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Balancing Chemical Equations

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1 Balancing Chemical Equations
Types of Chemical Reactions And Balancing Chemical Equations

2 Ex. 2H2 (g) + O2 (g) ------> 2H 2 O (l)

3 2H2 (g) + O 2 (g) -------> 2H2 O (l)
This reaction is interpreted as follows: 2 moles (or molecules) of hydrogen gas react with 1 mole (or molecule) of oxygen gas to form 2 moles (or molecules) of liquid water.

4 A chemical reaction consists of the following symbols:
Reactants are on the left side of the arrow. Products are on the right side of the arrow. The arrow means “yields” much like the equal sign means “equals”. (g), (s), (l), (aq) ; these represent the physical state of each substance. Gas, solid, liquid, or aqueous, respectively.

5 reaction symbols continued:
A down arrow represents a precipitate formed in the products. An up arrow represents a gas evolved in the products. A D means that energy or heat was added to the reaction.

6 Balancing Chemical Equations
The Law of Conservation of Matter states that the mass of the products coming out of a reaction must equal the mass of the reactants going into the reaction. Simply stated, the number of moles of a substance on the left of the arrow must equal the number of moles of that substance on the right of the arrow.

7 Balancing continued.... Coefficients are placed in front of elements or compounds in the equation to balance numbers of moles (or molecules) of the substance. Never use subscripts to balance an equation. Subscripts are ONLY used to write correct chemical formulas. Balancing is achieved by trial and error.

8 Ex. BiCl3 + H2S ------> Bi2S3 + HCl
Notice that there is one Bi on the left and two Bi on the right. There are 3 Cl on the left and one Cl on the right. There are 2 H on the left and one H on the right. There is one S on the left and 3 S on the right. continued.....

9 Balancing continued...... BiCl3 + H2S ------> Bi2S3 + HCl
If a coefficient of 2 is placed in front of BiCl3 , the Bi is then balanced but now there are 6 Cl on the left (2BiCl3 ) because the coefficient is multiplied by all elements in the compound. Fix this by placing a 6 in front of the HCl on the right. Now we have 6 H on the right. Place a 3 in front of H2S on the left. Fini !

10 The balanced chemical equation..
2 BiCl3 + 3 H2S --->Bi2S3 +6 HCl 2Bi, 6 Cl, 6 H, 3S ---> 2Bi, 6Cl, 6H, 3S Reactants = Products

11 2Al + 3F2 ----> 2AlF3 Ex.2 Al + F2 ----> AlF3
Hint ; when you see two different subscripts on the same element, find the least common multiple. F has a 2 on the left and a 3 on the right. 2 and 3 both fit into 6 so .... 2Al + 3F > 2AlF3 2Al, 6 F, > 2Al, 6 F Reactants = Products

12 Ex.3 Ca(OH)2 + HC2H3O2 ---> Ca(C2H3O2)2 + H2O
The polyatomic ions in this equation are hydroxide (2 on the left and one on the right) and acetate (one on the left and 2 on the right). Hint ; treat polyatomic ions as units that do not separate. This makes them easier to balance. Note that H2O is the same as HOH Ca(OH)2 + 2HC2H3O2 ---> Ca(C2H3O2)2 +2HOH

13 Types of Chemical Reactions
Combination Reaction (also know as a synthesis reaction)- two or more elements and/or compounds combine to form one product. Decomposition Reaction - A compound decomposes into two or more of its substituent elements and/or compounds. continued...

14 Types of Chemical Reactions continued..
Double Replacement Reactions - Where the positive ions in two compounds exchange places. There are several types of double replacement reactions. Single Replacement Reactions (oxidation / reduction reactions)- An element and a compound react. In so doing, either the metals or the non metals exchange places.

15 Reaction Types continued...
Combustion Reactions - The reaction of a substance with Oxygen. Acid / Base Reactions - An acid reacts with a base to form a salt and water. This is a type of double replacement reaction.

16 Ex.1 Combination Reaction
2K(s) + Cl2(g) ---> 2KCl(s) In this reaction, solid potassium combines with chlorine gas to form solid potassium chloride.

17 Ex.2 Decomposition Reaction
2 KClO >2 KCl + 3 O2 In this reaction, potassium chlorate decomposes into potassium chloride and oxygen gas.

18 Ex.3 Double Replacement Reaction
2AgNO3 + H2S ---->Ag2S(s)+ 2HNO3 In this reaction aqueous silver nitrate reacts with hydrogen sulfide gas to produce a precipitate of solid silver sulfide and aqueous nitric acid.

