CH 10 CHEMICAL REACTIONS

What’s the center or target?What’s the center or target?

Chemical Chemical ReactionsReactions

Evidence of Chemical Reaction

2. Energy change: absorbed or

released

3. Odor changes or production

4. Gas production

5. Precipitate formation

• Chemical equations are used to write/ represent chemical reactions– Show Reactants: the starting substances

– Show Products: resulting or ending substances

– Use yield (→) sign instead of equal sign (=)

ReactantReactant

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• Word equation: uses words– Setup: Reactant 1 + Reactant 2 Product 1– Ex: Iron(s) + Chlorine(g) Iron(III) Chloride(s)

• Skeleton equation: uses formulas– Ex: Fe(s) + Cl2(g) Fe Cl3(s)

• Chemical equation: balanced equation– Shows matter is conserved during a reaction (Law of Conservation of

Mass)– Ex: 2Fe(s) + 3Cl2(g) 2Fe Cl3(s)– Bolded NUMBER in FRONT are called Coefficients

1. Write the skeleton equation

2. Count the atoms of each element in the reactants

3. Count the atoms of each element in the products

4. Change the coefficients to make both sides of the equation the equal (=) (DO NOT CHANGE SUBSCRIPTS)

5. Write the coefficient(s) in the lowest ratio possible

6. Check your work– Ex:Ex: HH22(g) + O(g) + O22(g) (g) H H22O(g) O(g)

– Balanced: Balanced: 22HH2 2 + O+ O22(g) (g) 22HH22O(g) O(g)

• Practice website Practice website www.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html

Steps to Balance a Steps to Balance a Chemical EquationChemical Equation

Examples• Hydrogen gas and bromine gas form gaseous

hydrogen bromide– H2 + Br2 HBr

– Balanced: H2 + Br2 2HBr

• Balancing hints:– When odd numbers of oxygen needed use a fraction, then

multiply by two• Al + O2 Al2O3

• 2Al + O2 Al2O3 need three oxygens (odd number)

• 2Al + 1.5O2 Al2O3 fractions/ decimals cannot be left as coefficients

• 4Al + 3O2 2Al2O3 Whole equation multiplied by two to get rid of decimals or fractions

– Try the following:• C2H6 + O2 CO2 + H2O

Examples/ hints cont.• When balancing with polyatomic ions found both in

the reactants and products, replace the ions by a variable.– Fe2O3 + H2SO4 Fe2 (SO4)3+ H2O

– Fe2O3 + H2X Fe2 (X)3+ H2O

– Fe2O3 + 3H2X Fe2 (X)3+ 3H2O

– Fe2O3 + 3H2SO4 Fe2 (SO4)3+ 3H2O

• Balancing with elements that appear more than twice in different formulas (leave for last!)– CO2 + H2 CH4 + H2O hydrogen appears more than twice

– CO2 + H2 CH4 + 2H2O balance C and O

– CO2 + 4H2 CH4 + 2H2O

1.1. SynthesisSynthesis – – formation, combiningformation, combining

2.2. CombustionCombustion- - burns in air, reaction with Oburns in air, reaction with O22

3.3. DecompositionDecomposition – – breaking down into componentsbreaking down into components

4.4. Single-ReplacementSingle-Replacement – – exchange of 1 element in a compoundexchange of 1 element in a compound

5.5. Double-ReplacementDouble-Replacement – – exchange of ions b/w 2 compoundsexchange of ions b/w 2 compounds

AB AB → → A + BA + B

AB + CD AB + CD → → AD AD + BC+ BC

1. Synthesis Reaction– Chemical reaction in which two or more substances combine to form one single product.

– A + B A + B AB AB• Ex: 2Na(s) + Cl2(g) 2NaCl(s)• 4Fe(s) + 3O2(g) 2Fe2O3 (s)

2.2. Combustion ReactionCombustion Reaction– OxygenOxygen combined with a substance releasing energy in the form of light and heat combined with a substance releasing energy in the form of light and heat – Many combustion reactions are also synthesis reactionsMany combustion reactions are also synthesis reactions

– AA + + OO2 2 AO AO• Ex: 2HEx: 2H22(g) + O(g) + O2 2 2H 2H22O(g)O(g)

3. Decomposition reactions– Only one compound– Breaks down into 2 or more compounds or elements

– Setup: AB A + B– Often requires an energy source (i.e. heat, light, electricity)(i.e. heat, light, electricity)

• Ex: NH4NO3(s) N2O(g) + 2H2O(g)• The above reaction occurs when the reactant is heated

4. Replacement Reactions– involve the replacement of an element in compound– 2 kinds of replacement reactions:

• Single • Double

5. Single Replacement reactions – When atoms of one element replace the atoms of another element in a

compound

– Setup:

– Whether or not a metal will replace another metal or any other substance depends on the reactivity it has

A+ BX AX + B

6. Double Replacement reactions– Invovles the exchange of ions between 2 compounds

– Setup: AX + BY AY + BX

– If a solid is produced during the reaction it is called a precipitate

Practice – Classify the following reactions

Practice – Classify the Practice – Classify the following equations & following equations &

Balance themBalance them1. Cl2 + KBr KCl + Br2

2. Ag2O Ag + O2

3. Na + Cl2 NaCl

4. PbCl2 + Li2SO4 PbSO4 + LiCl

5. C6H12O6 + O2 → CO2 + H2O