Notes Notes

Chemical Equations and Chemical Equations and ReactionsReactions

Chemical Equation

• Reactants: original substances• Products: new or resulting

substances• Chemical EquationChemical Equation: represents, with

symbols & formulas, the identities & relative amounts of the reactants & products in a chemical reaction

Evidence of a Chemical Reaction

• Evolution of heat & lightEvolution of heat & light– Some give off light only or heat only

• Production of a gasProduction of a gas– Bubbles in a solution

• Formation of a precipitateFormation of a precipitate– A solid that is produced as a result of a

chemical reaction in solution and that separated from the solution

Characteristics of Equations

• Must represent known facts– All reactants & products must be included

• Must contain correct formulas– Some elements when uncombined

(elemental form) are diatomicdiatomic H2, N2, O2, F2, Cl2, Br2, I2

– Must always write their formulas this way!• Must satisfy law of conservation of mass

– Atoms are not created or destroyed– Coefficients are added in front of formula as

needed.

Word & Formula Equations

• Word equation:Word equation: reactants and products in a reaction are represented by words– Only has qualitative (descriptive) meaning

• Formula equation:Formula equation: represents the reactants and products by their symbols or formulas– Must be balanced to satisfy law of

conservation of massmore on this later!

Symbols Used in Equations

• “yields” indicates results

• used to indicate a reversible reaction

• (s) indicates solid state; precipitate– alternate to (s) but only for precipitate

• (l) indicates liquid state

• (aq) substance dissolved in water

• (g) indicates gaseous state– alternate to (g) but only for products

More Symbols Used in Equations

• heat reactants are heated• 2 atm pressure at which reaction is

carried out; 2 atm in this case• pressure pressure of reaction is greater

than atmospheric pressure• 0oC temperature at which reaction is

carried out; 0oC in this case• MnO2 formula of catalyst used to alter

rate of reaction; MnO2 in this case

Practice

• Convert word equation to formula equation– Methane gas and oxygen gas react to yield

carbon dioxide gas and water vapor

• Convert formula equation to word equation– 2HgO (s) 2Hg (l) + O2 (g)

  Synthesis A synthesis reaction involves two or more

substances combining to make a more complex substance.  The reactants may be elements or compounds, and the product will always be a compound.

A  +   B ---->  AB element or compound + element or compound -----> compound

Examples 2H2(g) + O2(g) ----> 2H2O(g)

C(s) + O2(g) ----> CO2(g)

CaO(s) + H2O(l) ----> Ca(OH)2(s)

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson82.htm

Synthesis (Composition) Reactions

Two or more substances combine to form a new compound.

A + X AX

Reaction of elements with oxygen and sulfur Reactions of metals with Halogens Synthesis Reactions with Oxides There are others not covered here!

Three types of Synthesis

Elements react w/ O or S Produces a metal oxide or metal sulfide Also can produce nonmetal oxide

Metals react w/ halogens Produces a metal halide compound

Oxides react w/ H2O Metal oxides produce metal hydroxide Nonmetal oxides produce oxyacids

Decomposition

In a decomposition reaction, one substance is broken down into two or more, simpler substances. 

AB ----> A  +  B Compound ------> element or compound + element or compound

Examples C12H22O11(s) ----> 12C(s) + 11H2O(g)

Pb(OH)2(cr) ----> PbO(cr) + H2O(g)

2Ag2O(cr) ----> 4Ag(cr) + O2(g)

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Decomposition Reactions

A single compound undergoes a reaction that produces two or more simpler substances

Decomposition of: Binary compounds H2O(l ) 2H2(g) +

O2(g) Metal carbonates CaCO3(s) CaO(s) +

CO2(g) Metal hydroxides Ca(OH)2(s) CaO(s) +

H2O(g) Metal chlorates 2KClO3(s) 2KCl(s) +

3O2(g) Oxyacids H2CO3(aq) CO2(g) + H2O(l

)

AX A + X

Five types of Decomposition All usually require heat

ElectrolysisElectrolysis:decomposition of a substance by an electric current

Binary Compounds Produces elements

Metal Carbonates Produces metal oxide & CO2

Metal Hydroxides Produces metal oxide & H2O

Metal Chlorates Produces metal chloride & O2

Acids Produces nonmetal oxide & H2O

Single Replacement

In this type of reaction, a neutral element becomes an ion as it replaces another ion in a compound.  

