Chapter 6 Chemical Reactions

What is the clue that a chemical reaction has occured? When colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, the solution turns blue A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.

What is the clue that a chemical reaction has occured?

Evidence for a Chemical Reaction Precipitate Energy change Color change Gas produced

Chemical Reactions Reactions involve chemical changes in matter that result in new substances. Reactions involve rearrangement and exchange of atoms to produce new molecules. Reactants  Products

Chemical Equations Shorthand way of describing a reaction CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(l) Reactants Products Provides information about the reaction: –Formulas of reactants and products –States of reactants and products –Relative numbers of reactant and product molecules that are required

Symbols Used in Equations Symbols used after chemical formula to indicate state: (g) = gas (l) = liquid (s) = solid (aq) = aqueous, dissolved in water Other symbols catalyst: heat added:

Physical states of substances Aqueous silver nitrate reacts with solid copper to form aqueous copper (II) nitrate and solid silver silver nitrate + copper  copper (II) nitrate + silver AgNO 3(aq) + Cu (s)  Cu(NO 3 ) 2(aq) + Ag (s)

Conservation of Mass Matter cannot be created or destroyed. In a chemical reaction, all the atoms present at the beginning are still present at the end. Therefore, the total mass cannot change. Equations must be balanced

2c brownie mix + 1/2c water + 1/4c oil + 2eggs  24brownies Coefficients- numbers written before formulas of reactants and products which indicate the relative amounts of each. Sb 2 S 3 + 6HCl  2SbCl 3 + 3H 2 S

Combustion of Methane Methane gas burns to produce carbon dioxide gas and liquid water CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(l) H H C H H OO + O O C + O HH

Combustion of Methane Balanced CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(l) H H C H H OO + O O C + O HH OO + O HH + 1 C + 4 H + 4 O

Writing Balanced Equations Use correct formulas for each reactant and product. Count the number of atoms of each type of element on both sides of the arrow Balance by changing the coefficient Balance uncombined elements last

Balancing by Inspection Count atoms of each element Mg(s) + O 2 (g)  MgO(s) Mg11 O21 Pick an element to balance Add coefficients to equation 2Mg(s) + O 2 (g)  2MgO(s)

Zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas and zinc chloride that remains in solution.

Solid potassium reacts with water to form hydrogen gas and potassium hydroxide that remains in solution. Hmco Photo File

Decorations on glass are produced by etching with hydrofluoric acid. Hmco Photo File The silicon dioxide in the glass reacts with the hydrofluoric acid to produce silicon tetrafluoride and liquid water.

Chapter 7

Reaction Types Synthesis Decomposition Single Displacement Double Displacement Combustion

Synthesis (Combination) Examples: 2Na + Cl 2  2NaCl CaO + SiO 2  CaSiO 3 NH 3 + HCl  NH 4 Cl 2H 2 + O 2  2H 2 O

Synthesis (Combination) 2 or more reactants combine to form a single product A + B  AB

4Fe + 3O 2  2Fe 2 O 3

Decomposition Examples: CdCO 3  CdO + CO 2 Pb(OH) 2  PbO + H 2 O N 2 O 4  2NO 2 2KClO 3  2KCl + 3O 2 2H 2 O  2H 2 + O 2

Decomposition A single reactant is broken down into two or more products AB  A + B

Single Displacement Examples: 2Al + 3CuSO 4  Al 2 (SO 4 ) 3 + 3Cu Cu + 2AgNO 3  Cu(NO 3 ) 2 + 2Ag Cl 2 + 2KBr  2KCl + Br 2

Single Displacement An uncombined element displaces an element which is part of a compound A + BX  AX + B element + compound  compound + element

Double Displacement Examples Pb(NO 3 ) 2 + 2NaI  PbI 2 + 2NaNO 3 ZnBr 2 + 2AgNO 3  Zn(NO 3 ) 2 + 2AgBr CaCO 3 + 2HCl  CaCl 2 + H 2 CO 3

Double Displacement Sometimes referred to as ionic reactions or precipitation reactions Ions from two different compounds replace each other AX + BY  AY + BX compound + compound  compound + compound

Combustion Examples: CH 4 + O 2  CO 2 + 2H 2 O C 3 H 8 + O 2  3CO 2 + 4H 2 O 2C 4 H O 2  8CO H 2 O C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O

Combustion Organic compounds (carbon, hydrogen compounds) burn in presence of oxygen to form water and CO 2 C x H y + O 2  CO 2 + H 2 O

Predicting Single Displacement Reactions A + BX  AX + B Not all SD reactions will occur. Why? –Depends on reactivity of element

Predicting Single Displacement Reactions A + BX  AX + B The reaction will proceed if “A” is more reactive than “B” –“A” must be higher on the reactivity series than “B” –Metal Reactivity series can be found on your periodic table –Reactivity series for halogens: F>Cl>Br>I

Examples Al + HCl  Al + HCl  AlCl 3 + H 2 Ni + Na 3 PO 4  Ni + Na 3 PO 4  NR NaI + F 2  NaI + F 2  NaF + I 2 NaCl + Br 2  NaCl + Br 2  NR