Cu(NO 3 ) 2 3H 2 O. Test Review # 5. Chemistry: Form TR5-6A

Transcription

1 Chemistry: Form TR5-6A TEST 5 REVIEW Name Date Period Test Review # 5 Polar Molecules. Electronegativity differences between 0.4 and 1.7 are found in molecules with polar bonds. These molecules can be polar depending on their shapes. Molecules with polar bonds distributed symmetrically are nonpolar. Asymmetrical molecules with polar bonds are polar. Water is polar. An imaginary line can be drawn through a water molecule separating the positive pole from the negative pole. This is because the charges are distributed asymmetrically. Carbon dioxide is nonpolar because the electronegative oxygens are distributed symmetrically around the carbon. (O=C=O) Chemical Formulas. A chemical formula consists of chemical symbols, subscripts, and, in some cases, a coefficient. The chemical Symbols Coefficient symbols show which elements are present in the compound. Subscripts are small numbers written to the lower right of the symbol to which they refer. Example (Atoms in a Formula) 5(NH 4 ) 3 PO 4...N = 15, H = 60, P = 5, O = 20, TOTAL = 100 Cu(NO 3 ) 2 3H 2 O Subscripts Formula Writing. The quickest way to determine the formula of a compound of two elements or polyatomic ions is to use the Cross- Over Rule. Look up the oxidation state of each element or ion and reduce to lowest terms. Then cross over the oxidation states in lowest terms without the sign to find the subscripts as shown in the diagram to the right and the example below. Finding the Charge on Polyvalent Metals. Many transition metals have more than one oxidation state. They are called polyvalent. The fact that a metal is polyvalent becomes important when the compound is named. In order to properly name a compound, it is necessary to first check the Periodic Table to see if the metal is polyvalent. If it is, you need to figure out the oxidation state of the metal by checking to see which one will make the sum of the oxidation states in the compound add up to zero. The process is shown in the box to the left. The process is only applied for metals that have more than one oxidation state. The Stock System.The stock system is a set of rules for naming compounds of metals and non metals. The metal always comes first in the name and the formula. Monatomic metal ions, those consisting of only one type of atom, come in two varieties univalent and polyvalent. For univalent metal ions, those having only one oxidation state, the name of the ion is exactly the same as that of the element that formed it. For polyvalent metal ions, those having multiple oxidation states, a roman numeral indicates the oxidation state. Polyatomic metal ions, those consisting of more than one type of element such as NH 4 +, ammonium, are found on Table E. Al +3 and CO 3 2 Al 2 (CO 3 ) 3 Finding the Charge on the Cation Prepare a table as shown to the right. Step 1: List the subscripts for the cations and the anions. Step 2: Look up the oxidation state of the anion on the Periodic Table. Step 3: Multiply the oxidation state of the anion by the its subscript to get the total charge. Step 4: Determine the total charge of the cations by calculating the number which, when added to the total charge of the anions, gives the compound a total charge of zero. Step 5: Divide the total charge of the cations by the subscript of the cation to get the oxidation state. The nonmetal always comes last in the name and in the formula. For monatomic nonmetal ions, delete the last part of the elements name and add "IDE". Polyatomic nonmetal ions, such as SO 4-2 (sulfate) or OH (hydroxide) are found on Table E. To write the name from the formula, it is necessary to first check the Periodic Table to see if the metal is polyvalent. If it is, you need to figure out the oxidation state of the metal by checking to see which one will make the sum of the oxidation states in the compound add up to zero. To write the formulas from the name, you need to look up the oxidation states of the ions, and apply the crossover rule

2 TEST 5 REVIEW Page 2 Binary Covalent Compounds. Two nonmetals can combine to form compounds. When two nonmetals combine, they form covalent bonds. The nonmetal with the lower electronegativity behaves like a metal and has a positive oxidation state. In carbon dioxide (CO 2 ), the carbon behave like a metal while the oxygen behaves like a nonmetal. The metal is written first in the name and the formula. The name of the metal is the same as the name of the element. If there is more than one atom of the metal, the number of atoms is indicated with a prefix. (See the list of prefixes to the right.) The nonmetal is written last in the name and formula..the name of the nonmetal is the same as the name of the element minus the final syllable or two, plus IDE.The number of nonmetal atoms is indicated with a prefix (even when there is only one). Writing formulas for these compounds is easy, because the prefix tells the subscript. Selecting a Naming System. See below. Number of Atoms Prefix Number of Atoms 1 mono 6 hexa 2 di 7 hepta 3 tri 8 octa 4 tetra 9 nona 5 penta 10 deca Prefix Metallic Bonding. Metals have low ionization energies. This means they hold onto electrons loosely. As a result, in a metal crystal, the valence electrons move easily and do not belong to any single atom. Since the atoms in the crystal do not hold on to their own valence electrons, they become like cations in a sea of mobile electrons. The attraction between the cations and the electrons holds the metal crystal together. Because of this, metals are lustrous, flexible, good conductors of heat and electricity, and are solids at room temperature except for mercury.

