Unit 7 Chemical Equations

Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. Objectives: To identify seven elements that occur naturally as diatomic molecules, H2, N2, O2, F2, Cl2, Br2, I2. [HOBrFINCL twins or BrINClHOF brothers] To write a chemical equation from the description of a chemical reaction. REACTANTS  PRODUCTS

Describing a Chemical Reaction Indications of a Chemical Reaction Evolution of heat, light, and/or sound Production of a gas Formation of a precipitate Color change Objective: To state four observations that are evidence for a chemical reaction. Photo of precipitate: http://www.geocities.com/chem_reactions/Images/Precipitate.jpg

Showing Phases in Chemical Equations H2O(s) H2O(l) H2O(g) Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s) Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l) Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g)

Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state And or added to (s) (l) (aq) (g) +

Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product or Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 oC Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction D heat 2 atm pressure 0 oC MnO2

Chemical Equations aluminum oxide Shows the kind of reactants and products, their relative amounts, and the physical states of compounds. 4 Al(s) + 3 O2(g) 2 Al2O3(s) A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products. Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution. A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms. http://img.alibaba.com/photo/10784694/Aluminum_Oxide_Abrasive_Fine_Grit_120_220_.jpg http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg/600px-Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg http://www.germes-online.com/direct/dbimage/50327532/Aluminum_Alloy_Pipe.jpg The letters (s), (g), and (l) are the physical states of compounds.

The numbers in the front are called Chemical Equations aluminum oxide The numbers in the front are called coefficients. 4 Al(s) + 3 O2(g) 2 Al2O3(s) A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products. Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution. A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms. http://img.alibaba.com/photo/10784694/Aluminum_Oxide_Abrasive_Fine_Grit_120_220_.jpg http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg/600px-Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg http://www.germes-online.com/direct/dbimage/50327532/Aluminum_Alloy_Pipe.jpg First: the coefficients give the number of molecules (or atoms) involved in the reaction. In the example reaction, four molecules of aluminum react with three molecules of oxygen and produce two molecules of aluminum oxide. Second: the coefficients give the number of moles of each substance involved in the reaction. In the example reaction, four moles of aluminum react with three moles of oxygen and produce two moles of aluminum oxide.

Chemical Equations The Law of Conservation of Mass Because the same atoms are present in a reaction at the beginning (reactants) and at the end (products), the amount of matter in a system does not change. Law of Conservation of Matter means that nothing is “lost” or “stolen”. In the real world retail theft costs business lots (much of the theft is actually employee theft)! The Law of Conservation of Mass states: Matter can neither be created or destroyed 20% 100% Chemical Factory 100% 80% Kotz web

Chemical Equations Because of the principle of the conservation of mass, all equations must be balanced. There must be the same number of atoms and the same kind of atoms on both sides of equation. Reactants = Products Lavoisier, 1788 What a ladies man!

Visualizing a Chemical Reaction 2 Na + Cl2 NaCl 2 ___ mole Na 10 10 ___ mole Cl2 5 5 ___ mole NaCl 10 10 ?

Balancing Chemical Equations Balanced Equation – the number of each atom of each element must be the same on each side (reactant and product) of the reactions. Objective: To write balanced chemical equations by inspection. Balanced chemical equation – Provides qualitative information about the identities and physical states of the reactants and products – Provides quantitative information because it tells the relative amounts of reactants and products consumed or produced in the reaction – The number of atoms, molecules, or formula units of a reactant or product in a balanced chemical equation is the coefficient of that species – Mole ratio of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Determine whether the following equation is balanced. 2 Na + H2O  2 NaOH + H2 2 Na + 2 H2O  2 NaOH + H2

Balancing Chemical Equations Counting atoms - to be able to write a balanced equation you must be able to count the number of atoms of each element in a substance correctly Example: 6 H2O Coefficient Subscript If no subscript is present, it is understood to be a “1”. The coefficient tells how many representative particles there are in a substance. If no coefficient is present, it is understood to be “1”.

Balancing Chemical Equations Coefficient Example: 6 H2O Subscript subscript ______ H x coefficient _____ = ______   subscript ______ O x coefficient _____ = ______ ______ Total atoms 2 6 12 1 6 6 18

CH4 + 2 O2  CO2 + 2 H2O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

Rules for Balancing Chemical Equations Count the number of each element on each side of the equation Balance each element one at a time Determine the coefficients that make the equation balance. Balance polyatomic ions, then metals, then hydrogen, then oxygen LAST Reduce if necessary

Balancing Chemical Equations Watch youtube video lessons for balancing equations by clicking the links below. https://www.youtube.com/watch?v=gskm-dfKv5g https://www.youtube.com/watch?v=_B735turDoM http://images.google.com/imgres?imgurl=http://www.uwec.edu/Images/UWECWeb/uwec8.gif&imgrefurl=http://www.uwec.edu/carneymj/Chem103.htm&h=67&w=359&sz=10&hl=en&start=101&tbnid=7T7IE0FudtwTnM:&tbnh=23&tbnw=121&prev=/images%3Fq%3Dgeneral%2Bchemistry%2Bpowerpoint%26start%3D90%26gbv%3D2%26ndsp%3D18%26hl%3Den%26sa%3DN Watch youtube video lessons for completing worksheets. Review: Counting Atoms in Compounds Review: Forming and Naming Compounds Which Equations are Balanced? Balancing Equations Word Equations

Don’t forget the Diatomic Elements Writing Word Equations Don’t forget the Diatomic Elements H2, N2, O2, F2, Cl2, Br2, and I2

Writing Word Equations See any diatomic??? Cl Br Mg H Ca S  Cl2 Diatomic  Br2 Diatomic  Mg  H2 Diatomic  Ca S

Writing Word Equations Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 1) Look up the oxidation number for each element in a compound. chlorine + sodium bromide  bromine + sodium chloride No charge Na1+ Br 1- No charge Na1+ Cl 1- 2) Write the correct formulas for all reactants and products by swapping and dropping. Don’t forget diatomic elements! Cl2 + NaBr  Br2 + NaCl 3) Determine the coefficients that make the equation balance. 1 Cl2 + 2 NaBr  1 Br2 + 2 NaCl

Writing Word Equations Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate. 1) Look up the oxidation number for each element in a compound. aluminum sulfate + calcium chloride  calcium sulfate + aluminum chloride Al3+ (So4) 2- Ca2+ Cl 1- Ca2+ (So4) 2- Al3+ Cl 1- 2) Write the correct formulas for all reactants and products by swapping and dropping. Al2(SO4)3 + CaCl2  CaSO4 + AlCl3 3) Determine the coefficients that make the equation balance. 1 Al2(SO4)3 + 3 CaCl2  3 CaSO4 + 2 AlCl3