Chapter 8 Chemical Reactions

Physical Change In a physical change, The identity and composition of the substance do not change The state can change or the material can be torn into smaller pieces Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Chemical Change In a chemical change, Reacting substances form new substances with different compositions and properties A chemical reaction takes place Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Chemical Reaction In a chemical reaction, Old bonds are broken and new bonds are formed Atoms in the reactants are rearranged to form one or more different substances Fe and O 2 form rust (Fe 2 O 3 )

Chemical Reaction In a chemical reaction, A chemical change produces one or more new substances There is a change in the composition of one or more substances Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Evidence of a Chemical Reaction Changes that can be seen are evidence of a chemical reaction.

Writing a Chemical Reaction Chemists use a shorthand approach when writing the specifics of a chemical reaction. This approach is called the chemical equation. Reactants -----> Products

Chemical Equations A chemical equation, Gives the chemical formulas of the reactants on the left of an arrow and the products on the right Reactants Product C(s) O 2 (g) CO 2 (g)

Symbols Used in Equations Symbols used in chemical equations show: The states of the reactants The states of the products The reaction conditions Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Chemical Equations Are Balanced In a balanced chemical reaction, Atoms are not gained or lost

Chemical Equations Are Balanced In a balanced chemical reaction, The number of reactant atoms are equal to the number of product atoms

Chemical Equations Chemical equations: symbolic descriptions of chemical reactions. Two parts to an equation: reactants and products H 2 + O 2  H 2 O A Chemical Equation must also be balanced. 2H 2 + O 2 --> 2H 2 O

Balanced Chemical Equations Chemical Equations must be balanced –There must be equal numbers of atoms of each element on both sides of the equation (both sides of the arrow) 1. Write the correct symbols and formulas for all of the reactants and products. 2. Count the number of each type of atom on BOTH sides of the equation. 3. Insert coefficients until there are the equal numbers of each kind of atom on both sides of the equation.

A Balanced Chemical Equation Al + S Al 2 S 3 Not Balanced How many atoms of Al and S are on each side of the equation? coefficients 2Al + 3S Al 2 S 3 Balanced 2 Al = 2 Al 3 S = 3 S

Learning Check State the number of atoms of each element on the reactant and on the product sides of the equations: P 4 (s) + 6 Br 2 (l) → 4 PBr 3 (g)

Learning Check State the number of atoms of each element on the reactant and on the product sides of the equations: 2Al(s) + Fe 2 O 3 (s) → 2Fe(s) + Al 2 O 3 (s)

Check the balance of atoms in the following: Fe 3 O 4 (s) + 4H 2 (g) 3Fe(s) + 4H 2 O(l) 1. Number of _____atoms in reactants. 2. Number of ____ atoms in reactants. 3. Number of ____ atoms in reactants. Learning Check

Determine if each equation is balanced or not. Na(s) + N 2 (g) → NaN 3 (s) C 2 H 4 (g) + H 2 O(l) → C 2 H 5 OH(l)

Balancing Equations Methane (CH 4 ) reacts with oxygen (combustion reaction) to form carbon dioxide and water. Write a properly balanced chemical equation 1. Write out chemical formulas CH 4 + O 2  CO 2 + H 2 O 2. Use coefficients to balance the equation

The Numbers in Chemical Equations

More Practice: Balancing Reactions C 2 H 6 + O 2  CO 2 + H 2 O C 3 H 6 + O 2  CO 2 + H 2 O NH 3 + O 2  NO + H 2 O

And more practice…. B 2 H 6 + H 2 O  H 3 BO 3 + H 2 C 3 H 8 O + O 2  CO 2 + H 2 O

Balancing with Polyatomic Ions MgCl 2 (aq) + Na 3 (PO 4 )(aq) NaCl(aq) + Mg 3 (PO 4 ) 2 (s) HINT: Balance PO 4 3- as a unit. Set of # coef?

Type of Reactions Chemical reactions can be classified as Combination reactions. Decomposition reactions. Single Replacement reactions. Double Replacement reactions. Combustion reactions.

Combination In a combination reaction, Two or more elements form one product. Or simple compounds combine to form one product. + 2Mg(s) + O 2 (g) 2MgO(s) 2Na(s) + Cl 2 (g)2NaCl(s) SO 3 (g) + H 2 O(l)H 2 SO 4 (aq) AB AB

Decomposition In a decomposition reaction, One substance splits into two or more simpler substances. 2HgO(s) 2Hg(l) + O 2 (g) 2KClO 3 (s) 2KCl(s) + 3O 2 (g) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Classify the following reactions as A) combination or B) decomposition:

Single Replacement In a single replacement reaction, One element takes the place of a different element in a reacting compound. Zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) Fe(s) + CuSO 4 (aq) FeSO 4 (aq) + Cu(s) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Double Replacement In a double replacement reaction, Two elements in the reactants exchange places. AgNO 3 (aq) + NaCl(aq)AgCl(s) + NaNO 3 (aq) ZnS(s) + 2HCl(aq)ZnCl 2 (aq) + H 2 S(g) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Classify the following reactions as A) single replacement B) double replacement

In a combustion reaction, A compound such as carbon reacts with oxygen, O 2. C(s) + O 2 (g)CO 2 (g) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) Fuels burned in oxygen produce CO 2, H 2 O, and energy. Combustion

Learning Check Balance the combustion equation ___ + ___

Summary of Reaction Types Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Heat of Reaction The heat of reaction, Is the amount of heat absorbed or released during a reaction at constant pressure. Is the difference in the energy of the reactants and the products. Is shown as the symbol ΔH. ΔH = H products − H reactants

Endothermic Reactions In an endothermic reaction, Heat is absorbed. The sign of ΔH is +. The energy of the products is greater than the energy of the reactants. Heat is a reactant. N 2 (g) + O 2 (g) kJ 2NO(g) ΔH = +181 kJ (heat added) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Exothermic Reactions In an exothermic reaction, Heat is released. The sign of ΔH is -. The energy of the products is less than the energy of the reactants. Heat is a product. C(s) + 2H 2 (g) CH 4 (g) + 75 kJ ΔH = - 75 kJ (heat given off) Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Identify each reaction as 1) exothermic or 2) endothermic. A. N 2 + 3H 2 2NH kJ B. CaCO kJCaO + CO 2 C. 2SO 2 + O 2 2SO 3 + heat