Chemical Reactions Chapter 9 Section 1

Indications of a Chemical Reaction Chemical changes alter the molecular structure of the substances involved. These observations suggest that a chemical change has occurred: 1. Evolution of heat(temp change) and light. 2. Production of a gas. 3. Formation of a precipitate (solid). 4. Color change.

Chemical Equations Used to represent chemical reactions (rx). Chemical equations have the following characteristics: a. equation must represent known facts. b. equation must contain the correct formulas for the reactants and products. c. law of conservation of mass must be satisfied.

Chemical Equations Let’s diagram the following equation: 2C 2 H 2 (g) + 5O 2 (g)  4CO 2 (g) + 2H 2 O(g) Reactants Products *  is read as yield (also produce or form) *number that appears in front of a formula is called a coefficient. It represents relative number of moles of a substance. *(g) refers to the state of matter of the substance.

The following is a list of symbols used in chemical equations: = reversible rx (aq) = aqueous (s) = solid phase (cr) crystalline (l) = liquid phase (g) = gas phase or = reactants are heated = pressure at which rx took place = temp at which rx took place = formula of catalyst (manganese dioxide) heatΔ 2 atm 0˚ MnO 2

*add to the right of rx conditions (s) if: single metal, solid, ash, ribbon, salt, crystalline. (g) if: any “big 7”, CO 2, CO. (aq) if: all acids, dissolved in water, solution. electric if electricity is added. *If substance does not fit above criteria, do not put any state of matter.

Balancing Chemical Equations Balancing Steps: Step 1: Identify reactants and products. Write a formula equation by substituting correct formulas for the names of reactants and products. EX: Water breaks down into hydrogen and oxygen gases when heated. H 2 O(l)  H 2 (g) + O 2 (g)

Diatomic Molecules There are seven elements that are not found as single atoms in nature. They are found bound to other elements or to themselves in molecular compounds. They are referred to as the “Big 7” because they form the shape of a 7 on the periodic table. These elements are: Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Bromine, and Iodine.

Balancing Chemical Equations Step 2: Balance the formula equation according to the law of conservation of mass. a. Balance the different types of atoms one at a time changing only the coefficients. b. Balance unique atoms first. c. Balance polyatomic ions if they appear on both sides as single units. d. Balance H atoms and O atoms last.

Write after d. **If an odd # on one side and even on another, make odd even with a 2**

EX: H 2 O(l)  H 2 (g) + O 2 (g) *Is this balanced? Why or Why not? Not balanced. Because there are more oxygen atoms on the product side. This violates the law of conservation of mass.

Balancing Chemical Equations H 2 O(l)  H 2 (g) + O 2 (g) How to balance: 1. There are no unique atoms to balance first. 2. H atoms are already balanced. 3. Balance the O atoms using only coefficients. 2H 2 O(l)  H 2 (g) + O 2 (g)

4. By adding a coefficient of 2 to H 2 0 to balance the O atoms, the H atoms became unbalanced. Balance the H atoms. 2 H 2 O(l)  2 H 2 (g) + O 2 (g)

Step 3: Count atoms to be sure that the equation is balanced. 2 H 2 O(l)  2 H 2 (g) + O 2 (g) (4H + 2O) = (4H) + (2O) 2:2:1 ratio

Avoid the following when balancing: 1. Writing incorrect chemical formulas. 2. Balancing equations by changing the subscripts. 3. Coefficients do not represent the smallest whole number ratio. EX: 4 H 2 O(l)  4 H 2 (g) + 2 O 2 (g) 4:4:2  2:2:1 * Factor out the LCM.

Hint to balancing hydrocarbons: -hydrocarbons are C x H x or C x H x OH + O 2 -place a coefficient of 2 in front of hydrocarbon and then balance. -may have to factor 2 out when finished.

Practice Balancing 1. Zn(s) + HCl(aq)  ZnCl 2 (aq) + H 2 (g) Zn(s) + 2 HCL(aq)  ZnCl 2 (aq) + H 2 (g) 2. HNO 3 (aq) + Mg(OH) 2 (s)  Mg(NO 3 ) 2 (aq) + H 2 O(l) 2 HNO 3 (aq) + Mg(OH) 2 (s)  Mg(NO 3 ) 2 (aq) + 2 H 2 O(l)

Types of Chemical Reactions There are 5 basic types of chemical rxs. 1. Synthesis rx 2. Decomposition rx 3. Single replacement (displacement) rx 4. Double replacement (displacement) rx 5. Combustion rx

Synthesis Rxs In a synthesis rx, two or more reactants combine to form a single new product. Represented by: A + X  AX EX: 2Mg(s) + O 2 (g)  2MgO(s) 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) 2Na(s) + F 2 (g)  2NaF(s)

Decomposition Rxs In a decomposition rx, a single reactant undergoes a rx that produces two or more simpler products. Represented by: AX  A + X EX: 2H 2 O(l)  2H 2 (g) + O 2 (g) 2KClO 3 (s)  2KCl(s) + 3O 2 (g) H 2 CO 3 (aq)  CO 2 (g) + H 2 O(l)

Decomposition Rxs A metal carbonate breaks down to form a metal oxide and carbon dioxide gas. CaCO 3 (s)  CaO (s) + CO 2 (g) A metal hydroxide (except those metals in Group 1) break down to form a metal oxide and water. Ca(OH) 2 (s)  CaO(s) + H 2 O A metal chlorate breaks down to form a metal chloride and oxygen gas. 2KClO 3 (s)  2KCl(s) + 3O 2 (g)

Single Replacement Rxs In a single replacement rx, one elemental reactant replaces a similar type element in a reactant compound to form a new product compound and a new elemental product. Represented by: A + BX  AX + B EX: 3Fe(s) +4H 2 O(l)  Fe 3 O 4 (s) + 4H 2 (g) 2Al(s) + 3Pb(NO 3 ) 2 (aq)  3Pb(s) + 2Al(NO 3 ) 3 (aq)

Wow! He’s the bomb. Stupid Jerk! Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g)

Single Replacement Rxs The ability of an element to react is referred to as the element’s activity. The more readily an element reacts with other substances, the greater its activity is. An activity series is a list of elements organized according to the ease with which the elements undergo single replacement reactions.