19 Double Replacement continued...
In order for a double replacement reaction to occur, one of the following must be true: An insoluble product must form (that is, a precipitate) A neutral, molecular product must form (like water)

20 Solubility Rules Soluble Exceptions Soluble Exceptions Ammonium salts
none Acetates Ag+, Al +3 Sulfates Group IA Ba +2 , Sr +2 , Ca +2 , Pb +2 , Ag+, Hg +2 , Hg +2 some Li+ compounds salts Nitrates none Halides Ag+, Hg +2 , Pb +2

21 Ex.4 Single Replacement Reaction (REDOX)
Zn(s) + Pb(NO3)2 ----> Zn(NO3)2 + Pb(s) In this reaction, solid zinc replaces aqueous lead in the lead nitrate, to form aqueous zinc nitrate. Solid lead is displaced. Zn must be more reactive than Pb in order to replace it. continued...

22 continued... In order for one substance to replace another in a single replacement reaction, that substance must be more reactive than the one that it will replace. We use a table called the activity series to determine if one substance is more active than another. increasing activity > Ag, Cu, H, Pb, Ni, Zn, Al, Mg, Na, Ca, Ba, K, Li

23 Single Replacement / Redox continued...
Zn(s) + Pb(NO3)2----> Zn(NO3)2 + Pb(s) An important feature of this type of reaction is that the charges (oxidation states ) on some of the elements change. Zn went from having a zero charge to having a +2 charge. Lead went from having a +2 charge to having a zero charge. The number of electrons lost by one substance must equal the number of electrons gained by the other substance.

24 Single Replacement /Redox continued....
Electrical current is defined as the flow of electrons. In an oxidation /reduction reaction, (Redox), electrons are exchanged from one substance to another, thus a flow of electrons is produced. The substance that gains the electrons is said to be reduced. The substance that loses electrons is said to be oxidized. continued...

25 continued... Li(s) + AlCl3(aq) ----> LiCl(aq) + Al(s)
Ex. Li(s) + AlCl3(aq) ----> LiCl(aq) + Al(s) In this reaction: Li is more active than Al so it replaces Al. Li goes from having a zero charge to +1, therefore it is oxidized. (It got more Positive) Al went from a +3 to a zero therefore it was reduced. (It got more negative, gained electrons)

26 COMBUSTION REACTIONS C 3 H 8 + 5 O 2 -----> 3 CO 2 + 4 H 2 O
In this reaction a hydrocarbon (that is a compound composed only of carbon and hydrogen) reacts with oxygen to form carbon dioxide and water. When a hydrocarbon is burned in oxygen (that is combustion) the products are always carbon dioxide and water.

27 Acid / Base Reactions Acid / Base reactions are a special type of
double replacement reactions. We must first be able to recognize an acid and a base. We will start with a simple Bronstedt definition for now. An acid gives up a hydrogen in the reaction and the base accepts that hydrogen. A hydrogen is referred to as a proton because it lost its electron and has a +1 charge. continued....

28 Acid / Base continued... Lets look at an example ....
So, we can say that an acid is a proton donar and a base is a proton acceptor. The products of this type of reaction are always a salt (which is simply an ionic compound) and water. Lets look at an example ....

29 H 3 PO 4 + 3 NaOH ----> Na 3 PO 4 + 3HOH
continued... Notice that the Na replaced the Hydrogens on the H 3 PO 4 . The H 3 PO 4 gave up its hydrogens therefore it is the acid. Also note that the new product was not H3OH but instead 3 HOH (or 3 H2O). This is because the OH has a -1 charge and a hydrogen without its electron (a proton) has a +1 charge. The correct formula is H+1 OH-1 (written HOH or H2O) continued....

30 H3PO4 + 3NaOH ---> Na3PO4 + 3HOH
For now, a simple way to recognize an acid is that it is a compound that has hydrogen as the first atom. A base is a compound composed of a metal hydroxide. continued...

31 Acids and Bases continued...
examples of acids: HCl , H3PO4 , H2SO4 ,H2CO3 , HNO3 , HC2H3O2 examples of bases: NaOH , Ca(OH)2 , KOH , LiOH ,Mg(OH)2 Al(OH)3

32 balance a chemical equation
You should be able to..... balance a chemical equation define and use the symbols in a chemical equation Identify and give examples of the different types of chemical reactions Identify and give examples of acids and bases

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