Chapter 11 Chemical Reactions Hingham High School Mr. Dan Clune

All chemical reactions have two parts: –Reactants - the substances you start with –Products- the substances you end up with The reactants turn into the products. Reactants  Products

In a chemical reaction The way atoms are joined is changed. Atoms aren’t created or destroyed.

In a chemical reaction Can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation Copper + chlorine  copper (II) chloride

Symbols in equations-p.323 the arrow (  ) separates the reactants from the products Read “reacts to form” The plus (+) sign is read “and” Cu + Cl 2  CuCl 2

Symbols used in equations (s) after the formula = solid (g) after the formula = gas (l) after the formula = liquid Cu( s ) + Cl 2 (g)  CuCl 2 (s)

Symbols used in equations (aq) after the formula - dissolved in water, an aqueous solution.  used after a product indicates a gas (same as (g))  used after a product indicates a solid (same as (s))

Symbols used in equations indicates a reversible reaction (more later) shows that heat is supplied to the reaction is used to indicate a catalyst is supplied, in this case, platinum.

What is a catalyst? A substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts.

Skeleton Equation Uses formulas and symbols to describe a reaction doesn’t indicate how many. All chemical equations are sentences that describe reactions.

Convert this to an equation Fe 2 S 3 (s)+ HCl(g)  FeCl 3 +H 2 S(g) Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.

HNO 3 (aq)Na 2 CO 3 (aq) +  H 2 O(l) + CO 2 (g)NaNO 3 (aq) + Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

Now, read these: Fe(s) + O 2 (g)  Fe 2 O 3 (s) Cu(s) + AgNO 3 (aq)  Ag(s) + Cu(NO 3 ) 2 (aq) NO 2 (g) N 2 (g) + O 2 (g)

Balancing Chemical Equations

Balanced Equation Atoms can’t be created or destroyed All the atoms we start with we must end up with A balanced equation has the same number of each element on both sides of the equation.

C + O 2  CO 2 This equation is already balanced What if it isn’t? C + O O  C O O

C + O 2  CO We need one more oxygen in the products. Can’t change the formula, because it describes what it is (carbon monoxide in this example) C + O  C O O

Must be used to make another CO But where did the other C come from? C + O  C O O O C

Must have started with two C 2 C + O 2  2 CO C + O  C O O O C C

Rules for balancing:  Determine the correct formulas for all the reactants and products.  Write a skeleton equation.

Rules for balancing: 3 Count the number of atoms of each type appearing on both sides of the equation. 4 Balance the elements one at a time by adding coefficients (the numbers in front) - save H and O until LAST!

Rules for balancing: 5 Check to make sure it is balanced. 6 Make sure the coefficients are in the lowest possible ratio.

Never change a subscript to balance an equation. –If you change the formula you are describing a different reaction. –H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula –2 NaCl is okay, Na2Cl is not. X X

Example H 2 +H2OH2OO2O2  RP H O

H2OH2OO2O2  22

_AgNO 3 + _Cu  _Cu(NO 3 ) 2 + _Ag RP Ag Cu NO

_Mg + _N 2  _Mg 3 N 2 RP Mg N

_P + _O 2  _P 4 O 10 RP P O

_Na + _H 2 O  _H 2 + _NaOH RP Na H O

_CH 4 + _O 2  _CO 2 + _H 2 O RP C H O

Section 8.2 Types of Chemical Reactions OBJECTIVES: –Identify a reaction as combination, decomposition, single-replacement, double-replacement, or combustion

#1 - Combination Reactions Combine - put together 2 or more substances combine to make one compound. A + B  Ca +O 2  CaO SO 3 + H 2 O  H 2 SO 4

#1 - Combination Reactions We can predict the products if they are two elements. Mg + N 2  Mg N 32 3

Write and balance Ca + Cl 2  Fe + O 2  iron (II) oxide Al + O 2  Remember that the first step is to write the correct formulas Then balance by using coefficients only CaCl 2  FeO Al 2 O

Homework Questions: 13, 14 Page: 331 Due: 2/8/05

#2 - Decomposition Reactions decompose = fall apart one reactant falls apart into two or more elements or compounds. AB  A + B

#2 - Decomposition Reactions NaCl Na + Cl 2 CaCO 3 CaO + CO 2 Note that energy is usually required to decompose

#2 - Decomposition Reactions Can predict the products if it is a binary compound Made up of only two elements Falls apart into its elements H 2 O HgO H 2 + O 2 Hg + O

#2 - Decomposition Reactions If the compound has more than two elements you must be given one of the products The other product will be from the missing pieces NiCO 3 CO 2 + ? H 2 CO 3 (aq)  CO 2 + ? Ni H2H2

Homework Questions: 15, 16 Page: 332 Due: 2/8/05