Chemical Equation 1 - Lecture notes Chapter 1 PDF

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Chemical Equations A chemical equation is a visual representation of what happened during a chemical reaction. It uses chemical formulas, symbols, and coefficients. When you look a chemical equation it should gives you information regarding the reagents, the products, and the stoichiometric ratios. The latter being probably the most important since it shows that the chemical reaction obeys the Law of Mass Conservation. In a chemical reaction there is a rearrangement of atoms, no atoms are lost, that's why it obeys the Law of Mass Conservation. Reagents are the substances whose chemical identity changes during a chemical reaction and the products are new substances created from the reagents. In a chemical equation the products are always to the side the arrow is point at. Consider the chemical equation below: CH4 + Cl2 ----------> CCl4 + HCl As written, this equation tells me: CH4 and Cl2 are reagents CCl4 and HCl are products What is missing? physical state of reagents and products CH4 (g) + Cl2 (g) ----------> CCl4 (l) + HCl (g) Now the equation is more complete, but not quite well yet. To make sure that the equation is following the Law of Mass Conservation, it MUST have the same number of each element's atoms on both sides of the equation. Now, look at the equation above, is it obeying the Law of Mass Conservation? Certainly, not. It means that we need to balance our equation. We use coefficients in front of the chemical formulas to balance the equation. Lets rewrite the equation:

CH4 (g) + 4 Cl2 (g) ----------> CCl4 (l) + 4 HCl (g) Now the equation is a BALANCED EQUATION. Reagents 1C , 4H, 8 Cl Products 1C, 4H, 8 Cl The stoichiometric coefficients on this equation are 1,4,1,4 , in that order from left to right. The number one in front of a formula is omitted on the equation but you should interpret it as being one. Now that my equation is balanced the information I get from it is as follows: " One mol of methane reacts with four moles of chlorine gas to produce one mole of carbon tetrachloride and four moles of hydrogen chloride gas" I'm reading the equation. Is important that you can read the equations and identify the coeffcients as we will use them eventually for stoichiometric calculations. Now is your turn: Balance and write down a sentence that describes what is happening in the following chemical equation: LiOH (aq) + Fe(NO3)3 (aq) ----------> Fe(OH)3 (s) + LiNO3 (aq)

Types of Reactions I. Precipitation Reactions A precipitation reaction is the combination of cations and anions of aqueous solutions of reagents that forms an insoluble material called a precipitate. The formation of this precipitate is explained by a double-displacement mechanism. It involves the exchange of partners (cations and anions

from each reagent) as seen in the general equation depicted below (Equation 1). Whether an insoluble material forms or not depend on the solubility of the product in water among other factors such as temperature and concentration of the solution. The formation of this precipitate can be predicted before doing the reaction using the solubility rules for inorganic compounds. A -B (aq)

+

C-D (aq)

------------->

A-D (s)

+

C- B (aq)

Eq. 1

For a double-displacement reaction be a precipitation reaction, one of the products must be a solid and the other product is soluble in water Consider the following chemical equations representing a doubledisplacement reaction. (Eq. 2)

2 NaCl (aq) + CuSO4(aq) ------->

(Eq. 3)

NaCl (aq) + AgNO3 (aq)

------->

Na2SO4 (aq) + CuCl2 (aq)

NaNO3 (aq) +

AgCl (s)

no precipitate

a precipitate

In both examples, the cation of one reagent combines with the anion of the other reagent. But only equation 3 is consider a precipitation reaction since a solid (AgCl) was formed from the combination of the two aqueous solutions, namely, NaCl and AgNO3. II. Acid -Base reaction A reaction is considered an acid-base reaction if the two reagents are an acid and a base. There are various acid and base definitions. For now, we will consider the Arrhenius acid-base model. In this model an acid is a susbtance that releases hydrogen ions (H+) when added to water and a base is a substance that when added to water releases hydroxide ions (HO- ). A neutralization reaction is then a reaction between an acid and a base that produces a salt and water. Ex. HCl (aq) + NaOH (aq) --------> NaCl (aq) + H2O (l)

In the above equation, HCl is the acid and NaOH is the base. NaCl is the soluble salt formed in this reaction. A neutralization reaction is also a double displacement reaction as you can noticed on the equation above. The salt is not always a water-soluble salt. III. Redox Reactions Oxidation-Reduction reactions, also known as Redox reactions, are reactions in which one reagent gains electrons and the other reagent loses electrons. This gaining and losing of electrons must occur simultaneously. Often, metals undergo this type of reactions but also some reactions that involve nonmetals can be redox reactions. An example of a redox reaction involving non-metals is combustion (burning wood or paper).Oxidation is defined as losing electrons or an increase on the number of oxygen atoms; reduction on the other hand, is defined as gaining electrons or gaining hydrogen atoms Ex. Zn (s) + 2 HCl (aq) ---------> ZnCl2 (aq) + H2 (g) CH4 (g) + 2 O2 (g) ----------> CO2 (g) + 2 H2O (g) (combustion rxn)

VOCAB TO KNOW: Chemical Reaction: When two or more substances combine and produce a new substance. - a rearrangement of atoms occurs -Law of Mass Conservation is obeyed. Ex. rusting of iron Chemical Equation: Represents the summary of a chemical reaction. It should includes:

Ex.



reagents



products



arrow



physical states of products and reagents



stoichiometric coefficients Pb(NO)3 (aq) + 2 NaBr (aq) ---------------> PbBr2 (s)

+ 2 NaNO3 (aq) Reagents

Products

Reagent: Substance in a chemical reaction that is transformed into a different substance. Its chemical identity changes Product: New substance formed during a chemical reaction. Has different chemical properties than the substances from which it formed. Precipitation Reaction: Occurs when two aqueous solutions combine and produce an insoluble material. Acid-Base Reaction: Chemical reaction between an acid and base. The products are a salt and water. Ex. HCl (aq) + NaOH (aq) -----> NaCl (aq) + H2O (l) Redox Reaction: Chemical reaction in which a reagent gains electrons and another reagent loses electrons, simultaneously. Often observed in reactions involving metal

Ex. NaCl (aq) + AgNO3 (aq) --------------> AgCl (s) + NaNO3 (aq)...