Indicators of chemical reactions Formation of a gas Emission of light or heat Formation of a precipitate Color change Emission of odor

Review….Count The Atoms CaCl 2 Zn(NO 3 ) 2 2NaOH 3H 2 O One Calcium/2 Chlorine 1 Zinc/2Nitrogen/6Oxygen 2 Na, 2 O, 2 H 6 H, 3 O

Unit 6 Describing Chemical Reactions OBJECTIVES: -Describe how to write a word equation. -Describe how to write a skeleton equation. - Describe the steps for writing a balanced chemical equation.

All chemical reactions: have two parts Reactants - the substances you start with Products- the substances you end up with The reactants turn into the products. Reactants  Products

- Page 321 Reactants Products

Anatomy of a Chemical Reaction CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) Reactants SUBSCRIPT: Tells how many atoms there are of an element PLUS SIGN: Used to separate each reactant or each product FORWARD ARROW: Means produces or yields, etc. DELTA: Shows that heat is supplied to the reaction ITALICIZED ABBREVIATION: Tells what physical state that reactant or product is in COEFFICIENT: Tells how many of each reactant or each product there is Products

Symbols used in equations (s) after the formula –solid Cu (s) (g) after the formula –gas H 2 (g) (l) after the formula -liquid H 2 O (l) (aq) after the formula - dissolved in water, an aqueous solution. CaCl 2 (aq)

Symbols used in equations indicates a reversible reaction. indicates a reversible reaction. shows that heat is supplied to the reaction. shows that heat is supplied to the reaction., or is used to indicate a catalyst used supplied, in this case, platinum., or is used to indicate a catalyst used supplied, in this case, platinum., indicates a pressure other than STP, indicates a pressure other than STP

Describing chemical reaction The way atoms are joined is changed Atoms are not created or destroyed. Can be described several ways 1. In a sentence ex: Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation ex: Copper + chlorine  copper (II) chloride Cu (s) + Cl 2 (g)  CuCl 2 (aq)

Warm-up: write these equations in a sentence: Fe (s) + O 2(g)  Fe 2 O 3(s) Fe (s) + O 2(g)  Fe 2 O 3(s) Cu (s) + AgNO 3(aq)  Ag (s) + Cu(NO 3 ) 2(aq) Cu (s) + AgNO 3(aq)  Ag (s) + Cu(NO 3 ) 2(aq) NO 2(g) N 2(g) + O 2(g) NO 2(g) N 2(g) + O 2(g)

#3. The Skeleton Equation Uses formulas and symbols to describe a reaction but doesn ’ t indicate how many; this means they are NOT balanced All chemical equations are a description of the reaction.

Write a skeleton equation for: 1.Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. 2.Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

(The Diatomic Elements!!!)

Diatomic elements There are 8 elements that never want to be alone. They form diatomic molecules. H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2, and At 2 The –ogens and the –ines pattern on the periodic table

Convert this to an equation Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas. Fe 2 S 3 (s) + HCl (g)  FeCl 2 (s) + H 2 S (g)

HNO 3 (aq) + Na 2 CO 3 (s)  NaNO 3 (aq) + H 2 O (l) Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. Convert this to an equation

The other way Fe (s) + O 2 (g)  Fe 2 O 3 (s) Solid iron reacts with oxygen gas to form solid iron oxide (rust).

A silver spoon tarnishes. The solid silver reacts with sulfur in the air to make solid silver sulfide, the black material we call tarnish. Ag (s) + H 2 S (g) + O 2 (g)  Ag 2 S (s) + H 2 O

#4. Balanced Chemical Equations Atoms can ’ t be created or destroyed in an ordinary reaction: All the atoms we start with we must end up with (meaning: balanced!) A balanced equation has the same number of each element on both sides of the equation.

Rules for balancing: 1)Assemble the correct formulas for all the reactants and products, using “ + ” and “→” 2)Count the number of atoms of each type appearing on both sides 3)Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) 4)Double-Check to make sure it is balanced.

Never change a subscript to balance an equation (You can only change coefficients) If you change the subscript (formula) you are describing a different chemical. H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula; they must go only in the front 2 NaCl is okay, but Na 2 Cl is not. 2 NaCl is okay, but Na 2 Cl is not.

Practice Balancing Examples _AgNO 3 + _Cu  _Cu(NO 3 ) 2 + _Ag _Mg + _N 2  _Mg 3 N 2 _P + _O 2  _P 4 O 10 _Na + _H 2 O  _H 2 + _NaOH _CH 4 + _O 2  _CO 2 + _H 2 O

Balancing Equations ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O(l) 22 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.

Aluminum metal reacts with liquid bromine to form solid aluminum bromide Translate Equation ___ Al(s) + ___ Br 2 (l) →___ AlBr 3 (s) 2 32