Chemical Equations. Physical Science

Transcription

1 Chemical Equations Physical Science

2 Chemical Equations A chemical equation is a shorthand way of expressing a chemical reaction.

3 CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

4 Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

5 Symbols represent elements. Formulas describe compounds. Chemical equations describe a chemical reaction

6 Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

7 Chemical Equations Their Job: Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) (reactants) (product) The numbers in the front are called Coefficients.

8 Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

9 Symbols Used in Equations Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) Catalyst H 2 SO 4 Escaping gas ( ) Change of temperature ( )

10 Counting Atoms Before you can balance an equation, you must be able to count the atoms on each side of the equation. **Subscripts are numbers written after an element that tell how many atoms of that element are in a compound. **If there is a parentheses, the subscript goes with all atoms inside the parentheses.

11 Counting Atoms How many atoms are in: C 6 H 12 O 6 - Mg(OH) 2-3H 2 SO 4 Al(PO 4 ) 3

12 Balancing Equations When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the charges.

13 Steps to Balancing Equations 1. Remember, DO NOT CHANGE FORMULAS! 2. Find the number of atoms for each element on both sides of the equation. 3. Place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element. 4.Check your answer to see if: the equation is balanced. the coefficients are reduced.

14 Some Suggestions to Help You Take one element at a time, except for H and O. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. Polyatomic ions that appear on both sides of the equation can be balanced as a unit.

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16 Balancing Equations 2 H 2 (g) + O 2 (g) ---> 2 H 2 O(l) What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule.

17 Balancing Equations 2 Al(s) + 3 Br 2 (l) ---> Al 2 Br 6 (s)

18 Balancing Equations C 3 H 8 (g) + O 2 (g) -- CO 2 (g) + H 2 O(g) B 4 H 10 (g)+ O 2 (g) - B 2 O 3 (g)+ H 2 O(g)

19 Balancing Equations Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphate Na 3 PO 4 + Fe 2 O 3 -- Na 2 O + FePO 4

20 Indicators of a Chemical Reaction 1. Color Change 2. Gas is formed (bubbles) 3. Change in temperature Endothermic (gets colder) Exothermic (gets hotter) 4. Solid is formed (called a precipitate) 5. Light is produced.

21 I. Synthesis Types of Reactions 1. Two smaller substances COMBINE to form one larger product. 2. Usu. Element + Element Compound 3. Examples: A. A + B AB B. Fe + O 2 Fe 2 O 3 C. Na 2 O + H 2 O NaOH ck

22 II. Decomposition 1. One larger reactant BREAKS DOWN to form two or more smaller products. 2. Usually compound element + element 3. Examples: A. AB A + B B. NaI Na + I 2 C. NaOH Na 2 O + H 2 O

23 III. Single Replacement (SR) 1. An element and a compound react to form a new element and a new compound. The element switches places with the element in the compound with the same charge. Use the Activity Series to determine if the reaction will occur. 2. Element + Compound Element + Compound 3. Examples: A. A + BC AC + B B. Al + Fe 2 O 3 Fe + Al 2 O 3

24 K Na Li Ca Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au Activity Series To be used with Single Replacement ONLY!!!! If element by itself is HIGHER on the list than the cation in the compound, they SWITCH!

25 IV. Double Replacement (DR) 1. Two compounds switch cations to form two new compounds. 2. Compound + Compound Compound + Compound 3. AB + CD AD + BC 4. 3NaCl + AlBr 3 AlCl 3 + 3NaBr 5. The solubility rules are used to determine if DR reactions will occur (song: don t break the law)

26 Double Replacement Reactions/Solubility Rules. Soluble Dissolves in Water; aqueous or (aq) Insoluble Does not dissolve in Water; Forms a Precipitate; solid (s) If both products are SOLUBLE, the reaction will NOT occur.

27 V. Combustion Burning, exploding, etc. The reactants are always hydrocarbons and oxygen. The products are always carbon dioxide and water. CxHx + O 2 CO 2 + H 2 O X can be any number.

28 Energy Changes Chemical Energy the energy stored in the chemical bond of a substance. Chemical reactions involve the breaking of chemical bonds in the reactants and forming new bonds in the products. Breaking chemical bonds requires energy (sometimes requires a spark like a gas grill to get the reaction started) Forming bonds releases energy.

29 During a chemical reaction, energy is either released or gained. Exothermic Releases energy. The energy released from forming products is greater than energy required to break the bonds. Ex. Combustion (burning propane) Endothermic Absorbs energy. More energy is required to break bonds in reactants than to form the products

30 Reaction Rates (How fast a reaction occurs) 1. Temperature bc particles move faster. (Ex. Frying an egg; milk) 2. Surface Area bc more collisions of reactants; Ex. newspaper 3. Concentration add more reactant 4. Stirring more exposure to reactants. Ex. Washing clothes 5. Catalysts chemical that brings reactants together

31 Rates of Reaction Song o6zm_km

32 Review of Types of Reactions ctionswebquest/home/reaction-videos Watch the reactions: y8hcsq