Chemical Equations and Reactions Notes Chemical Equations and Reactions

Chemical Equation Reactants: original substances Products: new or resulting substances Chemical 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 & light Some give off light only or heat only Production of a gas Bubbles in a solution Formation 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 diatomic 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: reactants and products in a reaction are represented by words Only has qualitative (descriptive) meaning Formula equation: represents the reactants and products by their symbols or formulas Must be balanced to satisfy law of conservation of massmore 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 D 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) D 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) http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson82.htm

Decomposition Reactions A single compound undergoes a reaction that produces two or more simpler substances AX  A + X 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 )

Five types of Decomposition All usually require heat Electrolysis: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) http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson82.htm

Single Replacement Reactions A + BX  AX + B BX + Y  BY + X Replacement of: Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

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 Hydrogen Bismuth Copper Mercury Silver Platinum Gold Metals can replace other metals provided that they are above the metal that they are trying to replace. Metals above hydrogen can replace hydrogen in acids. Metals from sodium upward can replace hydrogen in water

The Activity Series of the Halogens Fluorine Chlorine Bromine Iodine Halogens can replace other halogens in compounds, provided that they are above the halogen that 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 an aqueous 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 of solution, 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 large amount 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.