CHEMICAL EQUATIONS & REACTIONS

Describing Reactions  What is a Chemical Reaction?  Occurs when compounds are mixed (sometimes with help)  Chemical structures and properties are changed

Describing Reactions  Chemical equations: chemical formulas are used to express chemical and physical changes  Reactants are written on the left  Products written on right  Separate multiple reactants/products with plus sign(s)  Separate reactants from products with an arrow

Describing Reactions W + X  Y + Z Reactants Products X + Y  Z ReactantsProducts

Chemical Equations  Chemical equations follow a set of rules  Matter cannot be created or destroyed during a reaction, only changed  Law of Conservation of Matter!  Chemical equations will be balanced  Use coefficients (mole-mole ratio) so that atoms present in reactants are also found in products

Law of Conservation of Matter C + O 2  CO 2 Why is Oxygen written as “O 2 ”?? Some elements will only be stable when they can group in pairs (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ) Diatomic “Two atoms”

Are they balanced? H 2 O  H 2 + O 2 C 3 H 8 + O 2  H 2 O + CO 2 NaCl + H 2 SO 4  HCl + Na 2 SO 4

Rules for Balancing Equations 1. Count up starting atoms 2. Balance all non-H’s and O’s first  Look for element(s) that only appear in one reactant and one product! 3. Add a coefficient to the front of a substance with the fewer number of atoms.  NEVER change a subscript! 4. Re-count atoms after each coefficient you write 5. Balance H’s, then O’s 6. Re-check the count

Balancing Equations 1.Count up starting atoms ReactantsProducts H22 O12 H 2 O  H 2 + O 2

2.Balance all non-H’s and O’s first There aren’t any! Go to the next step! Balancing Equations H 2 O  H 2 + O 2

Balancing Equations 3.Add a coefficient to the front of a substance with the fewer number of atoms. _2_H 2 O  ___H 2 + ___O 2 H 2 O  H 2 + O 2

Balancing Equations 4.Re-count atoms after each coefficient you write ReactantsProducts H24 O22 H 2 O  H 2 + O 2 _2_H 2 O  ___H 2 + ___O 2

Balancing Equations 5.Balance H’s, then O’s H 2 O  H 2 + O 2 _2_H 2 O  ___H 2 + _2_O 2

Balancing Equations 6.Re-check the count ReactantsProducts H44 O22 H 2 O  H 2 + O 2 _2_H 2 O  ___H 2 + _2_O 2

More Balancing Tips  NEVER change a subscript!  The final equation only has whole numbers!  Leave single atoms and diatomic molecules until the end  If a polyatomic ion is on both sides, you don’t have to split up the atoms in your tally

Learning Check  Which of the following sets of coefficients correctly balances the following chemical equation? Mg + AgNO 3  Ag + Mg(NO 3 ) 2 1, 1, 1, 1 1, 3, 1, 2 2, 1, 1, 2 1, 2, 2, 1

Balancing Equations NaCl + H 2 SO 4  HCl + Na 2 SO 4 ReactantsProducts Na12 Cl11 H21 SO 4 11

Balancing Equations 2 NaCl + H 2 SO 4  2 HCl + Na 2 SO 4 ReactantsProducts Na22 Cl22 H22 SO 4 11

Balancing Equations C 3 H 8 + O 2  H 2 O + CO 2

Balancing Equations ReactantsProducts C31 H82 O23 C 3 H 8 + O 2  H 2 O + CO 2

Balancing Equations ReactantsProducts C33 H88 O10 C 3 H O 2  4 H 2 O + 3 CO 2

Balancing Equations Zinc + hydrogen sulfate (sulfuric acid)  zinc sulfate + hydrogen gas Zn + H 2 SO 4  ZnSO 4 + H 2

Classifying Equations  Combustion: A+ O 2  CO 2 + H 2 O  Single Displacement: A + BY  B + AY AX + Y  X + AY  Double Displacement:AX + BY  BX + AY  Composition: A + B  AB  Decomposition:AB  A + B Reaction TypeGeneral Formula