3 TEST 5 REVIEW Page 3 Formula Mass. The masses of ionic and covalent compounds are found the same way from the formula. The atomic masses of the elements in the compound and the formula are used to determine the mass. The mass determined from the formula is called a formula mass. A molecular mass is Finding the Formula Mass Find the formula mass of CuSO4 a type of formula mass. The terms are sometimes used interchangeably. Step 1: Look up the mass of each element on the Formula masses are determined by following the steps in the box to the Periodic Table and round it off. right. The results are in atomic mass units (amu) Step 2: Multiply each element s atomic mass by its Empirical Formulas. The chemical formula for a molecular compound subscript to get the product. shows the number and type of atoms present in a molecule. Ionic crystals Step 3: Add the products together to get the total are a collections of ions. The chemical formula for an ionic compound Atomic shows the ratio ions in the compound. The ratio of ions in the formula for Element Mass Subscript Product an ionic compound is always in lowest terms. A chemical formula in which the ratio of the elements are in lowest terms is called an empirical formula. Cu 64 1 = 64 The molecular formula for glucose (C 6 H 12 O 6 ) is not an empirical formula. All the subscripts are divisible by six. When the subscripts are divided by S 32 1 = 32 six, the empirical formula for glucose, CH 2 O, is obtained. Some molecular formulas, such as the one for carbon dioxide, CO 2, are already empirical O 16 4 = 64 formulas without being reduced. TOTAL 160 There are two skills you need to learn in order to work with empirical formulas: Finding the empirical formula from the molecular formula; and finding the molecular formula from the empirical formula and the molecular mass. To find the empirical formula from the molecular formula, divide all the subscripts by the greatest common factor. To find the molecular formula from the empirical formula and the molecular mass. Step 1: Step 2: Step 3: Procedural Steps Determine the empirical formula mass. Divide the molecular mass by the empirical formula mass to determine the multiple. Multiply the empirical formula by the by the multiple to find the molecular formula Sample Problem A compound with an empirical formula of CH2O has a molecular mass of 90 amu. What is its molecular formula? Step 1: CH 2 O C = 12 1 = 12 H = 1 2 = 2 O = 16 1 = Step 2: 90 = 3 30 Step 3: [CH 2 O] 3 = C 3 H 6 O 3 Percent Composition. Percentage composition is determined by finding the formula mass of a compound, multiplying the mass of each element by 100, and dividing the product by the formula mass of the compound. Use the periodic table to find the masses of individual elements. See the Sample Problem to the right. Chemical Change vs. Physical Change. Any change in which no new substances are formed, is only a physical change. When new substances form, there are not only physical changes, there are chemical changes as well. Typically, there are a number of changes that serve as evidence of a chemical change. They are: [1] energy changes; [2] release of gases; [3] formation of a solid in solution or the formation of water; [4] a change in Sample Problem: Find the percentage composition of MgCO 3. Formula Mass Percentage Composition Mg = 24 1 = 24 % Mg = = 29 C = 12 1 = 12 % C = = 14 O =16 3=48 % O = = color; and [5] a change in odor. These five changes serve only as evidence of a chemical change. There is no rule that identifies chemical changes unequivocally except that new substances are formed. Chemical Equations. Chemical equations provide a shorthand way to easily describe what occurs during a chemical reaction. In a typical chemical equation, the reactants are written on the left, while the products are written on the right. The reactants and products are separated by an arrow, or yield sign, which indicates that reactants yield products. ( REACTANTS ÿ PRODUCTS ) There are other symbols

