Unit 5: Chemical Reactions. Chapter 11

Transcription

1 Unit 5: Chemical Reactions Chapter 11

2 Objectives 35 Identify the five types of chemical reactions 36 Write word and chemical equations based on chemical reactions 37 Balance chemical equations 38 Predict the products of chemical equations

3 Chemical Reactions A chemical reaction is a chemical change in which the bonds of one or more reactants break and reform to create one or more products. The way a chemical reaction is written can tell a scientist a lot about the reaction. Over the course of this tutorial, we will look into the different aspects that are shown in a chemical reaction.

4 Basic information Coefficients CH 4 + 2O 2 2H 2 O + CO 2 Reactants Products

5 35. Identify the five types of chemical reactions There are five common types of chemical reactions. Synthesis Decomposition Single Replacement Double Replacement Combustion Click on each to get a description. The key identifying part will be highlighted in blue.

6 Synthesis A + X AX Return A synthesis reaction occurs when two or more reactants become one product. There are four situations to watch for: Two single elements added together: i.e.: 2Mg + O 2 2MgO An metal oxide added to water makes a metal hydroxide. i.e.: 2Na 2 O + H 2 O 2NaOH A metal chloride added to oxygen makes a metal chlorate. i.e.: 2NaCl + 3O 2 2NaClO 3 A metal oxide added to carbon dioxide makes a metal carbonate. i.e.: Na 2 O + CO 2 Na 2 CO 3

7 Decomposition AX A + X Return A decomposition reaction occurs when one reactant becomes many products. There are four situations to watch for: A compound breaking into single element: i.e.: 2MgO 2Mg + O 2 A metal hydroxide becoming a metal oxide and water. i.e.: 2NaOH 2Na 2 O + H 2 O A metal chlorate breaking into a metal chloride and oxygen. i.e.: 2NaClO 3 2NaCl + 3O 2 A metal carbonate breaking into a metal oxide and carbon dioxide. i.e.: Na 2 CO 3 Na 2 O + CO 2

8 36. Write word and chemical equations based on chemical reactions Word and chemical equations are the most common ways to see a chemical reaction. It is important to be able to go from one to the other. It is also important to show as much information as possible about the reaction.

9 Basic symbols in a chemical reaction Below is list a basic symbols that are used when writing chemical equations. Symbol + And or with (s) (l) (g) (aq) Meaning Yields, produces, or creates Solid Liquid Gas Aqueous In the presence of a catalyst In the presence of heat

10 Writing word equations from chemical equations Using a chemical equation, determine the reactants and the products and write their names as discussed in Units 5 and 6. Insert words to describe each symbol. NaCl (aq) + AgBr (aq) AgCl (s) + NaBr (aq) Aqueous sodium chloride and aqueous silver bromide yield solid silver chloride and aqueous sodium bromide.

11 Writing chemical equations from word equations From the word equation, identify the reactants and the products and write their formulas as discussed in Units 5 and 6. Add in the necessary symbols. Liquid carbon tetrahydride reacts with gaseous oxgyen in the presence of heat to produce gaseous water and carbon dioxide gas. CH 4 (l) +O 2 (g) H 2 O (g) + CO 2 (g)

12 Single Replacement Return A single replacement reaction occurs when one element replaces part of a compound to produce a single element and a new compound. There are two types of single replacement reactions Cationic Single Replacement A + BX B + AX i.e.: Na + AgCl NaCl + Ag Anionic Single Replacement AX + Y AY + X i.e.: 2NaCl + F 2 2NaF + Cl 2

13 Double Replacement AX + BY AY + BX A double replacement reaction occurs when two ionic compounds switch their cations. i.e.: NaCl + LiF LiCl + NaF Return

14 Combustion C x H y + O 2 CO 2 + H 2 O Combustion reactions occur when a hydrocarbon reacts with oxygen to form carbon dioxide and water. A hydrocarbon is a covalent molecule that contains carbon, hydrogen, and occasionally oxygen. i.e.: CH 4, C 2 H 6, or C 3 H 7 O Return

15 37. Balance chemical equations When writing chemical equations, it is important to correctly write each formula. This will make it appear as though the law of conservation of mass is not followed. To correct for this, coefficients are placed before each part of the reaction to show how many of each compound or element would be used or produced. When counting atoms with a coefficient, make sure to multiply the coefficient by the subscripts i.e.: 2MgCl 2 would have 2 magnesiums and 4 chlorine atoms

16 Mg + AlCl 3 MgCl 2 + Al Take a look at the equation above: Notice how each compound is correctly written to balance their charges. However, the chlorine is not balanced. The formulas cannot be changed so coefficients will be added. A coefficient is a number placed before the elements or compounds. The next two slides will work through how to balance this equation.

