in this chapter we'll talk about chemical reactions and we'll learn how to balance chemical equations so first we need to talk about physical and chemical changes in the physical change the substance does not change its chemical composition but it does undergo a change in physical appearance so examples of physical changes are boiling of water so if we start with water in liquid state heat up the the water which consists of H2O molecules and then the H2O molecules go into the gas phase in both the water and the gas the molecules are the same so there's no change in the molecular level if we take an ice cube and we crush it crushing is another example we crush ice cube before and after the process we still have H2O molecules tearing a piece of paper melting an ice cube all of these are physical changes chemical change on the other hand involves the following the atoms that make up the substance rearrange to make new compounds so another name for chemical changes is chemical reaction and I'm going to use the abbreviation rxn that you see right here for reactions so here's some examples of chemical changes or chemical reactions oxidation of iron is an example of a chemical change or chemical reaction burning of a candle which is combustion that's another example and we represent these chemical reactions by using chemical equations so here's what here's an example of a chemical equation the arrow represents that a chemical reaction is taking place so this is a chemical change or chemical reaction on the left of the arrow are the reactants these are the compounds that react they physically have to collide with each other they have to some atoms have to be exchanged and new compounds are formed and on the right hand side of the arrow are the products so in a chemical equation we have to include two things we have to include the formula of the reactants in this case iron oxygen feric oxide but we also have to include the states and here's where I'm showing the states s for solid G for gas s for solid so both the formula and the states have to be part of the chemo equation so in the next part of these notes I'm giving you a list of some of the states of the most common elements and molecules that you see in this class so s is the state for solid and it includes all metals there's one exception however Mercury Mercury is the only metal that's a liquid so for mercury we'll write L for liquid but all other metals are solids all ionic compounds are also solids and then here's some um non-metals that are also in solid form so that includes carbon silicon phosphor sulfur and iodine and then liquids Mercury is a liquid bromine is a liquid liid and obviously water at room temperature is also a liquid for gases hydrogen nitrogen oxygen Florine and chlorine are gases all the noble gases are gases and then ammonia the molecule ammonia is also a gas at from temperature now sometimes the ionic compounds or the molecules will be dissolved in water and if that's the case then we have to use the symbol AQ which stands for aquous this means water solution so if I take sodium chloride Crystal s think of table salt uh table salt is white shiny crystals solid it from temperature but if we dissolve them in water then the correct way to express it will be to use the aquous symbol so sodium chloride aquous is salt dissolved in water if I put s here instead of aquous then it's just salt no water all right so let's do the following example this is a chemical reaction that's described in word format and you have to converted to um the correct formulas oxygen and hydrogen form water so we write the formula for oxygen remember oxygen is one of the seven diatomic elements so O2 plus H2 another diatomic element here's the arrow for chemical reaction forms water H2O the state for oxygen and hydrogen are both gases water at room temperature is a liquid you can ignore these red numbers that I have the two the one one and the two and the one you can ignore it for now we'll talk about them later these are called coefficients that we use for balancing equations let's do another example calcium and sulfur form calcium sulfide so calcium is a metal it's a solid sulfur is a nonmetal it's one of the nonmetal solids when they combine they form calcium sulfite which is also a solid you might be wondering how do we get calcium sulfide this is an ionic compound and you get the formula by considering the charges of the non-metal sulfide and the metal calcium so figure out what their charges are do the criss-crossing method and that's how you get the ionic formula and then the next example sodium and Florine form sodium fluoride here's the formula for sodium it's a solid because it's a metal Florine is diatomic I write F2 and it's a gas and when they react they form the ionic compound sodium fluoride um please don't N2 Florine is datomic only if it's by itself once it's connected to other elements then it's no longer the atomic instead it carries a charge and you have to figure out what the charge is Florine has fluoride ion has a one minus charge so write down the formula for fluoride ion for sodium ion do the crisscross method and that's how we come up with sodium fluoride formula and since it's an ionic compound and there's no water around it's a solid okay so let's go back to the these numbers that I have here I have a two and a one and a two what does that mean let's look at the last reaction that we just discussed sodium and Florine forms sodium fluoride notice how there's two Florine atoms on the left hand side of the equation but there's only one Florine on the right hand side of the equation now the law of conservation of mass states that the number and the type of atoms before the reaction have to equal to the number and the type of atoms after the reaction so if there's two flines before the reaction we need to have have two flines on the product side as well so I'm not allowed to just change the formula put a two because that doesn't exist but I could use these big numbers these are called stochiometric coefficients and by putting a two here now I have two Florin on the product side so I have two Florin on the reactant side I have two flines on the product side I'm putting brackets here to indicate that the two applies for both the sodium and the Florine since I placed the coefficient two in front of sodium fluoride now I need to place in front of sodium to balance the sodium atoms so now let's double check we have two sodiums on the reactant side two sodiums on the product side two Florine atoms on the reactant side two Florine atoms on the product side this number here the coefficient you just multiply it by the subscripts of the formula so 2 * 1 is two flines 2 * 1 is two sodium atoms always check whether you've balanced the equation correctly go back to the previous equation two oxygen on the reaction re an side but only one oxygen on the product side that's why I need the two in front of water to balance the oxygen but since I place two that also applies two hydrogen so that's total of four hydrogens and on the left I only have two hydrogens so I'm placing two to balance the hydrogens and the middle equation is balanced so the coefficients are one to one to one so I've summarized this balancing of equations so the atoms before the reaction have to equal two atoms after the reaction and this is the law of conservation of mass we're going to use Co stochiometric coefficients which are the big numbers uh before the formulas in front of the formulas and then when you use the coefficients they have to be in the lowest possible ratio so let's go back I'll show you what I mean this last equation the coefficients are 2 to 1 to two ratio if you wrote 4 to 2 to two ratio that'll be incorrect you need to reduce these coefficients until you end up with the lowest possible ratio and then if you have poly in the reactants and the products treat them as a whole unit so if we have two sulfates on the left make sure you have two sulfates on the right of the reaction okay let's do another example magnesium and oxygen forms magnesium oxide so I've translated this word problem into the following equation magnesium solid plus O2 which is the atomic gas forms magnesium oxide that's the formula for magnesium oxide after you do the crisscrossing of the charges okay now we examine there's two oxygen on the left and only one on the right and that's why I'll place a coefficient of two in front of magnesium oxide and I need a two in front of magnesium to balance the magnesiums and here I'm checking my work I write down there's two oxygens this is just me checking to make sure that the number of oxygen on the left the number of atoms on the left equal to the number of atoms on the right so I'm writing two oxygens on the left and there's two oxygen on the right there's two magnesium atoms on the left on the reactant side and there's two magnesium atoms on the product side one more example ammonium chloride solution this abbreviation s o Ln stands for solution and this is the case where you have to use the aquous symbol so ammonium chloride solution Plus silver sulfate solution forms ammonium sulfate solution and Silver Sol chloride solid okay here's the formulas for the reactants and the products and notice how there's two ammonium ions this is an example where there's polyatomic ions so I keep the polyatomic ions together there's two ammonium ions on the product side and only one on the reactant side that's why I need the coefficient of two in front of ammonium chloride but since I put a two here now the chlorines are going to change I have to put two to balance the chlorines in front of sodium uh silver chloride then I checked to make sure I checked that there's two chlorines on each side two ammonium ions two silver ions and one sulfate ion