Chapter 7 - Chemical Reactions

Evidence of a Chemical Reaction

If we could see the atoms and molecules that compose matter, we could easily identify a chemical reaction:

Atoms combine with other atoms to form compounds. New molecules form. The original molecules decompose. Atoms in one molecule change places with atoms in another.

WE MUST LOOK FOR OTHER EVIDENCE !!

Heat evolution

NOT Evidence of a Chemical Reaction

Changes occurring at the atomic and molecular level determine whether a chemical reaction

has occurred.

Only chemical analysis that shows that the initial substances have changed into other substances conclusively proves that a chemical reaction has occurred.

Chemical reactions may occur without any obvious signs, yet chemical analysis may show that a reaction has indeed occurred.

Evidence of a Chemical Reaction

Chemical ReactionsReactions involve rearrangement and exchange

of atoms to produce new pure substances.

Reactants Products

Chemical EquationsShorthand way of describing a reaction

Provides information about the reaction

1. formulas of reactants and products 2. states of reactants and products 3. relative numbers of reactant and product

molecules

Combustion of Methane“Methane gas reacts with oxygen gas to produce carbon

dioxide gas and gaseous water.”

CH4(g) + O2(g) ➜ CO2(g) + H2O(g)

This equation reads “1 molecule of CH4 gas combines with 1 molecule of

O2 gas to make 1 molecule of CO2 gas and 1 molecule of H2O gas.”

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A reminder: Formulas for reactants and products must be correct!!

C + O2 -----------------> CO2 carbon dioxide

C + O2 -----------------> CO carbon monoxide

A reminder: Certain elements must always be written in their diatomic forms!!

N2, H2, O2, F2, Cl2, Br2, I2

What about conservation of mass ??

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1 C + 4 H + 2 O 1 C + 2 O + 2 H + OX

Combustion of Methane, BalancedTo show the reaction obeys the Law of Conservation of Mass,

the equation must be balanced.

“1 molecule of CH4 gas combines with 2 molecules of O2 gas to make 1 molecule of CO2 gas and 2

molecules of H2O gas.”

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We balance the equation by changing coefficients.

CH4(g) + O2(g) ➜ CO2(g) + H2O(g)

CH4(g) + 2 O2(g) ➜ CO2(g) + 2 H2O(g)

Equations must be balanced by mass. ( The number of reactant atoms of each element must equal the number of product atoms of each element.)

1. C + O2 -------------------> CO

2. N2 + H2 -------------------> NH3

2C + O2 -------------------> 2CO

N2 + 3H2 -------------------> 2NH3

Symbols used to indicate state after chemical:

(g) = gas; (l) = liquid; (s) = solid (aq) = aqueous = dissolved in water

Energy symbols used above the arrow for conditions for reactions:

Δ = heat hν = light shock = mechanical elec = electrical

Symbols Used in Equations

General Rules for Balancing Equations

1. Balance elements other than H, O, and polyatomic ions (which do not change in the equation)

Generally, start with the element that appears the most.

2. Balance the polyatomic ions

3. Balance the H atoms

4. Balance the O atoms

5. Check the coefficients for the lowest whole number ratio

}** You may have to back up after each of these steps if the results of a previous step are changed

How to Write Balanced Chemical Equations

1. Write a equation by writing correct chemical formulas for each of the reactants and products.

2. If an element occurs in only one compound on both sides of the equation, balance it first. If there is more than one such element, balance metals before nonmetals.

3. If an element occurs as a free element on either side of the chemical equation, balance it last. Always balance free elements by adjusting the coefficient on the free element.

4. If the balanced equation contains coefficient fractions, change these into whole numbers by multiplying the entire equation by the appropriate factor

5. Check to make certain the equation is balanced. 6. Remember, change only the coefficients to balance a chemical

equation; never change the subscripts.

Aluminum metal reacts with oxygen gas to produce solid aluminum oxide.

Al (s) + O2 (g) -------------------> Al2O3 (s)

2 Al (s) + O2 (g) --------------------> Al2O3 (s)

2 Al (s) + 3 O2 (g) --------------------> 2 Al2O3 (s)

4 Al (s) + 3 O2 (g) --------------------> 2 Al2O3 (s)

Al (s) + O2 (g) -------------------> Al2O3 (s)

Al (s) + 3 O2 (g) --------------------> 2 Al2O3 (s)

4 Al (s) + 3 O2 (g) --------------------> 2 Al2O3 (s)

Solid phosphorous (P4) reacts with hydrogen gas to produce phosphorous trihydride.

