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

Indicators of chemical Indicators of chemical reactionsreactions

Formation of a gas

Emission of light or heat

Formation of a precipitate

Color change

Emission of odor

Review….Count The AtomsReview….Count The Atoms

CaClCaCl22

Zn(NOZn(NO33))22

2NaOH2NaOH

3H3H22OO

One Calcium/2 ChlorineOne Calcium/2 Chlorine

1 Zinc/2Nitrogen/6Oxygen1 Zinc/2Nitrogen/6Oxygen

2 Na, 2 O, 2 H2 Na, 2 O, 2 H

6 H, 3 O6 H, 3 O

Unit 6Unit 6Describing Chemical ReactionsDescribing Chemical Reactions

OBJECTIVES:OBJECTIVES:-Describe-Describe how to write a how to write a word word equationequation..-Describe-Describe how to write a how to write a skeleton skeleton equationequation..- Describe- Describe the steps for writing a the steps for writing a balanced chemical equationbalanced chemical equation..

All chemical reactions:All chemical reactions:

have two partshave two parts

ReactantsReactants - the substances you start with - the substances you start with

ProductsProducts- the substances you end up with- the substances you end up with

The reactants turn into the products.The reactants turn into the products.

Reactants Reactants ProductsProducts

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Reactants

Products

Anatomy of a Chemical Anatomy of a Chemical ReactionReaction

CH4 (g) + 2O2 (g) CO2 (g) + 2H 2O (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 equationsSymbols used in equations

(s) after the formula –solid Cu(s) after the formula –solid Cu(s)(s)

(g) after the formula –gas H(g) after the formula –gas H2 (g)2 (g)

(l) after the formula -liquid H(l) after the formula -liquid H22OO(l)(l)

(aq) after the formula - dissolved (aq) after the formula - dissolved in water, an aqueous solution. in water, an aqueous solution. CaClCaCl2 (aq)2 (aq)

Symbols used in equationsSymbols used in equations

indicates a reversible reaction.indicates a reversible reaction.

shows that heat is shows that heat is supplied to the reaction.supplied to the reaction.

, or is used to , or is used to indicate a catalyst used supplied, in this case, indicate a catalyst used supplied, in this case, platinum.platinum.

, indicates a pressure other , indicates a pressure other than STPthan STP

Describing chemical Describing chemical reactionreactionThe way atoms are joined is changedThe way atoms are joined is changed

Atoms are notAtoms are not created or destroyed. created or destroyed.

Can be described several waysCan be described several ways

1. In a sentence 1. In a sentence

ex: Copper reacts with chlorine to form ex: Copper reacts with chlorine to form copper (II) chloride.copper (II) chloride.

2. In a 2. In a word equationword equation

ex: Copper + chlorine ex: Copper + chlorine copper (II) chloridecopper (II) chloride

CuCu(s)(s) + Cl + Cl22(g)(g) CuCl CuCl22(aq)(aq)

Warm-up: write these equations Warm-up: write these equations in a sentence:in a sentence:

FeFe(s)(s) + O + O2(g)2(g) Fe Fe22OO3(s)3(s)

CuCu(s)(s) + AgNO + AgNO3(aq)3(aq) Ag Ag(s)(s) + Cu(NO + Cu(NO33))2(aq)2(aq)

NONO2(g)2(g) N N2(g)2(g) + O + O2(g)2(g)

Pt

#3. The Skeleton Equation#3. The Skeleton Equation

Uses formulas and symbols to Uses formulas and symbols to describe a reactiondescribe a reaction

but doesnbut doesn’’t indicate how t indicate how many; this means they are many; this means they are NOT balancedNOT balanced

All chemical equations are a All chemical equations are a descriptiondescription of the reaction. of the reaction.

Write a Write a skeleton equationskeleton equation for: for:1.1. Solid iron (III) sulfide reacts with Solid iron (III) sulfide reacts with

gaseous hydrogen chloride to gaseous hydrogen chloride to form iron (III) chloride and form iron (III) chloride and hydrogen sulfide gas.hydrogen sulfide gas.

2.2. Nitric acid dissolved in water Nitric acid dissolved in water reacts with solid sodium reacts with solid sodium carbonate to form liquid water carbonate to form liquid water and carbon dioxide gas and and carbon dioxide gas and sodium nitrate dissolved in water.sodium nitrate dissolved in water.

