UNIT 7: Chemical Reactions

Chemical Reactions Why do we balance chemical reactions?

Writing Chemical Reactions Basic Format: Reactants  Products Word Equation: Iron metal reacts with oxygen gas and forms solid iron (III) oxide Skeleton Chemical Equation: Balanced Chemical Equation:

Writing Chemical Reactions Basic Format: Reactants  Products Word Equation: Iron metal reacts with oxygen gas and forms solid iron (III) oxide Skeleton Chemical Equation: Fe(s) + O2(g)  Fe2O3(s) Balanced Chemical Equation: 4Fe(s) + 3O2(g)  2Fe2O3(s)

Examples of Balanced Chemical Equations 2 H2 + O2  2 H2O

Recall Dalton’s Atomic Theory to understand Why Equations are Balanced All elements are composed of indivisible particles called atoms Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can physically mix together or chemically combine with one another in simple whole number ratios to form compounds.(law of definite composition) Chemical rxns occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.(law of conservation of mass)

Energy Changes in Chemical RXNS Endothermic energy is absorbed + change T drops Exothermic energy released - change T rises

Energy Changes in Chemical RXNS 1. When bonds are broken energy is absorbed. (endothermic process) 2. When bonds are formed energy is released. (exothermic process) The overall process depends on the magnitude of each part the process of a chemical rxn.

Energy Changes in Chemical RXNS Endothermic RXN – It takes more energy to break bond then energy released to form a bond

Energy Changes in Chemical RXNS Exothermic Reaction – it takes less energy to break a bond than to energy released to form a bond

Balancing Equations Guidelines 1. Write the correct chemical formulas for the reactants and the products 2. Include states of matter if necessary (s, l, g, aq) 3. Count the number of atoms of each element in the reactants and products

Balancing Equations Guidelines 4. Balance the elements one at a time using coefficients; never change subscripts a. Treat polyatomic ions appearing on both sides of rxn as one unit and balance first b. Balance atoms that appear once on each side of the equation c. Balance H and O last

Balancing Equations Guidelines 5. Check Work 6. Coefficients should be in lowest possible ratios

Balancing Equations Guidelines Quick Steps: 1. Write formulas 2. States of matter 3. Count atoms 4. Balance using coefficients 5. Check work 6. Coefficients in lowest ratios?

Reaction Types Combination = 2 pure substances forming 1 compound A + B  AB   Decomposition = 1 compound breaks down into 2 or more pure substances AB  A + B Single Replacement= An elements switches places with another element in a compound to form a new compound and element A + BC  B + AC or A + BC  C + BA    Double Replacement= 2 compounds trade elements to form 2 new compounds AB + CD  AD + CB Combustion = A compound of C, H, and maybe O combine with oxygen to form carbon dioxide and water CxHyOz + O2  CO2 + H2O 

Predicting Products Consider reaction type patterns to predict products of chemical reaction.

Activity Series of a Metal Lab Single Replacement Reactions = Example: _Al + _Fe(NO3)2 

Activity Series of a Metal Lab Single Replacement Reactions =An elements switches places with another element in a compound to form a new compound and element Example: 2Al + 3Fe(NO3)2  2Al(NO3)3 + 3Fe

Activity Series of a Metal Lab Oxidation-Reduction Reactions (REDOX) Reactions that involve a transfer of electrons from reactants to products Occur with single replacement reactions Oxidation = loss of electrons Reduction = gain of electrons

Activity Series of a Metal Lab Oxidations numbers: Elements have oxidation number of zero Ions have an oxidation number that matches its charge Follow the oxidation numbers of metals to determine what has been oxidized or reduced. 2Al + 3Fe(NO3)2  2Al(NO3)3 + 3Fe

Activity Series of a Metal Lab Activity Series – list of elements organized according to reactivity or decrease in ease of oxidation A more reactive metal will replace any metal listed below it in the activity series

Activity Series of a Metal Lab Will these rxns occur? Li + MgCl2  Pb + AlCl3  Activity Series of Metals: Li>K>Ba>Ca>Na>Mg>Al>Mn>Zn>Cr>Fe>Co>Ni>Sn>Pb>H>Cu>Ag>Hg>Pt>Au