Chemistry 2

Writing and Balancing Chemical Equations

Purpose:

The purpose of this lab is to practice writing and balancing chemical equations.

Introduction to Chemical Equations:

1) Chemical reactions are represented by chemical equations.

2) All chemical reactions contain reactants, products, the phase for each reactant and product and an arrow.

a. The reactants are species present on the left side of the arrow.

b. The products are species present on the right side of the arrow.

c. The physical state (phase) of each species is represented by the following:

(aq) – aqueous (dissolved in water)

(s) – solid

(l) – liquid

(g) – gas

3) An example of a chemical reaction, in which aqueous acetic acid (HC2H3O2) reacts with solid sodium bicarbonate (NaHCO3) to produce aqueous carbonic acid (H2CO3) and aqueous sodium acetate (NaC2H3O2), is as follows:

HC2H3O2 (aq) + NaHCO3 (s) ( H2CO3 (aq) + NaC2H3O2 (aq)

Carbonic acid readily decomposes to produce liquid water (H2O) and gaseous carbon dioxide (CO2). The reaction is as follows:

HC2H3O2(aq) + NaHCO3(s) ( H2O(l) + CO2(g) + NaC2H3O2 (aq)

Introduction to Balancing Chemical Equations:

1) All chemical equations must contain the same number of each type of atom on both sides of the arrow.

2) An equation is balanced by changing the coefficients (or numbers placed in front of a compound). This is done in a trial and error fashion.

3) When balancing an equation, only change the coefficients NEVER subscripts. If no coefficient is present, a 1 is assumed

4) Subscripts tell you how many atoms you have. If a number is not present, then there is one atom of that element.

a. Consider sulfuric acid (H2SO4). There are 2 hydrogen atoms, one sulfur atom, and 4 oxygen atoms.

5) When counting atoms, the subscript is multiplied by the number of atoms inside parentheses.

a. Consider calcium phosphate (Ca3(PO4)2) There are 3 calcium atoms, 2 phosphorous atoms, and 8 oxygen atoms.

b. If the number 2 served as the coefficient in front of calcium phosphate (2 Ca3(PO4)2), there would be 6 Ca atoms, 4 P atoms, and 16 O atoms present.

6) Consider the reaction between aqueous calcium hydroxide and aqueous phosphoric acid to produce solid calcium phosphate and liquid water.

1st – Write the chemical equation.

Ca(OH)2(aq) + H3PO4(aq) ( Ca3(PO4)2(s) + H2O(l)

2nd – Make a table that will allow you to count and keep track of the atoms. If a polyatomic ion appears on both sides of the equation, it can be balanced as a whole unit. If not, each atom must be balanced individually.

Reactants Side	Element	Products Side
1	Ca	3
2	O	1
5	H	2
1	PO4	2

The number of atoms, of each element, on the left does not equal the number of atoms, of each element, on the right. This equation is not balanced.

3rd – Pick out the compound with the most atoms and choose an element that is not hydrogen or oxygen or a polyatomic ion. Starting with calcium, there are 3 Ca atoms on the right and 1 Ca atom on the left. To balance the Ca atoms, place a 3 in front of Ca(OH)2.

3 Ca(OH)2(aq) + H3PO4(aq) ( Ca3(PO4)2(s) + H2O(l)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
3	Ca	3
6	O	1
9	H	2
1	PO4	2

4th – Balance the polyatomic ion, PO4. There are two phosphates on the right and one phosphate on the left. Put a 2 in front of H3PO4.

3 Ca(OH)2(aq) + 2 H3PO4(aq) ( Ca3(PO4)2(s) + H2O(l)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
3	Ca	3
6	O	1
12	H	2
2	PO4	2

The Ca and PO4 ions are both balanced, now balance the hydrogen and oxygen atoms.

5th – Balance H and then O. There are 12 H atoms on the left and 2 H atoms on the right. Put a 6 in front of H2O on the right to balance the H atoms.