Activity Series of Elements In an activity series, the most active metals are placed at the top. An active metal can replace each of the elements below it but not above it. Activity series are used to help predict whether certain rxs will occur.

Using the Activity Series of Elements Aluminum is more reactive than Zinc so the following rx will occur: 2Al(s) + 3ZnCl 2 (aq)  3Zn + 2AlCl 3 (aq) Cobalt is less reactive than Sodium so the following rx will not occur: Co(s) + 2NaCl(s)  CoCl 2 (s) + Na 2 (s) This rx should be written as: Co(s) + NaCl(s)  no reaction

Using the Activity Series of Elements Use the activity series to predict whether each of the following rxs will occur: 1. Zn(s) + H 2 O(l)  Will Occur 2. Cu(s) + HCl(aq)  No Reaction 3. Cd(s) + Pb(NO 3 ) 2 (aq)  Will Occur

Single Replacement Rxs Indicate whether the metal or nonmetal is being replaced. EX: Mg + FeCl 2  Fe + MgCl 2 SR-metal Cl 2 + KI  KCl + I 2 SR-nonmetal

Double Replacement Rxs In a double replacement rx, the ions of two reactant compounds exchange places in an aqueous solution to form two new product compounds. Represented by: AX + BY  AY + BX EX: 2KI(aq) + Pb(NO 3 ) 2 (aq)  PbI 2 (s) + 2KNO 3 (aq)

Combustion Rxs In a combustion rx, a hydrocarbon/alcohol (consisting of carbon and hydrogen) combines with oxygen, releasing a large amount of energy in the form of heat and light. Represented by: ( C x H x OH) or C x H x + O 2  CO 2 + H 2 O EX: C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(g) *combustion of hydrocarbons always result in the formation of CO 2 and water.

Identification of Rx Types Synthesis Combustion (combust) Double Replacement (DR) Single Replacement (SR) -Indicate whether metal or nonmetal Decomposition (decomp) -Indicate metal carbonate, metal hydroxide, or metal chlorate. -If none of the above, general.

Identifying Rxs Use your knowledge of rx types to classify the following rxs: 1. NH 3 (g) + HCl(g)  NH 4 Cl(s) 2. C 2 H 5 OH(l) + O 2 (g)  CO 2 (g) + H 2 O(g) 3. 2KClO 3 (s)  2KCl(s) + 3O 2 (g) 4. Zn + H 2 SO 4  ZnSO 4 + H 2 5. S(s) + O 2 (g)  SO 2 (g) 6. AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq)

Reaction Prediction Steps Step 1: Find the reactants and write them in formula form. Step 2: Identify the type of rx based on the reactants. Step 3: Write products -positive oxidations bond with negative oxidations. Step 4: Balance equation.

Hints: 1. Know the 3 types of decomp rxs and what products they produce. 2. Combustion products, CO 2 + H 2 O 3. H 2 O as a reactant, HOH 4. 2 single elements always synthesis 5. SR rxs, use Activity Series 6. If you need oxidation for metals, darker number on periodic table is most common ion so use that.

Predicting Products Use your knowledge of rx types to predict the products of the following rxs: 1. Na + H 2 O [HOH]  NaOH + H 2 2. HNO 3 + Ca(OH) 2  Ca(NO 3 ) 2 + H 2 O 3. Mg + O 2  MgO

Predicting Products 4. Ca + Cl 2  CaCl 2 5. C 6 H 14 + O 2  CO 2 + H 2 O 6. Br 2 + KI  KBr + I 2 7. MgO + Pb  No Reaction

8. HgO  Hg + O 2 9. Na + Cl 2  NaCl 10. NaI + Cl 2  NaCl + I 2 *Now, balance these rxs to make them correct. Δ

In Summary Five observations that suggest a chemical reaction is taking place are the evolution of light and heat, the production of gas, a change in color, and the formation of a precipitate. Reactants are the starting substances in a reaction. Products are the substances resulting from a reaction.

In Summary Physical states of substances are shown by; (g) = gas, (l) = liquid, (s) = solid, and (aq) = aqueous, which indicates substance is dissolved in water. Balancing an equation means adjusting coefficients so that there is the same number of atoms of each element on the left and right sides of the equation.

In Summary There are 5 basic types of chemical reactions: 1. Synthesis Rx: A + X  AX 2. Decomposition Rx: AX  A + X 3. Single Replacement Rx: A + BX  AX + B 4. Double Replacement Rx: AX + BY  AY + BX 5. Combustion Rx: C x H x + O 2  CO 2 + H 2 O

In Summary Activity series list the elements in order of their chemical reactivity and are useful in predicting whether a chemical reaction will occur. Products of reactions can be predicted by a general knowledge of the reaction types.

In Summary For the exam you must be able to: 1. Make a formula equation out of a word equation. (or the opposite) 2. Identify the reaction type. 3. Predict the products for the reaction. 4. Balance the formula equation. Be familiar with the entire contents of the notepacket and chapter in book.

A C6H12O6 production