(The Diatomic Elements!!!)

Diatomic elementsDiatomic elementsThere are 8 elements that never want to There are 8 elements that never want to be alone.be alone.

They form diatomic molecules.They form diatomic molecules.

HH22 , N , N22 , O , O22 , F , F22 , Cl , Cl22 , Br , Br22 , I , I22 , and At , and At22

The –ogens and the –inesThe –ogens and the –ines

1 + 7 pattern on the periodic table1 + 7 pattern on the periodic table

Convert this to an equationConvert this to an equationSolid iron (III) sulfide reacts with gaseous hydrogen Solid iron (III) sulfide reacts with gaseous hydrogen

chloride to form iron (II) chloride and hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas.sulfide gas.

Fe2S3 (s) + HCl(g) FeCl2 (s) + H2S(g)

HNO3 (aq) + Na2CO3 (s) NaNO3 (aq) + H2O(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 equationConvert this to an equation

The other wayThe other way

FeFe(s)(s) + O + O22(g)(g) Fe Fe22OO33(s)(s)

Solid iron reacts with oxygen gas to form solid iron oxide (rust).

A silver spoon tarnishes. The solid silver reacts A silver spoon tarnishes. The solid silver reacts with sulfur in the air to make solid silver sulfide, with sulfur in the air to make solid silver sulfide,

the black material we call tarnish. the black material we call tarnish.

Ag (s) + H2S (g) + O2 (g) Ag2S (s) + H2O

#4. Balanced Chemical Equations#4. Balanced Chemical Equations

Atoms canAtoms can’’t be created or t be created or destroyed in an ordinary reaction:destroyed in an ordinary reaction:

All the atoms we All the atoms we start withstart with we we must must end up withend up with (meaning: (meaning: balanced!)balanced!)

A balanced equation has the same A balanced equation has the same number of each element number of each element on both on both sidessides of the equation. of the equation.

Rules for balancing:Rules for balancing:1)1) Assemble the Assemble the correct formulascorrect formulas for all the reactants and for all the reactants and

products, using products, using ““++ ”” and and ““→→””

2)2) Count the number of atoms of each type appearing on Count the number of atoms of each type appearing on both sidesboth sides

3)3) Balance the elements Balance the elements one at a timeone at a time by adding by adding coefficientscoefficients (the numbers in front) where you need more (the numbers in front) where you need more - - save balancing the H and O until LAST! save balancing the H and O until LAST!

(hint: I prefer to save O until the (hint: I prefer to save O until the veryvery last) last)

1)1) Double-CheckDouble-Check to make sure it is balanced. to make sure it is balanced.

NeverNever change a change a subscriptsubscript to balance an equation (You can to balance an equation (You can only change only change coefficientscoefficients))

If you change the subscript (formula) you are describing a If you change the subscript (formula) you are describing a different chemical.different chemical.

HH22O is a different compound than HO is a different compound than H22OO22

NeverNever put a coefficient in the put a coefficient in the middlemiddle of a formula; they of a formula; they must go only in the must go only in the frontfront

22NaCl is okay, but NaNaCl is okay, but Na22Cl is not.Cl is not.

Practice Balancing ExamplesPractice Balancing Examples

_AgNO_AgNO33 + _Cu + _Cu _Cu(NO _Cu(NO33))22 + _Ag + _Ag

_Mg + _N_Mg + _N22 _Mg _Mg33NN2 2

_P + _O_P + _O22 _P _P44OO1010

_Na + _H_Na + _H22O O _H _H22 + _NaOH + _NaOH

_CH_CH44 + _O + _O22 _CO _CO22 + _H + _H22OO

2 2

3

4 5

2 22

2 2

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> ___ H(g) ---> ___ H22O(l)O(l)2 2

•What Happened to the Other Oxygen Atom?

•This equation is not balanced!

•Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

Aluminum metal reacts with liquid bromine Aluminum metal reacts with liquid bromine to form solid aluminum bromideto form solid aluminum bromide

Translate EquationTranslate Equation

___ Al(s) ___ Al(s) + ___ Br+ ___ Br22(l) (l) →→___ AlBr___ AlBr33(s)(s)2 3 2