4 TEST 5 REVIEW Page 4 as well that show the state of the chemicals involved in the reaction. They are: (s) or 9 for a solid precipitate; (R) for a liquid; (g) or 8 for a gas; and (aq) for dissolved in water or aqueous. Symbols can also be used to show other factors involved in the reaction such as sources of energy used. These include: for heat or # for light. These symbols are written above or below the yield sign because they are neither reactants nor products. The complete equation shows the identity of the reactants and products using chemical formulas and symbols, the phases of the reactants and products, any energy changes involved in the reaction, and the mole ratios of all the substances indicated by the coefficients. Equations may occasionally be written omitting information about phases or energy changes. The example below shows a complete chemical equation with all the components. 2KClO 3 (s) MnO 2 ( s) 2KCl(s) + 3O 2 (g) In the above reaction, the equation shows that the reactant is solid potassium chlorate, the products are solid potassium chloride and oxygen gas, manganese dioxide is a catalyst, and the reaction is endothermic. Symbols for manganese dioxide and heat are shown above and below the yield sign because they are neither reactants nor products. Reaction Types. Chemical reactions can be grouped into four basic types. They are direct combination or synthesis, decomposition, single replacement or substitution, and double replacement or exchange of ions. An example of synthesis is shown below: catalyst N2( g) + 3H2( g) 2NH3( g) Synthesis often results in the formation of only one product from two reactants, but not always. Combustion, as in the following example, CH 4 (g) + 2O 2 (g) ÿ CO 2 (g) + 2H 2 O, is also a form of synthesis because the oxygen combines with both the metal and the nonmetal to form two oxides. Decomposition is the reverse of synthesis. One reactant breaks apart to form several products. This is what happens when hydrogen peroxide decomposes over time to leave behind plain, ordinary water [2H 2 O 2 (aq) ÿ 2H 2 O(R) + O 2 (g)]. During a single replacement reaction, a more active metal replaces a less active metal in a compound, or a more active nonmetal replaces a less active nonmetal in a compound. This is what happens when a metal becomes corroded by an acid [2Fe(s) + 6HCl(aq) ÿ 2FeCl 3 (aq) + 3H 2 (g)]. In single replacement reactions, an element is reacting with a compound. Double replacement reactions occur between aqueous compounds. The cations and anions switch partners. If an insoluble precipitate forms, the reaction is an end reaction, otherwise the result is an aqueous mixture of ions. An example of a double replacement reaction is AgNO 3 (aq) + NaCl(aq) ÿ NaNO 3 (aq) + AgCl(s). Answer the questions below by circling the number of the correct response Patterns of the Reaction Types Legend: < A and C = metals < B and D = nonmetals Direct combination (synthesis) A + B ÿ AB or AB + D ÿ AD + BD Decomposition AB ÿ A + B Single Replacement (substitution) AB + C ÿ CB + A or AB + D ÿ AD + B Double Replacement (Exchange of Ions) AB + CD ÿ AD + CB 1. Which molecule is nonpolar and has a symmetrical shape? (1) HCI (2) CH 4 (3) H 2 O (4) NH 3 2. Which formula represents a polar molecule? (1) CH 4 (2) Cl 2 (3) NH 3 (4) N 2 3. In which compound does the bond between the atoms have the least ionic character? (1) HF (2) HCl (3) HBr (4) HI 4. Which substance contains a polar covalent bond? (1) Na 2 O (2) Mg 3 N 2 (3) CO 2 (4) N 2 5. In which pair do the members have identical electron configurations? (1) S 2- and Cl - (2) S 0 and Ar 0 (3) K 0 and Na + (4) Cl - and K 0 6. When a chlorine atom reacts with a sodium atom to form an ion, the chlorine atom will (1) lose one electron, (2) gain one electron, (3) lose two electrons, (4) gain two electrons. 