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

Learn how to write chemical equations correctly with reactants and products, balancing equations, and using symbols for different states of matter. Understand the rules for balancing equations to follow the Law of Conservation of Matter.

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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

  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) • Balance: coming right up!

  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

  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 • Can you think of another example from biology? • Decomposition • Breaking compounds down • 2H2O2H2 + O2 • What other rx type is also decomposition? • combustion

  20. Classifying Chemical Reactions • Single replacement: • One element “steals” partner from another • General format: AB + C  A + BC • 3SrO(s) + 2Al(s)  Sr(s) + Al2O3(s) • Double replacement: • like “Wife Swap” • AB + CD  AD + CB • (note: the cation always goes first) • 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 • Complete ionic equation 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 participate in the rx

  23. Writing Net Ionic Equations • Write the reactants with (aq) next to them • write the products, • balance equation • Determine if a reaction occurs by using solubility rules (for DR rx.) or activity series (for SR rxs.). If no reaction occurs, write No Reaction • Write complete ionic eqn., keeping precipitate together, but separating all other soluble ions. • Cancel out spectator ions from reactant & product sides • Rewrite eqn, leaving out canceled ions.

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