Chemical Reactions

Chemical Reactions

• Reactants:– A starting substance in a chemical reaction.

• Products:– A substance formed in a chemical reaction.

Reactants Products

• Reactants are always on the left of the arrow.

• Products are always on the right of the arrow.

• What does the arrow mean?– Yields or produces

For Review: Law of Conservation of Mass

• Are atoms ever created or destroyed in a chemical reaction?

NO!!!

Definitions

• Chemical Equation:– The arrow separates the formulas of the

reactants from the formulas of the products.

• Skeleton Equation:– A chemical equation that doesn’t indicate the

relative amounts of the reactants and products involved.

– Symbols you will see: (s), (l), (g), and/or (aq)

More Definitions:• Catalyst:

–Substance that speeds up the rate of a reaction but is not used up in the reaction.• If a catalyst is used, the symbol or formula

is written above the arrow.

Evidence of Chemical Reaction

• Energy change

– Temp increase/decrease

– Light emitted

• Precipitate produced

• Unexpected color change

• Gas released/bubbles

Types of Reactions

• There are 5 types of reactions.–1. Combination (a.k.a. synthesis)–2. Decomposition–3. Single replacement–4. Double replacement–5. Combustion

Combination (Synthesis)• Start with TWO elements/compounds and

merge to form ONE productTwo elements combine• A + B C• Mg(s) + O2(g) MgO(s)

• Element/cmpd + element/cmpd Compound

• Example: Complete the following combination reaction:

K(s) + Cl2(g)KCl

2 2

2 2

Combination, cont.

• Nonmetal oxides react with water to form acids–SO2(g) + H2O(l) H2SO3 (aq)

• Metallic oxides react with water to from bases–CaO(s) + H2O(l) Ca(OH)2

Synthesis ReactionsSynthesis Reactions

• Here is another example of a synthesis reaction

PracticePractice

• Predict the products. Write and balance the following synthesis reaction equations.

• Sodium metal reacts with chlorine gas

Na(s) + Cl2(g) • Solid Magnesium reacts with fluorine gas

Mg(s) + F2(g) • Aluminum metal reacts with fluorine gas

Al(s) + F2(g)

Decomposition:

• A compound breaks down into a two or more simpler substances.

• C A + B• CaCO3(s) CaO(s) + CO2(g)

• *Most decomposition reactions require energy in the form of heat, light or electricity.

• Cmpd element/cmpd + element/cmpd

Decomposition Rxn.• Start with ONE cmpd and it breaks into TWO

pieces (two elements or two smaller cmpds)

– Ionic Salt M + NM

NaCl

– Carbonate salt metal oxide + CO2

CaCO3

– Chlorate/perchlorate salt metal chloride + O2

KClO3

Decomposition ReactionsDecomposition Reactions

• Another view of a decomposition reaction:

PracticePractice

• Predict the products. Then, write and balance the following decomposition reaction equations:

• Solid Lead (IV) oxide decomposes PbO2(s)

• Aluminum nitride decomposes

AlN(s)

Single Replacement

• One element replaces a second element in a compound.

• A + BC AC + BElement + Compound Compound +

Element

Single Displacement Rxns.• Element reacts with a cmpd to produce a new element

and a new cmpd.– Metal + cmpd new metal + new cmpd

AgNO3 (aq) + Cu (s) – NM + cmpd new NM + new cmpd

Cl2 (g) + NaI (aq) – Metal + water H2 + metal hydroxide (base)

Li (s) + HOH (l)

• Use REACTIVITY SERIES to make sure these rxns will occur

• Element must be more reactive than the ion in the cmpd in order to replace it

Single Replacement, cont.• Whether or not a metal will displace

another metal depends on its reactivity to other metals. – A metal must be more reactive to “kick out”

another metal. If it is not, there is no reaction.

– Activity Series of Metals. Reactivity decreases as you go down, so metals will only replace another one below it.

• A nonmetal can also replace another nonmetal from a compound.– This is usually limited to halogen group.

– Activity of halogens decreases as you go down the column (F is more reactive than I).

