Chemical Bonds And Equations

Transcription

1 Chemical Bonds And Equations

2 Intro to Chemical Bonds

3 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?

4 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

5 Some Common Compounds Vinegar Acetic acid HC 2 H 3 O 2 Sand Silicon dioxide SiO 2 Stomach Acid Hydrochloric acid HCl Cane Sugar Sucrose C 12 H 22 O 8

6 Coefficient represents the # of units of each substance 2H 2 O SO 4 2- Subscripts represents # of atoms in a molecule of a particular element Superscript represents oxidation # or how many electrons has gained or lost 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.)

7 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 attraction between opposite charges of ions between metal & nonmetals shares electrons can form multiple bonds between a nonmetal and another nonmetal nonmetal

8 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).

9 Writing Formulas and Naming Compounds

10 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.

11 POLYATOMIC IONS Polyatomic Ions a positively or group of atoms negatively charged covalently bonded Charged Name Formula 1+ Ammonium NH Acetate C 2 H 3 O Chlorate ClO Hydroxide OH - The prefix poly means many, so the term polyatomic means having many atoms 1- Nitrate NO Carbonate CO Sulfate SO Phosphate PO 3-4

12 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 prefix Mono- di- tri- tetra penta hexa hepta octa

13 Chemical Reactions

14 Chemical reaction - a reaction in which one or more are changed to new substances Reactants - substances that are about to react Products - new substances produced Reactants Produced Products Law of Conversation of Mass total starting mass of all reactants equals the total final mass all products.

15 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 yields (aq) Aqueous (solid dissolved in H2O) + plus Heat Reactants heated (s) solid Light Reactants exposed to light (l) liquid Elec Electric current (g) gas Applied to reactants

16 Coefficients # s which represents the number of units of each substance in a rxn 4Al (s) + 3O 2 (g) 2Al 2 O 3 Subscripts # s which represent the number of atoms in a molecule of a particular element

17 BALANCING CHEMICAL EQUATIONS

18 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.

19 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.

20 Step 4: Recheck the numbers of each atom on both sides of and adjust coefficients if necessary remember NEVER change subscripts. Ex. 2NaBr + Cl 2 2NaCl + Br 2 STEP 3 STEP 1 Na Br Cl STEP 2 STEP 4

21 CHEMICAL REACTION TYPES

22 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

23 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

24 CHEMICAL REACTIONS RATES AND ENERGY

25 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

26 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 CO H 2 O + energy C 6 H 12 O O 2. Ex. Chemical Ice pack, photosynthesis

27 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.

28 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.

29 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