1. UNIT 7: Chemical Reactions

2. Chemical Reactions
Why do we balance chemical reactions?

3. 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:

4. 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)

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

6. 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)

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

8. 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.

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

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

11. 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

12. 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

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

14. 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?

15. 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 

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

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

18. 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

19. 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

20. 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

21. 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

22. 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