Single Replacement ReactionsSingle Replacement Reactions

• Another view:

Single Replacement Practice

• Complete the following reactions:

• 1. Zn + H2SO4 • 2. Sn + NaNO3

• 3. Cl2 + NaBr

ZnSO4 + H2

No Reaction

NaCl + Br2

Single Replacement Practice

• Complete the following reactions:

• 1. Mg + KCl • 2. Ag + HCl• 3. l2 + KF• 4. Zn + LiCl• 5. Ca + PbO2

Double Replacement• Involves an exchange of positive ions

between two reacting compounds.

• AB + CD AD + CB

• Cmpd + Cmpd Cmpd + Cmpd

For a double replacement reaction to occur, one of the following is usually true:

1. A precipitate is formed.2. A gas is produced and bubbles out of solution.3. One product is a molecular compound such as water.

Double Replacement ReactionsDouble Replacement Reactions

• Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together

• Example:

AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)

• Another example:

K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s) 2

PracticePractice

Predict the products

1. HCl(aq) + AgNO3(aq)

2. CaCl2(aq) + Na3PO4(aq)

3. Pb(NO3)2(aq) + BaCl2(aq)

4. FeCl3(aq) + NaOH(aq)

5. H2SO4(aq) + NaOH(aq)

6. KOH(aq) + CuSO4(aq)

Combustion• An element or a compound reacts with oxygen (O2) often

producing energy as heat and light.

X + O2 XO

Often involves hydrocarbons: C and H• C#H#, C#H#O# or C#H#OH

Complete combustion of a hydrocarbon looks like this:

C3H8 + O2(g) CO2 + H2O

Combustion ReactionsCombustion Reactions• In general:

CxHy + O2 CO2 + H2O

• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

Identify the following reactions1. S + O2 → SO3

2.CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl3. K + Cl2 → KCl4. H2O2 → H2O + O2

5. Na + H2O → NaOH + H2

6. KClO3 → KCl + O2

7. C2H6 + O2 → CO + H2O8.KI + Cl2 KCl + l2

9. CH4 + O2 → CO2 + H2O10. N2 + H2 → NH3

11. P + O2 → P4O10

12. Fe(OH)3 → Fe2O3 + H2O

PracticePractice

Predict the products. Identify the type of reaction.1. S + O2

2. CaCl2 + Na3PO4)

3. Pb(NO3)2 + BaCl2

4. KI + Cl2

5. CH4 + O2

6. C3H8 + O2

7. CaCO3

8. Na + H2O

Balancing Combustion Balancing Combustion ReactionsReactions

When balancing combustion reactions, start with Carbon and balance Oxygen last.

If you end up needing an odd number of diatomic oxygen, multiply the coefficient in front of the hydrocarbon by 2 and rebalance.

• Here is what I mean…

– Balance the following equation:

C3H8 + O2 CO2 + H2O

C4H10 + O2 CO2 + H2O

C4H10 + O2 CO2 + H2O

3 45

4 5

2 8 1013

6.5

Combustion ReactionsCombustion Reactions

Combustion Practice

• Combust the following compounds:

• C6H6

• CH3OH

• Be sure to balance these!

+ 15 O2

+ 3 O2

12 CO2 + 6 H2O

2 CO2 + 4 H2O

2

2

Balancing Equations

• Balanced Equation:

– Each side of the equation has the same number of atoms of each element.

• Why balance an equation?

– To obey the law of conservation of mass.

How do we balance?

• Use Coefficients:– “The big numbers in front.” Place the

coefficient in front of the symbols for the respective parts. If no number is written, it is understood to be one.

Rules for Balancing

• 1. Determine the correct formulas– If states are given, include these in

parenthesis.

• 2. Write the formula for reactants on left and products on the right.– Separate with a yield sign.– If there are two or more reactants or

products, separate with a plus sign.

Rules for Balancing, cont.

• 3. Count the number of atoms of each element.– A polyatomic ion appearing unchanged on

both sides of the equation is counted as a single unit.

– Example:

CaS(s) + NaOH(aq) Ca(OH)2(aq) + Na2S(s)

Versus: NaHCO3(s)+HCl(aq) NaCl(aq)+H2O(l)+ CO2(g)

Rules for Balancing, cont.

• 4. Balance the elements one at a time.• 5. Check to be sure each atom or

polyatomic ion is balanced.• 6. Make sure all coefficients are in the

lowest possible ratio.