Writing Chemical EquationsText 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? The cake – duh! What symbol do we use to show

processes?

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 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)

Elements from periodic table How to write ionic and covalent

compound names How to interpret word problems

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

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

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

Rules for Writing Chem Equations

1. Figure out which are reactants and which are products.

2. Write chemical formulas for all substances, separated by the

3. Add state of matter to each substance

4. Balance equation (to learn Friday)

Writing Chem. Equation ExamplesZinc and aqueous lead(II)nitrate

are combined in an aqueous solution to produce zinc nitrate and a lead precipitate.

1. zinc and lead(II)nitrate are reactants; zinc nitrate and lead are products

2. Zn + Pb(NO3)2 Zn(NO3)2 + Pb3. Zn(s) + Pb(NO3)2(aq) Zn(NO3)2(aq)

+ Pb(s)4. Balancing Friday

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.

1. Carbon tetrachloride and hydrochloric acid are the products; methane and chlorine are the reactants

2. CH4 + Cl2 CCl4 + HCl3. CH4(g) + Cl2(g) CCl4(s) + HCl(aq)4. To Balance Friday

Balancing Chemical EquationsBig 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. 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 How many oxygen atoms? O2 2 H2O 1 PO4

3-

4 2H2SO4 8 2Ca(OH)2 4 3Ca3(PO4)2 24

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

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

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

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 Reactions Synthesis: Putting things together 2H2 + O2 2H2O (releases energy) Can you think of another example from

biology?

Decomposition Breaking compounds down 2H2O2H2 + O2 (requires much energy) What other rx type is also

decomposition? combustion

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

Solubility Ch 8 section 1Soluble 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 EquationsMolecular 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

Writing Net Ionic Equations

1. Write the reactants with (aq) next to them2. write the products, 3. balance equation4. 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

5. write complete ionic eqn., keeping precipitate together, but separating all other soluble ions.

6. cancel out like ions from reactant & product sides

7. rewrite eqn, leaving out canceled ions.