7. When a calcium atom loses its valence electrons, the ion formed has an electron configuration that is the same as the configuration of an atom of (1) Cl (2) Ar (3) K (4) Sc 8. Which molecule is nonpolar and contains a nonpolar covalent bond? (1) CCl 4 (2) F 2 (3) HF (4) HCl 9. Which structural formula represents a nonpolar symmetrical molecule? H (1) (2) H C H (3) H F (4) N O H H H H H H

5 TEST 5 REVIEW Page Why is NH 3 classified as a polar molecule? (1) It is a gas at STP. (2) H H bonds are nonpolar. (3) Nitrogen and hydrogen are both nonmetals. (4) NH 3 molecules have asymmetrical charge distributions. 11. Which statement best explains why carbon tetrachloride (CCl 4 ) is nonpolar? (1) Each carbon-chloride bond is polar. (2) Carbon and chlorine are both nonmetals. (3) Carbon tetrachloride is an organic compound. (4) The carbon tetrachloride molecule is symmetrical. 12. What is the total number of oxygen atoms in the formula MgSO 4 7H 2 O? [The represents seven units of H 2 O attached to one unit of MgSO 4.] (1) 11 (2) 5 (3) 7 (4) In the formula for water, H 2 O, the number 2 refers to the number of (1) hydrogens and oxygens, (2) waters, (3) hydrogens only, (4) oxygens only. 14. The number of atoms in Cu 3 (PO 4 ) 2 is (1) 13, (2) 9, (3) 10, (4) Which of the following has the most oxygen? (1) 4Fe 2 O 3 (2) 3Ba 3 (PO 4 ) 2 (3) 2(NH 4 ) 2 CO 3 (4) 3Al(CO 3 ) In which compound is the oxidation state of iron +3? (1) FeCl 2 (2) FeO (3) FePO 4 (4) FeS 2 O What is the formula for a compound of NH 4 and CO 3? (1) NH 4 CO 3 (2) (NH 4 ) 2 CO 3 (3) NH 4 (CO 3 ) 2 (4) NH 3 CO What is the correct formula for copper II nitrate? (1) Cu(NO 3 ) 2 (2) Cu 3 N 2 (3) Cu 2 NO 3 (4) Cu 2 N What is the correct name for BaO? (1) barium oxide (2) barium oxygen (3) barium II oxide (4) barium oxalate 20. The formula for zinc hydroxide is (1) Zn(OH) 2, (2) ZnOH 2, (3) ZnH 2, (4) Zn 2 H. 21. The formula for ammonium carbonate is (1) (NH 3 ) 2 (CO 4 ) 3, (2) NH 2 (CO 3 ) 4, (3) (NH 4 ) 3 CO, (4) (NH 4 ) 2 CO The formula for iron II sulfide is (1) Fe 2 (SO 4 ) 3, (2) FeS, (3) Fe 2 S 3 (4) FeSO The name of the compound CuCO 3 is (1) copper II carbonate, (2) copper I carbonate, (3) copper III carbonate, (4) copper oxide. 24. The formula for barium nitrate is (1) Ba 3 NO 2, (2) Ba 3 N 2, (3) Ba(NO 3 ) 2, (4) BaN. 25. The name of the compound H 2 S is (1) hydrogen II sulfate, (2) hydrogen sulfate, (3) helium I sulfide, (4) hydrogen sulfide. 26. Which is the compound whose formula is P 2 O 5? (1) potassium dioxide (2) dipotassium pentoxide (3) phosphorus dioxide (4) diphosphorus pentoxide 27. The formula for sulfur hexafluoride is (1) SHF, (2) SF, (3) SF 6, (4) S 6 F. 28. The IUPAC name for N 2 O 3 is (1) dinitrogen trioxide, (2) nitrogen oxide, (3) nitrogen trioxide, (4) dinitrogen oxide. 29. The prefix used to show there are four atoms of an element in a binary covalent compound is (1) quadra, (2) recta, (3) hepta, (4) tetra. 