Chapter 8. Chemical Equations

Transcription

1 Chapter 8 Chemical Equations 1

2 chemical equations provide us the followings: 1. summarize the reaction 2. display the substances that are reacting 3. show the products 4. indicate the amounts of all component substances in a reaction 2

3 8.1 The chemical equation reactants the substances entering the reaction products the substances formed In a chemical reaction atoms are neither created nor destroyed. All atoms present in the reactants must also be present in the products ex. thermite reaction 1. Al + Fe 2 O 3 Fe + Al 2 O 3 reactants products 2. 2Al + Fe 2 O 3 2Fe + Al 2 O 3 balance the equation 3. Δ 2Al + Fe 2 O 3 2Fe + Al 2 O 3 heat is supplied 4. Δ 2Al(s) + Fe 2 O 3 (s) 2Fe(s) + Al 2 O 3 (s) show the physical states of the substances 3

4 8.2 Writing and balancing equations disproved Thomson s model 4

5 ex. 8.1 Mg is burned in air to produce MgO Mg + O 2 MgO Mg is balanced, balance O Mg + O 2 2MgO Mg is not balanced 2Mg + O 2 2MgO all balanced ex. 8.2 methane CH 4 undergoes complete combustion to produce H 2 O and CO 2 CH 4 + O 2 H 2 O + CO 2 C is balanced, balance H CH 4 + O 2 balance O 2H 2 O + CO 2 CH 4 + 2O 2 2H 2 O + CO 2 all balanced ex. 8.3 heating potassium chlorate to form oxygen and potassium chloride Δ KClO 3 O 2 + KCl balance O Δ KClO 3 3/2 O 2 + KCl Δ 2KClO 3 3O 2 + 2KCl 5

6 ex. 8.4 silver nitrate reacts with hydrogen sulfide to produce silver sulfide and nitric acid AgNO 3 + H 2 S Ag 2 S + HNO 3 balance Ag 2AgNO 3 + H 2 S Ag 2 S + HNO 3 balance H 2AgNO 3 + H 2 S Ag 2 S + 2HNO 3 all balanced ex. 8.5 aluminum hydroxide is mixed with sulfuric acid to produce aluminum sulfate and water Al(OH) 3 + H 2 SO 4 Al 2 (SO 4 ) 3 + H 2 O balance Al 2Al(OH) 3 + H 2 SO 4 Al 2 (SO 4 ) 3 + H 2 O balance SO 4 2Al(OH) 3 + 3H 2 SO 4 balance H 2Al(OH) 3 + 3H 2 SO 4 all balanced Al 2 (SO 4 ) 3 + H 2 O Al 2 (SO 4 ) 3 + 6H 2 O 6

7 8.3 Information in a chemical equation a balanced equation gives us the following information: 1. what the reactants and products are 2. the formulas of the reactants and products 3. the number of molecules or formula units of reactants and products in the reaction 4. the number of atoms of each element involved in the reaction 5. the number of moles of each substance ex. 2H 2 + O 2 2H 2 O ex. 7

8 8.4 Types of chemical equations four principal reaction types: combination reaction two reactants combine to give one product A + B AB 8

9 decomposition reaction a single substance is decomposed to give two or more different substances AB A + B single-displacement reaction one element reacts with a compound to replace one of the elements of that compound, a different element and a different compound are yielded A + BC B + AC or A + BC C + AB 9

10 K, Ca, Na displace hydrogen from cold water, steam and acid Mg, Al, Zn, Fe displace hydrogen from steam and acid Ni, Sn, Pb displace hydrogen only from acid Cu, Ag, Hg, Au do not displace hydrogen 10

11 ex. 8.7 will a reaction occur between (a) Ni and HCl(aq) yes, Ni(s) + 2HCl(aq) NiCl 2 (aq) + H 2 (g) (b) Sn and AlCl 3 (aq) No double-displacement reaction two compounds exchange partners with each other to produce two different compounds AB + CD AD + CB accompanied by 1. the evolution of heat 2. the formation of an insoluble precipitate 3. the production of gas bubbles 11

12 ex. 8.8 write the equation for HBr + KOH neutralization of acid & base HBr(aq) + KOH(aq) KBr(aq) + H 2 O(l) ex. 8.9 write the equation for BaCl 2 + Na 2 SO 4 formation of precipitation BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + 2NaCl(aq) 12

13 8.5 Heat in chemical reactions energy changes always accompany chemical reactions when the reaction leads to a more stable state, energy is released to the surroundings as heat exothermic reaction liberates heat endothermic reaction absorbs heat H 2 (g) + Cl 2 (g) 2 HCl(g) kj N 2 (g) + O 2 (g) kj 2 NO(g) heat of reaction the quantity of heat produced by a reaction (kj or kcal) C(s) + O 2 (g) CO 2 (g) kj major source of energy of modern technology is fossil fuel coal, petroleum, natural gas (mixture of hydrocarbons) activation energy the amount of energy that must be supplied to start a chemical reaction even in an exothermic reaction, this activation energy is needed 13

14 8.6 Global warming: the greenhouse effect carbon dioxide CO 2 is a minor component in the atmosphere use of fossil fuel has led to an increase of 7.4% in CO 2 between 1900 and 1970, and an additional 3.5% increase during 1980s. rain forests are being destroyed other greenhouse gases: methane and water 14

15 greenhouse effect global warming efforts to prevent greenhouse effect: development of new energy source end to deforestation worldwide intense efforts to improve conservation 15