17 Balancing the equation with pictures Mg + AlCl 3 MgCl 2 + Al To be balanced, there must be the same number of each element on the reactant side and the product side. Since the chlorine is not balanced, we will add another MgCl 2 and another Mg. Chlorine is still not balanced so we will add a AlCl 3 this time and another Al. Since chlorine is still not balanced, we will add another MgCl 2 and another Mg. Now, there is the same number on each side. We can put the number of each in the blank in front of each chemical.

18 Balancing equations: Just numbers Mg + AlCl 3 MgCl 2 + Al Mg: 1 3 Mg: 1 3 Al: 1 2 Al: 1 2 Cl: 6 Cl: Without pictures, counting the elements works just as well. Start with the unbalanced equation. Notice how the chlorine is not balanced. We could add one at a time or look for the least common multiple. In this case, the least common multiple of 2 and 3 is 6. Therefore, if we put a 3 in for the coefficient for MgCl 2 and a 2 in for the coefficient for AlCl 3, the chlorine will be balanced. We can finish by balancing the Al and Mg.

19 38. Predicting products In chemistry, we want to know what products we will get if we mix two chemicals. The first step in this process is identify the chemical reaction from only the reactants. A flow chart has been created to help you work through this process. The next step is to correctly write the products. Do not worry at this point if the equation is not balanced. Finally, go back and balance the equation.

20 Start How many reactants? Two Is one a hydrocarbon? Yes Combustion One No Decomposition Is there a single element? No Yes No Is it oxygen? Are both ionic? Single Replacement (check the activity series) Yes No Yes Return Synthesis Double Replacement

21 Predicting products Select each of the following for a description of how to predict its products. Synthesis Decomposition Single Replacement Double Replacement Combustion

22 Predicting-Synthesis Return Once a reaction is determined to be synthesis, recall the 4 types of synthesis reactions. Two single elements will combine to make one compound. 2Na + Cl 2 2NaCl Metal chloride and oxygen make the metal chlorate. 2NaCl + 3O 2 2NaClO 3 Metal oxide and water make the metal hydroxide. Na 2 O + H 2 O 2NaOH Metal oxide and carbon dioxide make the metal carbonate. Na 2 O + CO 2 Na 2 CO 3

23 Predicting-Decomposition Return Once a reaction is determined to be decomposition, recall the 4 types of decomposition reactions. One compound breaking into its individual elements. 2NaCl 2Na + Cl 2 Metal chlorate breaking into metal chloride and oxygen. 2NaClO 3 2NaCl + 3O 2 Metal hydroxide breaking into metal oxide and water. 2NaOH Na 2 O + H 2 O Metal carbonate breaking into metal oxide and carbon dioxide. Na 2 CO 3 Na 2 O + CO 2

24 Predicting Single Replacement A single replacement reaction is identified by having a single element and a compound. Determine the charges on all the elements. The two with like charges will switch places. With single replacement, there is another step Na + K Cl NaCl + K

25 The activity series Not all single replacement reactions occur. An element must have enough reactivity to replace an element already in the compound. The activity series lists the elements in order of decreasing reactivity. An element can replace any element below it on the list.

26 Activity Series Return Assume you have a reaction between silver and potassium chloride. Because both silver and potassium are positive, silver will attempt to replace potassium. Check the portion of the activity series to the right. Potassium is higher on the list which means it can replace silver but silver cannot replace potassium in a reaction. Therefore, the equation would be written as: Ag + KCl No Reaction Activity Series Lithium Potassium Calcium Sodium Magnesium Silver Li K Ca Na Mg Ag

27 Predicting-Double Replacement A double replacement reaction occurs when two ionic compounds react. Determine the charges on all elements. The products will switch partners. Remember to correctly write each formula by putting the positive ion first. Na + OH - + H + Cl - H + OH - + Na + Cl - Return

28 Predicting-Combustion Combustion reactions always have a hydrocarbon (C x H y ) being added to O 2. Combustion reactions are probably the easiest to predict. The products are always carbon dioxide and water. CH 4 + O 2 CO 2 + H 2 O Return

29 This concludes the tutorial on measurements. To try some practice problems, click here. To return to the objective page, click here. To exit the tutorial, hit escape.

30 Definitions-Select the word to return to the tutorial Anion: the negative portion of an ionic compound Aqueous: dissolved in water Catalyst: a chemical added a reaction to increase the reaction rate without changing the products Cation: the positive portion of an ionic compound Law of conservation of mass: Mass can neither be created or destroyed; the masses of the reactants must equal the masses of the products