Positive ion being replaced:  A + BC ----> B + AC Negative ion being replaced:  A + BC ----> C + BA element + compound ----> element + compound Examples

Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)

2Al(s) + 3CuCl2(aq) ---> 2AlCl3(aq) + 3Cu(s)

Cl2(g) + KBr(aq) ----> KCl(aq) + Br2(l)

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Single Replacement Reactions

Replacement of:

Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

A + BX AX + B

BX + Y BY + X

Four types of Single Replacement Replacement of a metal by another metal

A more active metal will replace a less active metal Replacement of hydrogen in water by a metal

Only most active metals (Group 1) Produces metal hydroxide & H2

Replacement of hydrogen in an acid by a metal Produces a salt & H2

Replacement of halogens A more active halogen will replace a less active

halogen Activity decreases down group

The Activity Series of the Metals

Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead HydrogenHydrogen Bismuth Copper Mercury Silver Platinum Gold

Metals can replace other metalsprovided that they are above themetal that they are trying to replace.

Metals above hydrogen can replace hydrogen in acids.

Metals from sodium upward canreplace hydrogen in water

The Activity Series of the Halogens

Fluorine Chlorine Bromine Iodine

Halogens can replace other halogens in compounds, providedthat they are above the halogenthat they are trying to replace.

2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)

MgCl2(s) + Br2(g) ???No Reaction

???

Double Replacement

Like dancing couples, the compounds in this type of reaction exchange partners. 

AB + CD ----> CB + AD Compound + Compound ----> Compound + Compound

Examples AgNO3(aq) + NaCl(aq) ----> AgCl(s) + NaNO3(aq)

ZnBr2(aq) + 2AgNO3(aq) ----> Zn(NO3)2(aq) + 2AgBr(cr)

H2SO4(aq) + 2NaOH(aq) ----> Na2SO4(aq) + 2H2O(l)

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson82.htm

Double Replacement Reactions

The ions of two compounds exchange places in anaqueous solution to form two new compounds.

AX + BY AY + BX

One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out ofsolution, or a molecular compound, usually water.

Three types of Double Replacement

Formation of a Precipitate One product is solid & the other aqueous

Formation of a Gas One product is gas & the other aqueous

Formation of Water Acid-base neutralization is double

replacement H+ & OH- give water

Combustion When organic compounds like propane are

burned, they react with the oxygen in the air to form carbon dioxide and water. Combustion reactions will stop when all available oxygen is used up because oxygen is one of the reactants.

hydrocarbon + oxygen ----> carbon dioxide + water Examples

CH4(g) + 2O2(g) ----> 2H2O(g) + CO2(g)

2C2H6(g) + 7O2(g) ----> 6H20(g) + 4CO2(g)

C3H8(g) + 5O2(g) ----> 4H2O(g) + 3CO2(g)

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson82.htm

Combustion Reactions

A substance combines with oxygen, releasing a largeamount of energy in the form of light and heat.

Reactive elements combine with oxygen

P4(s) + 5O2(g) P4O10(s)

(This is also a synthesis reaction)

The burning of natural gas, wood, gasoline

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Steps to Balancing a Chemical Equation

• 1 ) Write all reactants on the left and all products on the right side of the equation arrow. Make sure you write the correct formula for each element.

• 2) Use coefficients in front of each formula to balance the number of atoms on each side.

Steps to Balancing a Chemical Equation

3 ) Multiply the coefficient of each element by the subscript of the element to count the atoms. Then list the number of atoms of each element on each side.

Steps to Balancing a Chemical Equation

4) It is often easiest to start balancing with an element that appears only once on each side of the arrow. These elements must have the same coefficient.

5) Next balance elements that appear only once on each side but have different numbers of atoms. Finally balance elements that are in two formulas in the same side.

***Note**It helps to balance hydrogen and oxygen

last.