30. Which of the following is a binary covalent compound? (1) Na 2 O (2) P 2 S 5 (3) Hg 2 Cl 2 (4) KI 31. What is the name of FeCO 3? (1) iron monocarbon trioxide (2) iron carbonate III (3) iron I carbonate (4) iron II carbonate 32. Which substance will conduct electricity in both the solid phase and the liquid phase? (1) AgCl (2) H 2 (3) Ag (4)HCl 33. What type of bonds are present in a strip of magnesium ribbon? (1) covalent (2) metallic (3) ionic (40 van der Waals 34. Which substance, in the solid state, is the best conductor of electricity? (1) Ag (2) NaCl (3) I 2 (4) CO Which substance exists as a metallic crystals? (1) Ar (2) SiO 2 (3) Au (4) CO Mobile electrons are a distinguishing characteristic of (1) an ionic bond (2) a metallic bond (3) an electrovalent bond (4) a covalent bond 37. Which element consists of positive ions immersed in a "sea" of mobile electrons.? (1) sulfur (2) calcium (3) nitrogen (4) chlorine 38. The molecular mass of CO 2 is the same as the molecular mass of (1) CO (2) C 2 H 6 (3) SO 2 (4) C 3 H Which is an empirical formula? (1) C 2 H 2 (2) Al 2 Cl 6 (3) C 2 H 4 (4) K 2 O 40. A 60. gram sample of LiCl H 2 O is heated in an open crucible until all of the water has been driven off. What is the total mass of LiCl remaining in the crucible? (1) 18 g (2) 42 g (3) 24 g (4) 60 g 41. Which is an empirical formula? (1) CH 2 (2) C 3 H 6 (3) C 2 H 4 (4) C 4 H A compound with a molecular mass of 34 contains hydrogen and oxygen in a ratio of 1:1. The molecular formula of the compound is (1) HO (2) OH (3) H 2 O 2 (4) HOH 43. The empirical formula of a compound is CH. Its molecular mass could be (1) 21 (2) 51 (3) 40 (4) What is the percentage by mass of bromine in CaBr 2? (1) 20% (3) 40% (3) 60% (4) 80%

6 TEST 5 REVIEW Page The percent by mass of Li in LiNO 3 (formula mass = 69) is closest to (1) 6% (2) 10% (3) 18% (4) 20% 46. The percent by mass of oxygen in CO is approximately (1) 73% (2) 57% (3) 43% (4) 17% 47. The mass in amu of CaSO 4 2H 2 O is (1) 172 amu (2) 154 amu (3) 136 amu (4) 118 amu 48. What is the empirical formula of the compound whose molecular formula is C 6 H 12 O 6? (1) C 12 H 24 O 12 (2) C 2 H 4 O 2 (3) C 6 H 12 O 6 (4) CH 2 O 49. The percent by mass of aluminum in Al 2 O 3 is approximately (1) 18.9 (2) 35.4 (3) 47.1 (4) A compound contains nitrogen and oxygen in a ratio of 1:1. The molecular mass of the compound could be (1) 14 (2) 16 (3) 30 (4) The percent by mass of oxygen in Na 2 SO 4 (formula mass = 142) is closest to (1) 11% (2) 22% (3) 45% (4) 64% 52. What is the ratio by mass of sulfur to oxygen in SO 2? (1) 1:1 (2) 1:2 (3) 1:3 (4) 1:4 53. What is the mass in amu of 1.00 molecule of O 2 gas? (1) 11.2 (2) 16.0 (3) 22.4 (4) What is the formula mass of CuSO 4 5H 2 O? (1) 160. amu (2) 178 amu (3) 186 amu (4) 250. amu 55. What is the molecular formula of a compound whose empirical formula is CH 4 and molecular mass is 16? (1) CH 4 (2) C 4 H 8 (2) C 2 H 4 (4) C 8 H The percent by mass of hydrogen in NH 3 is equal to (1) (2) (3) 100 (4) The formula mass of NH 4 Cl is (1) 22.4 amu (2) 53.5 amu (3) 28.0 amu (4) 95.5 amu 58. An example of an empirical formula is ( 1) C 2 H 2, (2) H 2 O 2, (3) C 2 Cl 2, (4) CaCl A 10.0 gram sample of a hydrate was heated until all the water of hydration was driven off. The mass of anhydrous product remaining was 8.00 grams What is the percent of water in the hydrate? (1) 12.5% (2) 20.0% (3) 25.0% (4) 80.0% 60. A compound has the empirical formula NO 2. Its molecular formula could be (1) NO 2, (2) N 2 O, (3) N 4 O 2, (4) N 4 O The percent by mass of oxygen in Ca(OH) 2 (formula mass = 74) is closest to (1) 16, (2) 22, (3) 43, (4) What is the percent by mass of oxygen in NaOH (formula mass = 40.)? (1) 80. (2) 40. (3) 32 (4) A compound whose empirical formula is CH 2 O could be (1) HCOOH, (2) CH 3 OH, (3) CH 3 COOH, (4) CH 3 CH 2 OH. 64. A compound has an empirical formula of CH 2 and a molecular mass of 56. Its molecular formula is (1) C 2 H 4, (2) C 3 H 6, (3) C 4 H 8, (4) C 5 H What is the percent by mass of hydrogen in NH 3 (formula mass = 17.0)? (1) 5.9% (2) 17.6% (3) 21.4% (4) 82.4% 66. The empirical formula of a compound is CH 2 and its molecular mass is 70. What is the molecular formula of the compound? (1) C 2 H 2 (2) C 2 H 4 (3) C 4 H 10 (4) C 5 H The percent by mass of nitrogen in Mg(CN) 2 is equal to (1) 14 / , (2) 14 / , (3) 28 / , (4) 28 / What is the percent by mass of oxygen in Fe 2 O 3 (formula mass = 160)? (1) 16% (2) 30.