10.1 Reactions and Equations Evidence of Chemical Reactions Chemical Reaction a process in which one or more substances are converted into new substances with different physical and chemical properties. Chemical reactions occur so that substances can reach more stable arrangements of electrons in an ion, atom or molecule. Signs of Reactions Temperature change rising or dropping Color change Odors Gas bubbles Appearance of solids (precipitates) Representing Chemical Reactions A chemical reaction can be represented by a chemical equation Reactants substances present before reaction Products substances present after reaction Spectators substances present before and after the reaction Basic Chemical Equation Conventions Reactants are placed on the left Products are placed on the right Multiple reactants or products are joined by a + An arrow represents the direction of the reaction Word Equations equations describing a reaction using words hydrochloric acid + zinc zinc chloride + hydrogen Skeleton Equations equations describing a reaction using formulas HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) Complete chemical equations. A chemical equation shows how a reaction occurs. Part of describing a reaction is describing the physical state or phase of each reactant and product. o Symbols (g) = gas (l) = liquid (s) = solid (aq) = aqueous solution Chemical Equations The Law of Conservation of Matter Matter may be neither created nor destroyed in a chemical reaction. A chemical equation must reflect this law.
All the atoms in the reactants must be present in the products. All the atoms in the products must have a source in the reactants. HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) Balancing Chemical Equations Balancing an equation is really about finding ratios that occur in reactions that satisfy the Law of Conservation of Matter. The single set of ratios will exist for every chemical reaction Coefficients are numbers used to show how many molecules, ions or atoms are present in a reaction. 2 is a coefficient. 2HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) Steps for Balancing Equations Write the skeleton equation for the reaction Count the number of atoms of the first element listed in the reactants Check the number of atoms of that element in the products If they don t match, add a coefficient in front of the formula with the fewest of that atom to make them equal Move to the next element, working left to right, until you have equal number of each atom on either side of the table Some rules/guidelines for balancing equations Balanced equations must use the lowest whole number ratio of coefficients This is balanced, AND the lowest ratio. 4KNO 3 4KNO 2 + 2O 2 2KNO 3 2KNO 2 + O 2 NEVER, EVER, EVER change a subscript in a formula while balancing equations This is unbalanced. This is balanced, but WRONG! The product should be water. H 2 + O 2 H 2 O H 2 + O 2 H 2 O 2 2H 2 + O 2 2H 2 O Balance single species substances last. This is unbalanced. This is balanced. This is balanced, but not the lowest ratio. This is correct. If I change the coefficient here, I only affect O. C 3 H 8 + O 2 CO 2 + H 2 O If I change the coefficient here, I affect O and C.
Examples Examples Examples 10.2 Classifying Chemical Reactions Synthesis Reactions Two or more substances combine to form one new substance. NH 3 (aq) + HCl(aq) NH 4 Cl(g) H 2 (g) + O 2 (g) H 2 O(l) Synthesis reactions may be recognized by the general form of the equation: Substance + Substance Compound Combustion Reactions Most organic substances and some inorganic substances will burn in air. This is actually a reaction with oxygen and is often called an oxidation reaction. Typically the products of oxidation of a hydrocarbon are carbon dioxide and water. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) Combustion reactions may be recognized by the general form of the equation: Hydrocarbon + Oxygen Carbon Dioxide +Water Decomposition Reactions A compound will break up or decompose into simpler substances when energy (light, heat, mechanical shock, or electricity) is supplied. CdCO 3 (s) CdO(s) + CO 2 (g) Pb(OH) 2 (s) PbO(s) + H 2 O(l) Decomposition reactions may be recognized by the general form of the equation: Compound Two or more substances* * Remember, a substance is defined as an element or a compound. Replacement Reactions (a.k.a. Displacement Reactions) Single Replacement Reactions One element replaces another in a compound.
Examples Examples Cl 2 (g) + 2KBr(aq) 2KCl(aq) + Br 2 (l) 2Al(s) + Fe 2 O 3 (aq) 2Fe(s) + Al 2 O 3 (aq) Single replacement reactions may be recognized by the general form of the equation: Element + Compound Element + Compound Double Replacement Reactions The positive and negative portions of two compounds are interchanged. CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + H 2 CO 3 (aq) 3NiSO 4 (aq) + 2Li 3 PO 4 (aq) Ni 3 (PO 4 ) 2 (s) + 3Li 2 SO 4 (aq) Double replacement reactions may be recognized by the general form of the equation: Compound + Compound Compound + Compound Solids formed in an aqueous double replacement reaction are called precipitates. Products will include a precipitate, a gas or water. 10.3 Reactions in Aqueous Solutions Solutions Substances dissolved in water form aqueous solutions Solvent water Solute substances dissolved Aqueous Solutions Solutes may be molecular or ionic Molecular solutes Most will exist as molecules even when dissolved in water Acids behave like ionic compounds when dissolved in water o Acids dissolve to form hydrogen cations and one or more anions Ionic solutes Ionic substances may separate into their component ions Ions from combined solutions may react (double replacement) Water is a spectator in the reaction and is not involved Reactions that Form Precipitates When ionic substances are dissolved in solution, the ions will act independently
The independency of the ions in an ionic aqueous solution may be shown using an ionic equation Chemical Equation K 2 S(aq) + CoCl 2 (aq) 2KCl(aq) + CoS(s) Complete Ionic Equation 2K + (aq) + S 2- (aq) + Co 2+ (aq) + 2Cl - (aq) 2K + (aq) + 2Cl - (aq) + CoS(s) Net Ionic Equation Co 2+ (aq) + S 2- (aq) CoS(s) In the net ionic equation the spectator ions K + and Cl - are left out. Reactions that Form Water In acid-base neutralization reactions water is a product which may not be outwardly noticeable The amount of solvent increases Chemical Equation HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) Complete Ionic Equation H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) Na + (aq) + Cl - (aq) + H 2 O(l) Net Ionic Equation H + (aq) + OH - (aq) H 2 O(l) In the net ionic equation the spectator ions Na + and Cl - are left out. Reactions that Form Gases Double replacement reactions may also produce gases Net ionic equations may be written for these as well.