Science 1206 Chemistry Unit #11

Transcription

1 Science 1206 Chemistry Unit #11 Types of Chemical Reactions Types of Chemical Reactions Five types of chemical reactions: Formation (synthesis) Decomposition Single Displacement Double Displacement Combustion 1

2 Formation (Synthesis) A synthesis reaction (also known as a formation reaction or combination reaction) is one in which two or more reactants combine to form a single product. General Form: reactant A + reactant B product C Two people joining together for a dance We will use the phrase simple combination reaction to describe reactions in which the reactants are elements. Recall: metal + non-metal ionic compound non-metal + non-metal molecular compound Synthesis example #1 sodium metal reacts with chlorine gas. element + element compound {general formula} sodium + chlorine sodium chloride {word equation} Na (s) + Cl 2 (g) NaCl (s) {chemical equation} 2 Na (s) + Cl 2 (g) 2 NaCl (s) {balanced C.E.} 2

3 Synthesis example #2 hydrogen gas is mixed with chlorine gas: element + element compound {general formula} hydrogen + chlorine hydrogen chloride {word equation} H 2 (g) + Cl 2 (g) HCl (g) {chemical equation} H 2 (g) + Cl 2 (g) 2 HCl (g) {balanced} Synthesis example #3 Carbon disulfide is produced by the reaction of carbon with sulfur: element + element compound {general formula} carbon + sulfur carbon disulfide {word equation} C (s) + S 8 (s) CS 2 (g) {chemical equation} 4 C (s) + S 8 (s) 4 CS 2 (g) {balanced} 3

4 Decomposition Reactions A chemical reaction that involves separating a compound into simpler substances is classified as a decomposition reaction. Synthesis reactions and decomposition reactions are opposites. General Form: reactant A product B + product C +... a dancing couple separating When a compound is decomposed into its elements, the reaction is termed simple decomposition.. Compound element + element Decomposition reactions example #1 Water is decomposed into its elements compound element + element {general formula} water hydrogen + oxygen {word equation} H 2 O (l) H 2 (g) + O 2 (g) {chemical equation} 2 H 2 O (l) 2 H 2 (g) + O 2 (g) {balanced C.E.} 4

5 Decomposition reactions example #2 Write a balanced chemical equation for the decomposition of nitrogen trichloride into its elements.: compound element + element {general formula} nitrogen trichloride nitrogen + chlorine {word equation} NCl 3 (s) N 2 (g) + Cl 2 (g) {chemical equation} 2 NCl 3 (s) N 2 (g) + 3 Cl 2 (g) {balance C.E.} Decomposition reactions example #3 Iron(III) oxide is broken down into its elements: compound element + element {general formula} iron (III) oxide iron + oxygen {word equation} Fe 2 O 3 (s) Fe (s) + O 2 (g) {chemical equation} 2 Fe 2 O 3 (s) 4 Fe (s) + 3 O 2 (g) {balanced C.E.} 5

6 Single Displacement Reactions A single displacement reaction (also known as a single replacement reaction) occurs when an element and a compound react to produce different element and different compound. General Form: element A + compound BC element B + compound AC someone cutting in on a dancing couple How do you know which ones switch? In order to determine which elements switch we can look at their charges. All we need to keep in mind that opposite charges attract and like charges repel. Sample exercise 1: Consider the reaction of zinc with copper (II) sulfate: In this case: zinc will form Zn 2+, copper will form Cu 2+ and sulfate is the complex ion SO 2-4 The positive ions will repel each other so there is no way that Zn and Cu will combine! Zinc + copper (II) sulfate copper + zinc sulfate Notice the names: The zinc and copper switched places! 6

7 How do you know which ones switch? (cont ) Consider the reaction of Chlorine with sodium bromide. In this case: chlorine will form Cl - sodium will form Na + and bromide is Br - This time the negative ions will repel each other so there is no way that Cl - and Br - will combine! How do you know which ones switch? (cont ) In reactions involving water, the water dissociates into H+ and OH -. So we usually like to write water as HOH. Consider the reaction of calcium with water. In this case, calcium will form Ca 2+ water will form H + and hydroxide OH - This time the positive ions will repel each other so there is no way that Ca 2+ and H + will combine! Calcium + water calcium hydroxide + water 7

8 Solubility Tables How do you know if the new ionic substance formed will be (s) or (aq)? When one of the reactants is a solution (aq), the (aq) means water is present as a solvent. Since water is present we have to decide if any of the new substances will dissolve in water. In order to determine this we use the solubility table found on the reverse of your periodic table. Solubility Table: Example 1 Consider the compound barium hydroxide Ba(OH) 2, will it dissolve in water? Find the anion OH - Figure out which row Ba 2+ is in (aq) or (s)? From this table when Ba(OH) 2 is placed in water it will dissolve hence Ba(OH) 2 (aq) 8

9 Solubility Table: Example 2 Will silver acetate (AgCH 3 COO) dissolve in water? The answer is NO, thus in water silver acetate is insoluble, AgCH 3 COO (s) Double Displacement Reactions In chemistry, double displacement reactions (also known as double replacement reactions) involve the replacement of a cation in one compound by the cation in another compound and vice versa. In other words, there is an exchange of cations. These reactions tend to involve one or more aqueous ionic compounds. General Form: compound AB + compound CD compound AD + compound CB 9

