Writing chemical equations
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Text Reference: Ch 7, Section 2 - 3 PowerPoint PPT Presentation


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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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Text Reference: Ch 7, Section 2 - 3

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Writing chemical equations

Writing Chemical Equations

Text Reference: Ch 7, Section 2 - 3


Chemical reactions

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?


Chemical reactions1

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


Chemical reactions2

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.


Old prerequisites for writing chemical equations things you should must already know

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


New prerequisites for writing chemical equations

New Prerequisites for writing chemical equations

  • (l) = liquid state

  • (s) = solid state

  • (g) = gas

  • (aq) = aqueous (dissolved in water; solution)

  • The BrINClHOF’s(for a family name)

  • Guess what elements these are

  • bromine,

  • iodine,

  • nitrogen,

  • chlorine,

  • hydrogen,

  • oxygen,

  • fluorine


New prerequisites for writing chemical equations1

New Prerequisites for writing chemical equations

  • The BrINClHOF’s(for a family name)

  • 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: HONey and the Halogens (for a band name)

  • AKA: Hyd-7 (for a card game)


Other things helpful to know

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 always “aqueous”


Rules for writing chem equations

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


Writing chem equation examples

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!


Writing chem equation examples1

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…(explained in the following slides)

  • CH4(g) + 4Cl2(g)  CCl4(s) + 4HCl(aq)


Balancing chemical equations

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


Rules for balancing chem eqns

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


Review reading molecular numbers

Review: reading molecular numbers

  • How many oxygen atoms in each of the following?

  • O2

  • 2

  • H2O

  • 1

  • PO43-

  • 4

  • 2H2SO4

  • 8

  • 2Ca(OH)2

  • 4

  • 3Ca3(PO4)2

  • 24


Balancing examples

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


Balancing a combustion example

balancing a 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 eq’n 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


Classifying chemical reactions

Classifying Chemical Reactions

  • You will need to be able to identify and balance the following types of chemical equations:

    1) Synthesis

    2) Decomposition

    3) Single Replacement

    4) Double Replacement

    5) Combustion


Classifying chemical reactions1

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


Classifying chemical reactions2

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


Classifying chemical reactions3

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


Solubility ch 8 section 1

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


Writing net ionic equations

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


Writing net ionic equations1

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 rxn) or activity series (for SR rxn). If no reaction occurs, write No Reaction

  • Write complete ionic eq’n., keeping precipitate together, but separating all other soluble ions.

  • Cancel out spectator ions from reactant & product sides

  • Rewrite eq’n, leaving out canceled ions.


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