Chemical Reactions

1. Principles of Chemistry and Physics Chemical Reactions

2. Chemical Reactions • When one or more substances (reactants) are changed into one or more new substances (products), a CHEMICAL REACTION has occurred and can be represented as a chemical equation Reactants  products

3. Chemical Equations • Replaces words with chemical formulas skeleton equations- does not indicate the relative amounts of reactants and products • Here is the skeleton equation for rusting: • Fe + O2 Fe2O3

4. Evidence a reaction has occurred: • Color change • Gas released • Precipitate formed • Temperature change (endo or exothermic) • Odor produced • Smoke • Light • Flames • pH change • Flammable to nonflammable or vice versa

5. Types of Reactions • There are five types of chemical reactions: • Synthesis reactions • Decomposition reactions • Single displacement reactions • Double displacement reactions • Combustion reactions • Unit objective: identify the type of reaction and predict the product(s).

6. 1. Synthesis reactions • Synthesis reactions occur when two substances (generallyelements) combine to form a compound. reactant + reactant  1 product • Basically: A + B  AB • Example: 2H2 + O2  2H2O • Example: C+ O2  CO2

7. Synthesis Reactions • Another example of synthesis:

8. 2. Decomposition Reactions • Decomposition reactions occur when a compound breaks up into the elements or into a few simpler compounds • 1 Reactant  Product + Product • In general: AB  A + B • Example: 2 H2O  2H2 + O2 • Example: 2 HgO 2Hg + O2

9. Decomposition Reactions • Another view of a decomposition reaction:

10. Decomposition Reaction AB  A + B

11. 3. Single Replacement Reactions • Single Replacement Reactions occur when one element replaces another in a compound. • A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). • element + compound element + compound A + BC  AC + B (if A is a metal) OR A + BC  BA + C (if A is a nonmetal) (remember the cation always goes first!) When H2O splits into ions, it splits into H+ and OH- (not H+ and O-2 !!)

12. Single Replacement Reactions • Another view:

13. + +  Cl Cl Cl Cl Zn Zn Cu Cu Single Replacement Example Example: Zn + CuCl2 General: AB + C  AC + B LIKE replaces LIKE

14. lists metals in order of decreasing reactivity. As a general rule, more reactive metals replace less reactive metals in a compound Activity Series Li K Ba Ca Na Mg Al Zn Fe Cd Ni Sn Pb H Cu Hg Ag Au

15. 4. Double Replacement Reactions • Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound • Compound + compound  compound+ compound • AB + CD  AD + CB

16. Decomposition reactions cont.. • Solubility rules- • The formation of a precipitate is a driving force of a double replacement reaction

17. 5. Combustion Reactions • Combustion reactions occur when a fuel reacts with oxygen gas, which produces heat! Fuel + O2 (+ Heat)  Product

18. Hydrocarbon Combustion Reactions • Hydrocarbon Combustion: CxHy+ O2  CO2 +H2O • Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide) • Combustion is used to heat homes (CH4)and run automobiles (octane: C8H18)

19. Carbon Monoxide Effects Edgar Allen Poe’s drooping eyes and mouth are potential signs of CO poisoning.

20. Exothermic process – a process that results in the evolution of heat- energy flows out of the system • Endothermic process- a process that absorbs energy from the surroundings- energy flows into the system

21. Exothermic or endothermic? • 1. Your hand gets cold when you touch ice • 2. ice melts when you touch it • 3. Ice cream melts • 4. Propane is burning in a propane torch. • 5. Water drops on your skin evaporate after swimming • 6. Two chemicals mixing in a beaker give off heat

22. Exothermic • Endothermic • Endothermic • Exothermic • Endthermic • exothermic

23. Writing Chemical Equations • Word Equations • Names of reactants on the left of an arrow separated by plus signs • Names of products to the right of the arrow separated by plus signs • Ex: flour + water + yeast + salt  bread - Ex: carbon + oxygen  carbon dioxide

24. Diatomic Elements • Some elements exist naturally in pairs, as diatomic molecules. You will be expected to memorize these: Br2, I2, N2, Cl2, H2, O2, F2.

25. Law of Conservation of Mass • Mass is never created or destroyed-ALL must be conserved and accounted for during a chemical reaction • The same number of atoms of reactant elements must equal the atoms of product elements

26. Rules for balancing equations: • Write correct skeleton formula • Determine number of atoms of each element of reactants and products. COUNT POLYATOMIC ION AS A SINGLE UNIT if it appears unchanged on both sides of the equation • Balance elements one at a time by using coefficients-never change subscripts • Begin with the easiest elements first • Check both sides to see if they match • Make sure coefficients are in the lowest possible ratio

27. Counting with Moles • Chemists use the unit mole to measure the amounts of small particles • 1 mole = 6.02 x 1023 particles (atoms, molecules) 6.02 x 1023 is known as Avogadro’s number

29. The molar mass of any two elements contain the same number of atoms • Ex: a dozen apples – 12 apples • a dozen oranges – 12 oranges

30. Molar mass • Molar mass is the amount of one mole of that element or compound (use the periodic table) • Once you know the molar mass of the compound, you can convert moles of that substance into moles

31. Molar Mass • Ex: the molar mass of one mole of CO2 is 44 g this means that 44 g CO21 mole 1 mole 44 g CO2 So, if I have 55 g of CO2, how many moles do I have?