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Writing Chemical Equations Text Reference: Ch 7, Section 2 - 3

Writing Chemical Equations Text Reference: Ch 7, Section 2 - 3. Chemical Reactions. A chemical reaction is a process. What does this mean? Give some examples of processes Is baking a cake a process? Name some ingredients of baking a cake What is the product of this baking a cake process?

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Writing Chemical Equations Text Reference: Ch 7, Section 2 - 3

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  1. Writing Chemical EquationsText Reference: Ch 7, Section 2 - 3

  2. Chemical Reactions • A chemical reaction is a process. • What does this mean? • Give some examples of processes • Is baking a cake a process? • Name some ingredients of baking a cake • What is the product of this baking a cake process? • The cake – duh! • What symbol do we use to show processes? • 

  3. Chemical Reactions • In a chemical reaction, the  can be read as “produces” or “yields” • With a chem. Rx. (chemical reaction) something(s) new is produced from other chemical ingredients. • The arrow indicates the process of burning, combining, exploding, disintegrating…. • To the left of the arrow is what goes into the reaction: • Reactants, separated by + signs • On the right: ? • Products, also separated by + signs

  4. Chemical Reactions • Convert baking a cake into a chemical reaction • Flour(s) + water(l) + eggs(s) + milk(l), etc  cake(s) + good aroma filling the kitchen!(g) • Ingredients are reactants • Cake + aroma = products •  = heat (produces, yields) • You know a chemical reaction has occurred because you can’t reverse it.

  5. Old Prerequisites for Writing Chemical Equations(things you should/must already know) • Elements from periodic table • How to write ionic and covalent compound names • How to interpret word problems

  6. New Prerequisites for writing chemical equations • (l) = liquid state • (s) = solid state • (g) = gas • (aq) = aqueous (dissolved in water; solution) • The BrINClHOF’s • Guess what elements these are • bromine, • iodine, • nitrogen, • chlorine, • hydrogen, • oxygen, • fluorine

  7. New Prerequisites for writing chemical equations • The BrINClHOF’s • Always exist in pairs, when not combined with other elements • Gas at room temp. • Referred to by their normal chemical name even though they’re combined • ie. “chlorine” = Cl2(g) • “oxygen” = O2(g) • AKA: “HON and the halogens” • Or Hydrogen + 7

  8. Other Things Helpful to Know • When metals are just named (i.e., “lead”) this is simply the solid pure element (i.e., Pb(s)) • When things are “bubbled in” that means a gas form of that element is being added: • Ex: “hydrogen is bubbled into a solution of…” • Is written as: H2(g) + …  … • “Solutions” are aqueous. Ex: “ a solution of lead(II)chloride…” = • PbCl2(aq) • Acids are aqueous

  9. Rules for Writing Chem Equations • Figure out which are reactants and which are products. • Write chemical formulas for all substances, separated by the  • Add state of matter to each substance • Balance equation (to learn Friday)

  10. Writing Chem. Equation Examples • Zinc and aqueous lead(II)nitrate are combined in an aqueous solution to produce zinc nitrate and a lead precipitate. • zinc and lead(II)nitrate are reactants; zinc nitrate and lead are products • Zn + Pb(NO3)2  Zn(NO3)2 + Pb • Zn(s) + Pb(NO3)2(aq) Zn(NO3)2(aq) + Pb(s) • Balancing Friday

  11. Writing Chem. Equation examples • Carbon tetrachloride may be prepared by the reaction of natural gas, methane, and chlorine in the presence of ultraviolet light. Hydrochloric acid (HCl) is also a product of this reaction. • Carbon tetrachloride and hydrochloric acid are the products; methane and chlorine are the reactants • CH4 + Cl2  CCl4 + HCl • CH4(g) + Cl2(g)  CCl4(s) + HCl(aq) • To Balance Friday

  12. Balancing Chemical Equations • Big Idea: Law of Conservation of Matter • Matter cannot be created or destroyed • Number of atoms of a certain element must be equal on reactant and product side. • A balanced chemical equation shows the ratio of elements from one side to other

  13. Rules for Balancing Chem. Eqns. • Write unbalanced equation • Once unbalanced equation is written, NEVER CHANGE THE SUBSCRIPTS • Only add coefficients to front of compound • Coefficients multiply everything in the compound by that amount • Work with most complex molecule first, and save simplest for last • If you have an odd # of elements on one side, multiply entire eqn by 2, and continue

  14. Review: reading molecular numbers • How many oxygen atoms? • O2 • 2 • H2O • 1 • PO43- • 4 • 2H2SO4 • 8 • 2Ca(OH)2 • 4 • 3Ca3(PO4)2 • 24

  15. Balancing examples • O3  O2 • You need the same number of O’s on both sides • 2O3  3O2 • 6 = 6 • Balance: H2 + O2  H2O • 2H2 + O2  2H2O • H’s : 4 O’s : 2

  16. Combustion example • Combustion of ethane • C2H6 + O2 CO2 + H2O • Which is most complex? • Ethane – so balance those elements first • C2H6 + O2 2CO2 + 3H2O • Now you have odd number of O’s • Multiply entire eqn by 2 • 2(C2H6 + O2 2CO2 + 3H2O) • 2C2H6 + 2O2 4CO2 + 6H2O • Balance O’s • 2C2H6 + 7O2 4CO2 + 6H2O • Make a T chart to double check balance of all elements on both sides

  17. Classifying Chemical Reactions • SC2 Students will relate how the Law of Conservation of Matter is used to determine chemical composition in compounds and chemical reactions. • a. Identify and balance the following types of chemical equations: • Synthesis • Decomposition • Single Replacement • Double Replacement • Combustion

  18. Classifying Chemical Reactions • Combustion - “burning” (but not necessarily with flames) to release energy from a compound • The energy is stored in the bonds of the compound being “burned” • Oxygen is almost always one of the reactants • Produces carbon dioxide and water • Ex: cellular respiration • Glucose + oxygen  water + carbon dioxide + ENERGY

  19. Classifying Chemical Reactions • Synthesis: • Putting things together • 2H2 + O2  2H2O (releases energy) • Can you think of another example from biology? • Decomposition • Breaking compounds down • 2H2O2H2 + O2 (requires much energy) • What other rx type is also decomposition? • combustion

  20. Classifying Chemical Reactions • Single replacement: • One element “steals” partner from another • 3SrO(s) + 2Al(s)  Sr(s) + Al2O3(s) • Double replacement: • like “Wife Swap” • CaF2 + H2SO4 CaSO4 + 2HF

  21. Solubility Ch 8 section 1 • Soluble dissolves (in water) • Insoluble doesn’t dissolve (in water) • Precipitate solids that form when two ions react in water to form an Insoluble compound • Precipitation/ing forming a solid

  22. Writing Net Ionic Equations • Molecular equation shows the complete (undissociated) forms of all reactants and products • Complete ionic equations shows all strong electrolytes (soluble, aqueous) as ions. • Spectator Ion an ion present in a solution that does not participate in a rx. • Net ionic equation includes only those components that change in the rx

  23. Writing Net Ionic Equations • Write the reactants with (aq) next to them • write the products, • balance equation • Use solubility rules to write in (aq) for products that are soluble and (s) for insoluble products. If neither product precipitates out, write No Reaction • write complete ionic eqn., keeping precipitate together, but separating all other soluble ions. • cancel out like ions from reactant & product sides • rewrite eqn, leaving out canceled ions.

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