Chemical Bonds And Equations Intro to Chemical Bonds EQ: Why do atoms have a strong desire to form chemical bonds in nature? What are these called when different types of atoms bond to form a chemically different substance? Compound – substance made of the combined atoms of two or more elements. Chemical formula – tells what elements a compound contains and the exact number of the atoms of those elements Some Common Compounds Vinegar Acetic acid HC2H3O2 Sand Silicon dioxide SiO2 Stomach Hydrochloric HCl Acid acid Cane Sugar Sucrose C12H22O8 Superscript – represents 2H2O oxidation # or 2- how many Coefficient SO4 – represents the # electrons has of units of each gained or lost substance Subscripts – represents # of atoms in a molecule of a particular element Atoms form compounds to become chemically stable. An atom is chemically stable when the outer energy level is complete, or full. (Why it is hard to find these elements alone / unbonded.) Chemical bond – force that holds together the atoms in a substance ION – positive or negatively charged atom Ionic Bond vs. Covalent Bond loses or gains electrons shares electrons attraction between can form multiple bonds opposite charges of ions between a nonmetal and another nonmetal between metal & nonmetal nonmetals EQ Answer: Compounds have different properties than the atoms that male them up. Atoms form compounds to create a chemically stable molecule or compound. The chemical bonding stabilizes atoms by filling their outer most shell (valence). Writing Formulas and Naming Compounds Binary compound – compound composed of two elements Oxidation number – positive or negative number (on PTable) which indicates how many electrons an element has gained, lost or shared when bonding with another atom. * When writing a formulas for binary ionic compounds, it is important to remember that compounds formed have a net charge of zero. POLYATOMIC IONS Charged Name Formula Polyatomic Ions a positively or group of 1+ Ammonium NH4+ - atoms negatively charged 1- Acetate C2H3O2 covalently bonded - 1- Chlorate ClO3 1- Hydroxide OH- 1- Nitrate NO - The prefix poly means 3 2- Carbonate CO 2- “many”, so the term 3 2- Sulfate SO 2- polyatomic means 4 3- Phosphate PO 3- “having many atoms” 4 Hydrate – a compound that has water chemically attached to its ions and written into chemical formula. Covalent compounds – can form more than one compound with each other. Scientist use Greek prefixes to indicate # of atoms of each element in binary compound. Prefixes For Binary Covalent Compounds #atoms 1 2 3 4 5 6 7 8 prefix Mono- di- tri- tetra penta hexa hepta octa Chemical Reactions Chemical reaction - a reaction in which one or more are changed to new substances Reactants - substances that are about to react Produced Products - new substances Reactants Products produced Law of Conversation of Mass – total starting mass of all reactants equals the total final mass all products. Chemical equation - uses chemical formulas and symbols to describe a chemical reaction and the product it produces (see below) Symbol Meaning Symbol Meaning produces or (aq) Aqueous (solid yields dissolved in H2O) + plus Heat Reactants heated (s) solid Light Reactants exposed to light (l) liquid Elec Electric current (g) gas Applied to reactants Coefficients – #’s which represents the number of units of each substance in a rxn 4Al (s) + 3O2 (g) 2Al2O3 Subscripts – #’s which represent the number of atoms in a molecule of a particular element BALANCING CHEMICAL EQUATIONS Balanced chemical equations – have the same number of atoms of each element on both sides of equation * When balancing chemical equation NEVER change the subscripts, instead change the coefficient in front of the compounds needing balanced. Steps to Balancing a Chemical Equation Step 1: Write a chemical equation for the reaction using formulas and symbols – make sure reactants are on left side and products on right. Step 2: Count the atoms in reactants and products. Step 3: Choose coefficients that balanced the equation. HINT: Generally, if there is an even # of an element on one side and an odd on the other side, place a 2 in front of compound containing odd number of atoms. Step 4: Recheck the numbers of each atom on both sides of and adjust coefficients if necessary – remember NEVER change subscripts. Ex. 2NaBr + Cl2 2NaCl + Br2 STEP 3 STEP 1 Na 1 2 1 2 Br 1 2 2 Cl 2 1 2 STEP 2 STEP 4 CHEMICAL REACTION TYPES Synthesis reaction - reaction where two or more substances combine to form another substance A+B AB Decomposition reaction - reaction where one substance breaks down, or decomposes, into two or more simpler substances AB A+B Double – displacement reaction - reaction that results in a precipitate, water or gas when a positive ion of one compound swaps with another positive ion of another compound. AB + CD AC + BD Single – displacement reaction - reaction in which one element replaces another in a compound A + B AC + B or A + BC AB + C CHEMICAL REACTIONS –RATES AND ENERGY • To determine if the overall reaction releases or absorbs energy, you must compare the energy content of the reactants and the products. • Exothermic reaction - releases energy; reactants contain more energy than the products. (reactants = products + energy) Ex: Zn + 2HCl → ZnCl2 + H2 + energy Ex. burning wood, fireworks • Endothermic Reaction - Absorbs energy; products contain more energy than the reactants. (reactants + energy = products) • Energy must be applied to get the reaction going. Energy will appear on the reactants side of the equation. • Ex: 6 CO2 + 6 H2O + energy → C6H12O6 + 6 O2. Ex. Chemical Ice pack, photosynthesis Rate of Reaction – Speed at which reaction occurs or new substances form. Activation Energy • Activation energy is the energy that molecules need to break the bonds and begin the reaction. • Sources for AE can be friction, electricity, light, etc. • The rate at which molecules come into contact with each other determines the reaction rate. This is known as the “collision theory”. • There are 3 factors that influence the rate at which particles run into each other: • 1. Temperature – The higher the temperature the faster the particles move. • 2. Concentration – The greater the concentration of the reactants, the more particles there are to collide. • 3. Surface Area - As surface area increases (crushing) there are more reactant particles exposed. Other factors that affect reactions… Catalyst – speeds up a chemical reaction without itself being permanently changed (lowers activation energy) Inhibitor – prevents or slows a chemical reaction or interferes with the catalyst.