3 Ca(OH)2(aq) + 2 H3PO4(aq) ( Ca3(PO4)2(s) + 6 H2O(l)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
3	Ca	3
6	O	6
12	H	12
2	PO4	2

The equation is balanced!

7) Consider the reaction for the decomposition of aqueous nitric acid, which yields gaseous nitrogen dioxide, liquid water, and gaseous oxygen.

1st – Write out the chemical equation.

HNO3(aq) ( NO2(g) + H2O(l) + O2(g)

2nd – Make a table that will allow you to count and keep track of the atoms. If a polyatomic ion appears on both sides of the equation, it can be balanced as a whole unit. If not, each atom must be balanced individually.

Reactants Side	Element	Products Side
1	H	2
1	N	1
3	O	5

The number of atoms, of each element, on the left does not equal the number of atoms, of each element, on the right. This equation is not balanced.

3rd- Pick a single element in the largest compound that is not H, O or a polyatomic ion. In this case, NO3 is a polyatomic ion, but is not present on both sides of the equation. Therefore, each atom must be counted. HNO3 is the largest compound and N is first atom to balance. There is 1 N atom on the left and 1 N on the right. N is balanced, so move on to H.

4th - There is one H atom on the left and 2 H atoms on the right. Put a 2 in front of HNO3 to balance the H atoms.

2 HNO3(aq) ( NO2(g) + H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
2	H	2
2	N	1
6	O	5

5th - The N atoms are no longer balanced; there are 2 N atoms on the left and 1 N atom on the right. Put a 2 in front of NO2 to balance the N atoms.

2 HNO3(aq) ( 2 NO2(g) + H2O(l) + O2(g)

Reactants Side	Element	Products Side
2	H	2
2	N	2
6	O	7

6th – Move on to balance the O atoms. There are 6 O atoms on the left and 7 on the right. Since O is present in all 3 products, the easiest thing to do is change the coefficients in front of the lone reactant. Try a 3 in front of HNO3.

3 HNO3(aq) ( 2 NO2(g) + H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
3	H	2
3	N	2
9	O	7

The N atoms are no longer balanced, so place a 3 in front of NO2.

3 HNO3(aq) ( 3 NO2(g) + H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
3	H	2
3	N	3
9	O	9

The N and O atoms are now balanced, but the H atoms are not. Try a 4 in front of HNO3.

4 HNO3(aq) ( 3 NO2(g) + H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
4	H	2
4	N	3
12	O	9

To balance the N put a 4 in front of NO2.

4 HNO3(aq) ( 4 NO2(g) + H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
4	H	2
4	N	4
12	O	11

Now the N atoms are balanced and the H atoms can easily be balanced by placing a 2 in front of H2O.

4 HNO3(aq) ( 4 NO2(g) + 2 H2O(l) + O2(g)

Re-count the atoms and determine which atom needs to be balanced.

Reactants Side	Element	Products Side
4	H	4
4	N	4
12	O	12

The equation is balanced!

Procedure:

Part 1: Writing Chemical Equations

1) Write a chemical equation pertaining to each of the following descriptions. Balance each chemical equation (if necessary). Include the physical state of each species.

Part 2: Balancing Chemical Equations

1) Balance the following chemical equations using coefficients.

Part 3: Writing and Balancing Chemical Equations

1) Determine the chemical formula for each reactant and product, write a balanced chemical equation pertaining to each of the following descriptions. Include the physical state of each species.

Conclusion

1) Complete parts one, two and three in your lab notebook.

2) Explain why chemical equations must be balanced.

3) Provide examples of observations that provide evidence for a chemical change.

4) What do the terms in parentheses represent in a chemical equation? Why are they important?

Data Table

Part 1

1) Solid potassium bromide (KBr) reacts with aqueous sulfuric acid (H2SO4) to produce gaseous hydrogen bromide (HBr) and solid potassium hydrogen sulfate (KHSO4).

2) Solid calcium carbonate (CaCO3) is heated to produce solid calcium oxide (CaO) and gaseous carbon dioxide (CO2).

