carbon monoxide gas produces solid iron and carbon dioxide gas.

12 Chemical Equation Symbols Reversible reaction Reaction pressure Reactants are heated Reaction temperature Catalyst used to speed rxn

13 Coefficients Coefficients show the relative amounts of reactants and products in a chemical equation. They tell us the ratio of molecules or moles. H 2 (g) + Cl 2 (g) à 2HCl(g) Molecule ratio: Mole ratio:

14 Chemical equations do not tell us: Whether a reaction will happen How fast a reaction will happen

15 Balancing equations Balance different atoms one at a time Usa a chart if necessary to stay organized First balance atoms that appear only once on each side of the rxn Treat PAIs that appear on both sides of the equation as one single unit Balance H and O atoms last Reduce coefficients if possible

16 8-2 Types of Chemical Reaections

17 5 basic types of chemical reactions 1. Synthesis 2. Decomposition 3. Single-displacement 4. Double-displacement 5. Combustion

18 1. Synthesis Reactions A + X à AX One product is formed Synthesis = to put together

19 4 Examples of synthesis reactions 1. Reactions with oxygen Most metals will react with O 2 2Mg(s) + O 2 (g) à 2MgO(s) 2. Reactions with sulfur S 8 (s) + 8O 2 (g) à 8SO 2 (g) 3. Reactions of metals + halogens 2Na(s) + Cl 2 (g) à 2NaCl(s) 4. Reactions of metal oxides with water Oxides of active metals react with water to make metal hydroxides CaO(s) + H 2 O(l) à Ca(OH) 2 (s)

20 2. Decomposition Reactions AX à A + X Most require energy 2H 2 O à 2H 2 + O 2 Example of electrolysis

21 6 Examples of Decomposition Reactions 1. Metal oxides into elements 2HgO(s) à 2Hg(l) + O 2 (g) 2. Metal carbonates into metal oxide and CO 2 CaCO 3 (s) à CaO(s) + CO 2 (g) 3. Metal hydroxide into metal oxide and H 2 O Ca(OH) 2 (s) à CaO(s) + H 2 O(g) 4. Metal chlorates into metal chloride and oxygen 2KClO 3 (s) à 2KCl(s) + 3O 2 (g) 5. Acids into nonmetal oxides and water H 2 CO 3 (aq) à CO 2 (g) + H 2 O(l)

22 3. Single-Displacement Reactions A + BX à AX + B Y + BX à BY + X Most take place in aqueous solutions

23 4 Examples of Single-Displacement Reactions 1. Displacement of a metal in a compound by another metal 2Al(s) + 3Pb(NO 3 ) 2 (aq) à 3Pb(s) + 2Al(NO 3 ) 3 (aq) 2. Displacement of H in water by a metal Produces H 2 gas 3Fe(s) + 4H 2 O(g) à Fe 3 O 4 (s) + 4H 2 (g) 3. Displacement of H in an Acid by a metal Mg(s) + 2HCl(aq) à H 2 (g) + MgCl 2 (aq) 4. Displacement of Halogens Cl 2 + 2KBr(aq) à 2KCl(aq) + Br 2 (l)

24 4. Double-Displacement Reactions AX + BY à AY + BX Ions in 2 compounds rearrange Remember! positve ions are attracted to negative ions

25 3 Examples of Double-Displacement Reactions 1. Formation of a Precipitate 2KI(aq) + Pb(NO 3 ) 2 (aq) à PbI 2 (s) + 2KNO 3 (aq) 2. Formation of a Gas FeS(s) + 2HCl(aq) à H 2 S(g) + FeCl 2 (aq) 3. Formation of Water HCl(aq) + NaOH(aq) à NaCl(aq) + H 2 O(l)

26 5. Combustion Reactions Substance combines with oxygen Lots of energy released Burning is a combustion reaction Water is usually a product

27 Examples of Combustion Reactions Hydrocarbons burning Hydrocarbon a compound made of hydrogen and carbon atoms C 3 H 8 (g) + 5O 2 (g) à 3CO 2 (g) + 4H 2 O(g) propane tank

28 8-3 Activity Series of the Elements

29 Activity An element s activity is the ability of an element to react with another substance. High activity = high reactivity Low activity = low reactivity Activity Series = lists elements in order of reactivity in certain reactions Most often used to predict single-displacement reactions

30 Activity Series Most active element is at the top It will replace the elements below it on the list from a compound in a S-D reaction Elements can replace ONLY elements BELOW them on the activity series list used to predict whether S-D reactions will happen Based on experiment Ex: 2Al(s) + 3ZnCl 2 (aq) à 3Zn(s) + 2AlCl 3 (aq)

31 Use your activity series on page 272 to predict whether the following reactions will occur. If a reaction occurs, write the products and balance the equation. Cr(s) + H 2 O(l) à Pt(s) + O 2 (g) à Cd(s) + 2HBr(aq) à Mg(s) + steam à

32 Identify the element that replaces hydrogen from acids but cannot replace tin from its compounds. Do #2 a through e on page 273 in your notes.