P4 (s) + H2 (g) ----------> PH3 (g)

P4 (s) + H2 (g) ----------> 4 PH3 (g)

P4 (s) + 6 H2 (g) ----------> 4 PH3 (g)

P4 (s) + H2 (g) ----------> PH3 (g)

P4 (s) + 3 H2 (g) ----------> 2 PH3 (g)

½ P4 (s) + 3 H2 (g) ----------> 2 PH3 (g) P4 (s) + 6 H2 (g) ----------> 4 PH3 (g)

Solid potassium chlorate decomposes to produce oxygen gas and potassium chloride.

KClO3 (s) -------------------> O2 (g) + KCl (s) 2 KClO3 (s) -------------------> 3 O2 (g) + KCl (s)

2 KClO3 (s) -------------------> 3 O2 (g) + 2 KCl (s)

H2SO4 (aq) + NaCN (s) ------> Na2SO4 (aq) + HCN(g)

H2SO4 (aq) + 2 NaCN (s) ------> Na2SO4 (aq) + HCN(g)

H2SO4 (aq) + 2 NaCN (s) ------> Na2SO4 (aq) + 2 HCN(g)

Aqueous sulfuric acid reacts with solid sodium cyanide to produce aqueous sodium sulfate and hydrogen cyanide gas.

K3PO4 (aq) + Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + KNO3 (aq)

K3PO4 (aq) + 3 Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + KNO3 (aq)

K3PO4 (aq) + 3 Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + 6 KNO3 (aq)

2 K3PO4 (aq) + 3 Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + 6 KNO3 (aq)

Aqueous potassium phosphate reacts with aqueous calcium nitrate to produce solid calcium phosphate and aqueous potassium nitrate.

K3PO4 (aq) + Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + KNO3 (aq)

2 K3PO4 (aq) + Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + KNO3 (aq)

2 K3PO4 (aq) + Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + 6 KNO3 (aq)

2 K3PO4 (aq) + 3 Ca(NO3)2 (aq) ----------> Ca3(PO4)2 (s) + 6 KNO3 (aq)

Propane (C3H8) burns in oxygen gas to produce carbon dioxide and water.

C3H8 + O2 ----------> CO2 + H2O

C3H8 + O2 ----------> 3 CO2 + H2O

C3H8 + O2 ----------> 3 CO2 + 4 H2O

C3H8 + 5 O2 ----------> 3 CO2 + 4 H2O

C2H2 + O2 ----------> CO2 + H2O

C2H2 + O2 ----------> 2 CO2 + H2O

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2 C2H2 + O2 ----------> 4 CO2 + 2 H2O

2 C2H2 + 5 O2 ----------> 4 CO2 + 2 H2O

Acetylene (C2H2) burns in oxygen gas to produce carbon dioxide and water.

Classifying Chemical Reactions by What Atoms Do

Classification of Reactions

Synthesis reaction

Single displacement reaction

Double displacement reaction

Decomposition reaction

4 Al (s) + 3 O2 (g) 2 Al2O3 (s)

2 H2 (g) + O2 (g) ---------> 2 H2O (g)

C2H4 (g) + H2O2 (aq) C2H6O2 (l)

Synthesis Reactions

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Decomposition Reactions

2 HgO (s) ---------> 2 Hg (l) + O2 (g)

CaCO3 (s) ---------> CaO (s) + CO2 (g)

2 NaCl (s) ---------> Cl2 (g) + 2 Na (l)

+

Cu (s) + 2 AgNO3 (aq) ---------> 2 Ag (s) + Cu(NO3)2 (aq)

2 Al (s) + Fe2O3 (s) ---------> Al2O3 (s) + 2 Fe (l)

Mg (s) + 2 HCl (aq) ---------> H2 (g) + MgCl2 (aq)

Single Displacement Reactions

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2 AgNO3(aq) + Cu(s) ➜ 2 Ag (s) + Cu(NO3)2(aq)

Ba(NO3)2 (aq) + Na2SO4 (aq) ---------> BaSO4 (s) + 2 NaNO3 (aq)

PCl3 (l) + 3 AgF (s) ---------> PF3 (g) + 3 AgCl (s)

HCl (aq) + NaOH (aq) ---------> H2O(l) + NaCl (aq)

Double Displacement Reactions

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Chemical Reactions Classified by

Reaction Type