3) Gaseous propane (C3H8) is burned in air (O2) to produce gaseous carbon dioxide (CO2) and gaseous water (H2O).

4) Aqueous zinc chloride (ZnCl2) reacts with aqueous silver nitrate (AgNO3) to produce solid silver chloride (AgCl) and aqueous zinc nitrate (Zn(NO3)2).

Part 2

1) C2H6(g) + O2(g) ( CO2(g) + H2O(g)

2) Hg(NO3)2(aq) + HBr(aq) ( HgBr2(s) + HNO3(aq)

3) Al(s) + Cl2(g) ( AlCl3(s)

4) CO2(g) + H2O(l) ( C6H12O6(aq) + O2(g)

5) Fe(s) + HCl(aq) ( FeCl3(aq) + H2(g)

6) Na2CO3 (aq) + CoCl2(aq) ( CoCO3(s) + NaCl(aq)

7) Li3PO4(aq) + Mg(ClO3)2(aq) ( LiClO3(aq) + Mg3(PO4)2(s)

8) BaO(s) + HNO3(aq) ( Ba(NO3)2(aq) + H2O(l)

9) K(s) + O2(g) ( K2O(s)

10) ZnCl2(aq) + (NH4)2S (aq) ( NH4Cl (aq) + ZnS (s)

11) Ni(s) + S8(s) ( NiS(s)

12) CH4(g) + O2(g) ( CO2(g) + H2O (g)

13) Cu2O(s) ( Cu(s) + O2(g)

14) Mg(s) + CuNO3(aq) ( Mg(NO3)2(aq) + Cu(s)

15) Li(s) + F2(g) ( LiF(s)

16) Al2(SO4)3(aq) + NaOH(aq) ( Al(OH)3(s) + Na2SO4(aq)

17) HI(aq) + Ca(OH)2(aq) ( CaI2(aq) + H2O(l)

18) Mg(s) + N2(g) ( Mg3N2(s)

19) H2SO3(aq) + KOH(aq) ( K2SO3(aq) + H2O(l)

20) Fe(C2H3O2)3(aq) + Ba(OH)2(aq) ( Fe(OH)3(s) + Ba(C2H3O2)2(aq)

21) Pb(NO3)2(aq) + Na3PO4(aq) ( Pb3(PO4)2(s) + NaNO3(aq)

22) Fe(NO3)3(s) ( Fe2O3(s) + NO2(g) + O2(g)

23) H3PO4(aq) + Ca(s) ( Ca3(PO4)2(s) + H2(g)

24) K(s) + H2O(l) ( KOH(aq) + H2(g)

25) C4H10(g) + O2(g) ( CO2(g) + H2O(g)

Part 3

1) Aqueous chromium (III) nitrate reacts with aqueous sodium carbonate to produce solid chromium (III) carbonate and aqueous sodium nitrate.

2) Elemental phosphorous (P4(s)) reacts with gaseous oxygen to produce solid tetraphosphorous decaoxide.

3) Aqueous cadmium chloride reacts with solid aluminum to produce aqueous aluminum chloride and solid cadmium.

4) Aqueous aluminum sulfate reacts with aqueous sodium chromate to produce solid aluminum chromate and aqueous sodium sulfate.

5) Aqueous carbonic acid reacts with aqueous lithium hydroxide to produce liquid water and aqueous lithium carbonate.

6) Solid potassium oxide is heated to produce solid potassium and gaseous oxygen.

7) Aqueous hydrochloric acid reacts with aqueous potassium carbonate to produce aqueous potassium chloride, liquid water and gaseous carbon dioxide.

8) Aqueous sodium bromide reacts with aqueous silver sulfite to produce solid silver bromide and aqueous sodium sulfite.

9) Solid copper reacts with gaseous nitrogen to produce solid copper (II) nitride.

10) Solid cadmium reacts with aqueous nitric acid to produce aqueous cadmium nitrate and hydrogen gas.

Jones

Introductory Chemistry Lab