CHEMICAL REACTIONSCHEMICAL REACTIONSBalancing all thingsBalancing all things

Reactants: Zn + IReactants: Zn + I22 Product: ZnIProduct: ZnI22

Chemical reactions occur when bonds between the outermost parts of atoms (valence electrons) are broken then reformed.

Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

Symbols represent elements (C, O2), Formulas describe compounds (H2O, NaCl), Chemical equations describe a chemical reaction.

2H2 + O2 2H2O

IntroductionIntroduction

Chemical Chemical EquationsEquations

Their Job: a chemical shorthand for Their Job: a chemical shorthand for the the reactantsreactants and and productsproducts and and their relative amounts in a reaction.their relative amounts in a reaction.

44 AlAl(s)(s) + + 33 OO22(g)(g) →→ 22 AlAl22OO33(s)(s)

The The numbers in the frontnumbers in the front are called are called

““coefficientscoefficients””The letters The letters (s)(s) and and (g)(g) are the physical are the physical

states of the compounds during the states of the compounds during the reaction.reaction.

Chemical equations show the conversion of • ReactantsReactants - the atoms and molecules on the left of the arrow (ORDER DOES NOT MATTER) , into

• ProductsProducts - the atoms and molecules on the right of the arrow. (ORDER DOES NOT MATTER)

• A + sign separates molecules on the same side• The arrow is read as “yields” or “reacts to form”• Example:

2Na(s) + Cl2(g) 2NaCl(s)

• This reads “two atoms of solid sodium plus one molecule of chlorine gas react to form two molecules of solid sodium chloride”

• Note that the coefficients are understood to be 1 when no coefficients are written.

Parts of a Reaction Parts of a Reaction EquationEquation

Reactants Products

• Solid = (s)• Pure Liquid = (l)• Gas = (g)• Aqueous solution = (aq)

–Dissolved in water

• Catalyst = Pt

• Escaping gas = ( )

• Precipitate (solid) formed = (↓)• Heat is required = Δ

Symbols Used in Symbols Used in EquationsEquations

Because of the principle of the Because of the principle of the

Conservation of MatterConservation of Matter, ,

“ “Cannot create or destroy matter”Cannot create or destroy matter”

Equations must be Equations must be balancedbalanced.. It must have the same It must have the same number of atoms on both number of atoms on both reactant and product sides.reactant and product sides.

Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

John Dalton, 1790

77Chemical Equation = Chemical Shorthand

• 2H2(g) + O2(g) → 2H2O(l)

• The subscript 2 on hydrogen tells how many atoms of hydrogen are in one molecule of hydrogen gas

• The 2 in front of hydrogen is called a coefficient and shows there are two molecules of hydrogen required to maintain conservation of mass in this reaction.

When balancing a chemical reaction you

may change the value of coefficients in front of the compounds

but you may notnot change the

subscripts on the compounds.• Changing the subscripts changes the

compound. Subscripts are determined by the ratio of charges on the anion and cation in

the compound.

Balancing EquationsBalancing Equations

Subscripts vs. Subscripts vs. CoefficientsCoefficients

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

1010The Seven Diatomic ElementsN2, O2, F2, Cl2, Br2, I2, H2

• Seven of the elements on the periodic table are diatomic (two atoms) naturally.

• The theme of the seven diatomics is “7”– There are 7, – They begin with element atomic number 7,

Nitrogen– They form a “7” from Nitrogen to Iodine– The 7th is Hydrogen

• Remember, you will always have a periodic table so just remember the pattern of 7.

Chemical EquationsChemical EquationsChemical EquationsChemical Equations 2Al2Al(s)(s) + Fe + Fe22OO3(s) 3(s) →→AlAl22OO3(s) 3(s) + 2Fe(s)+ 2Fe(s)

This equation means:This equation means:

2 Al atoms + 1 Fe2 Al atoms + 1 Fe22OO33 molecule produces molecule produces

1 molecule of Al1 molecule of Al22OO3 3 and 2 molecules of Feand 2 molecules of Fe

This reaction is called the thermite reaction and This reaction is called the thermite reaction and is reported to create a temperature of 3000 C!is reported to create a temperature of 3000 C!

There are four basic steps to balancing an equation:

1. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:– The numbers of atoms on both sides of the

equation are now balanced.– The coefficients are in the lowest possible

whole number ratios. (No fractions)

Steps to Balancing Steps to Balancing EquationsEquations

Some Suggestions to Help Some Suggestions to Help YouYou

• Take one element at a time, working left side to right side.

• Start with the most complicated and progress to the simplest.

• Leave free elements in the reaction for last.

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Balancing EquationsBalancing Equations

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) (l) →→ ___ Al ___ Al22BrBr66(s)(s)2 3

1515

Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

___C___C44HH88(g) + ___ O(g) + ___ O22(g) (g) →→ ___CO ___CO22(g) +___H(g) +___H22O(g)O(g)6 4 4

___C___C66HH1212 + ___O + ___O22 →→ ___CO ___CO22 + ___H + ___H22OO9 6 6

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Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

___C___C33HH66 + ___O + ___O22 →→ ___CO ___CO22 + ___H + ___H22OO

__Na__Na33POPO44 + __Fe + __Fe22OO33 → __Na→ __Na22O + __FePOO + __FePO44 3 22

92 3 3

___C___C33HH66 + ___O + ___O22 → ___CO → ___CO22 + ___H + ___H22OO2 9 6 6

Can’t leave a fraction so multiply by 2x

Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

__B__B44HH1010(g) + ___O(g) + ___O22(g) (g) →→ ___B ___B22OO33(g) + ___H(g) + ___H22O(g)O(g)11 42 10

2 511 2

Can’t leave a fraction so multiply by 2x

__B__B44HH1010(g) + ___O(g) + ___O22(g) (g) →→ ___B ___B22OO33(g) + ___H(g) + ___H22O(g)O(g)

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