10 Double Displacement Reactions example 1 Consider the reaction of a solution of silver nitrate with a solution of sodium chloride. In this case: sodium will form Na + chlorine will form Cl - silver will form Ag + and nitrate is the complex ion NO - 3 The positive ions will repel each other so there is no way that Na + and Ag + will combine! So the Na + will react with the NO - 3 and the Ag + will react with the Cl - We need to balance it and decide on the states of matter for the products. Since water is present in the reactants (aq) we have to consult the solubility tables! Silver Nitrate + Sodium chloride Silver chloride + Sodium Nitrate Notice the names: The silver and sodium switched places! Double Displacement Reactions example 2 Write a balanced chemical equation for the double replacement reaction involving aqueous calcium chloride and aqueous sodium carbonate. In this case: calcium will form Ca 2+ chlorine will form Cl - sodium will form Na + and carbonate will form CO 3 2- The positive ions will repel each other so there is no way that Na + and Ca 2+ will combine! So the Na + will react with the Cl - and the Ca 2+ will react with the CO 2-3 We need to balance it and decide on the states of matter for the products. Since water is present in the reactants (aq) we have to consult the solubility tables! Calcium Chloride + Sodium Carbonate Calcium Carbonate + Sodium Chloride Notice the names: The calcium and sodium switched places! 10

11 Combustion Reactions Combustion reactions typically require oxygen gas as a reactant. Energy in the form of heat and light is usually released. Any reaction which releases energy is said to be exothermic. Note that the reactants have more energy than the products because energy has been released. The opposite of an exothermic reaction is called an endothermic reaction. Any reaction in which energy is absorbed is said to be endothermic. Notice that the energy of the products is higher than the energy of the reactants because energy has been absorbed.. Hydrocarbon Combustion The examples of combustion that you are probably most familiar with are the ones that involve hydrocarbons - substances like diesel, gasoline, and propane. Hydrocarbons - consist of hydrogen and carbon atoms. Propane - C 3 H 8 Octane - C 8 H 18 Candle wax - C 25 H 52 General Equation for Combustion: hydrocarbon + oxygen oxide of carbon + water There are two possible oxides of carbon produced by hydrocarbon combustion. Carbon dioxide is produced when the combustion reaction is complete. The evidence for complete combustion is a blue flame. hydrocarbon + oxygen carbon dioxide + water Carbon monoxide and carbon in the form of soot are produced when combustion is incomplete resulting in an orange, smoky flame. hydrocarbon + oxygen carbon monoxide + carbon + water The factor that determines whether a combustion reaction is complete or incomplete is the amount of oxygen available for the reaction. If there is insufficient oxygen, deadly carbon monoxide is produced. 11

12 Balancing Hydrocarbon Combustion Reactions A great method to use when balancing combustion reactions is The Method of C H O. That is: balance C balance H balance O last. Balancing Hydrocarbon Combustion Reactions (example 1) Write a balanced chemical equation for the complete combustion of propane gas - C 3 H 8 (g). hydrocarbon + oxygen carbon dioxide + water C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O (g) 1. Balance C first: 3 C on the left -- multiply CO 2 by 3 to get 3 C on the right C 3 H 8 (g) + O 2 (g) 3 CO 2 (g) + H 2 O (g) 2. Balance H second: 8 H on the left -- multiply H 2 O by 4 to get 8 H on the right C 3 H 8 (g) + O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) 3. Balance O last: We have two sources of O on the right: 3 x CO 2 = 6 O 4 x H 2 O = 4 O Thus we have 10 O on the right -- multiply O 2 by 5 to get 10 O on the left. C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) Do a recheck of the whole equation: Left Right: 3 C 3 C 8 H 8 H 10 O 10 O 12

13 Balancing Hydrocarbon Combustion Reactions (example 2) Write a balanced chemical equation for the complete combustion of butane gas C 4 H 10 (g). hydrocarbon + oxygen carbon dioxide + water C 4 H 10 (g) + O 2 (g) CO 2 (g) + H 2 O (g) 1. Balance C first: 4 C on the left -- multiply CO 2 by 4 to get 4 C on the right C 4 H 10 (g) + O 2 (g) 4 CO 2 (g) + H 2 O (g) 2. Balance H second: 10 H on the left -- multiply H 2 O by 5 to get 10 H on the right C 3 H 10 (g) + O 2 (g) 4 CO 2 (g) + 5 H 2 O (g) 3. Balance O last: We have two sources of O on the right: 4 x CO 2 = 8 O 5 x H 2 O = 5 O We have 13 O on the right -- multiply O 2 by 6 1/2 to get 13 O on the left. C 4 H 10 (g) + 6 1/2 O 2 (g) 4 CO 2 (g) + 5 H 2 O (g) To remove the fraction we multiply the whole equation by 2. 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O (g) Do a recheck of the whole equation: Left Right: 8 C 8 C 20 H 20 H 26 O 26 O (16+10) Other Reactions Not all chemical reactions fit into the five reaction types described to this point. The purpose of learning the five reaction types is to learn how to predict the products that should form when one or more substances react. If you encounter unbalanced equations that do not appear to fit the patterns established up to this point, use the skills you have learned to assign coefficients. Here are some of the key points: always make sure you have written the chemical formulas correctly never balance an equation by changing the numerical subscripts in chemical formulas whenever possible, treat polyatomic ions as units balance the most numerous species first (unless the reaction is a combustion reaction) if you place a coefficient in front of a formula to balance one of its atoms or ions, then check to see if you have changed the number of other atoms or ions in the formula if you are having trouble balancing an equation, check the chemical formulas to make sure they are correct. Incorrect chemical formulas are the number one cause of incorrectly balanced equations. when a species occurs in two or more formulas on the same side of an equation, balance it last. (e.g. O 2 in combustion reactions) 13