LECTURE 4.1 – BALANCING CHEMICAL EQUATIONS

What is the law of conservation of mass?

I. Law of Conservation of Mass The law of conservation of mass states that matter cannot be created or destroyed, it can only change forms. Therefore, we cannot destroy something. All we can do is change the form.

How are states indicated in chemical reactions?

TNT Explosion C7H5N3O6 → N2 + H2O + CO + C

I. Chemical Reactions Substances react to form new substances Chemical equations are used to explain a reaction that occurs.

TNT Reaction C7H5N3O6 → N2 + H2O + CO + C ✔ Element Before Reaction? After Reaction? C ✔ H N O

I. Chemical Reactions Reactants  Products C7H5N3O6 → N2 + H2O + CO + C Reactants Products

II States of Molecules A molecule can be in a solid, liquid, gas, or aqueous state Aqueous means dissolved in water We abbreviate these as: (s) = solid (l) = liquid (g) = gas (aq) = aqueous

NH4OH (aq) + FeCl3 (s)  NH4Cl (s) + Fe(OH) 3 (aq) Table Talk With your table, label the products, the reactants, and the state of each compound. NH4OH (aq) + FeCl3 (s)  NH4Cl (s) + Fe(OH) 3 (aq)

H2O2 (aq) + NaBr (s)  NaOH (aq) + H2O (l) + O2 (g) Stop and Jot For the following reaction, label the products, the reactants, and the state of each compound: H2O2 (aq) + NaBr (s)  NaOH (aq) + H2O (l) + O2 (g)

How do you balance chemical reactions?

I. Balancing Chemical Reactions Need same amount of products and reactants to satisfy law of conservation of mass We balance chemical equations to achieve this

II. Tips for Balancing Chemical Reactions We CANNOT change the subscripts. We can change the number in front of the substance, known as the coefficient. Coefficient 2 NaCl This means we have 2 Na and 2 Cl

III. Organizing Our Thoughts Element/Compound # Atoms in Reactants # of Atoms in Products Balanced?

Class Example Balance the following chemical reaction: ____H2O2  ___ H2O + ____O2

__AgNO3 + ___Na2CrO4  __ Ag2CrO4 +___ NaNO3 Class Example Balance the following equation: __AgNO3 + ___Na2CrO4  __ Ag2CrO4 +___ NaNO3

Table Talk Balance the following equation: Mg + FeCl3  MgCl2 + Fe

Extra Examples ___ C3H6 + ____ H2O  ___ C3H8 + __ O2 ___ Mg + ___ FeCl3  ___ MgCl2 + ___ Fe __ H2O + __ Fe2 (SO4) 3  __ Fe2O3 + __ H2SO4 + ___ H2O + ___ H2