% (3) 56% (4) 70.% 69. Which formulas could represent the empirical formula and the molecular formula of a given compound? (1) CH 2 O, C 4 H 6 O 4 (2) CHO C 6 H 12 O 6 (3) CH 4, C 3 H 8 (4) CH 2, C 3 H The percent by mass of carbon in CO 2 is equal to (1) 44 / , (2) 12 / , (3) 28 / , (4) 12 / The empirical formula of a compound is CH 4. The molecular formula of the compound could be (1) CH 4, (2) C 2 H 6, (3) C 3 H 8, (4) C 4 H What is the percent by mass of oxygen in CH 3 OH? (1) 50.0 (2) 44.4 (3) 32.0 (4) The approximate percent by mass of potassium in KHCO 3 is (1) 19 %, (2) 24 %, (3) 39 %, (4) 61 % 74. A compound has an empirical formula of CH 2 and a molecular mass of 56. What is its molecular formula? (1) CH 2 (2) C 3 H 6 (3) C 2 H 4 (4) C 4 H What is the percent by mass of hydrogen in CH 3 COOH (formula mass = 60.)? (1) 1.7% (2) 6.7% (3) 5.0% (4) 7.1% 76. What is the percentage by mass of oxygen in CuO? (1) 16% (2) 25% (3) 20% (4) 50% 77. What is the approximate percent composition by mass of CaBr 2 (formula mass = 200)? (1) 20% calcium and 80% bromine (2) 25% calcium and 75% bromine (3) 30% calcium and 70% bromine (4) 35% calcium and 65% bromine 78. Which compound contains the greatest percentage of oxygen by mass? (1) BaO (2) MgO (3) CaO (4) SrO

7 TEST 5 REVIEW Page The precent by mass of oxygen in MgO (formula mass = 40) is closest to (1) 16% (2) 40% (3) 24% (4) 60% 80. The fact that burning wood gives off heat is evidence of a (1) change in mass, (2) chemical change, (3) physical change, (4) phase change. 81. A reaction in which heat is given off is (1) exothermic, (2) endothermic, (3) caloric, (4) acaloric. 82. Which of the following is NOT evidence of a chemical change? (1) release of a gas (2) change in color (3) change in odor (4) change in shape 83. When silver nitrate solution is mixed with sodium chloride solution, a white solid forms immediately with no noticeable change in temperature. Which of the following is a true statement regarding the observed change. (1) The change is probably only physical because there is no change in temperature. (2) The change is probably only physical because no gas is released. (3) The change is probably chemical because a precipitate forms. (4) The change is probably chemical because the reaction is exothermic. 84. The symbol (aq) after a chemical formula means (1) solid or precipitate, (2) liquid, (3) gas, (4) aqueous or dissolved. 85. In the reaction, AgNO 3 + NaCl! AgCl + NaNO 3, the reactants are (1) AgCl and NaNO 3, (2) AgNO 3 and NaCl, (3) Ag and Na, (4) Cl and NO 3. Answer questions by referring to the equation below: 2KClO 3 (s) MnO 2 ( s) 2KCl(s) + 3O 2 (g) 86. The symbol under the yield sign indicates that (1) the reaction is exothermic, (2) the reaction is endothermic, (3) a solid precipitate forms, (4) heat is a product of the reaction. 87. MnO 2 (s) is written above the yield sign because MnO 2 (s) is (1) a reactant, (2) a product, (3) neither a reactant nor a product, (4) both a reactant and a product. For each of the reactions described in questions 88-94, write the correct number to indicate whether the reaction type is (1) DECOMPOSITION, (2) DIRECT COMBINATION, (3) SINGLE REPLACEMENT, or (4) DOUBLE REPLACEMENT 88. A reaction occurs in which only one reactant is present. 89. A metal reacts with an acid. (2Fe + 6HCl! 2FeCl 3 + 3H 2 ) 90. Magnesium burns. 91. Two salt solutions react with each other. 92. Two elements unite to form a compound. 93. A compound breaks down. 94. HCl + NaOH! NaCl + H 2 O Answers