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CHAPTER 6 SECTION 1 CHEMICAL EQUATION

Writing Chemical EquationsGr.10

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Writing Chemical Equations reactant : is a substance that undergoes a reaction.   When reactants undergo a chemical change, each

new substance formed is called a product.

For example, a familiar chemical reaction involves the reaction between iron and oxygen (the reactants) that produces rust, which is iron (III) oxide (the product).

The simplest reactions involve a single reactant or a single product, but some reactions involve many reactants and many products.

CHEMICAL REACTIONS

Reactants: Zn + I2 Product: Zn I2

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Chemical Equations

4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)

The numbers in the front are calledStoichiometric _CoefficientsThe letters (s), (g), and (l) are the

physical states of compounds.

Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

Introduction

Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow).A + sign separates molecules on the same sideThe arrow is read as “yields”Example

C + O2 CO2This reads “carbon plus oxygen react to yield

carbon dioxide”

Parts of a Reaction Equation

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Chemical Change

Atoms in the reactants are rearranged to form one or more different substances

Old bonds are broken; new bonds formExamples: Fe and O2 form rust (Fe2O3)Ag and S form tarnish (Ag2S)

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Learning Check E1

Classify each of the following as a 1) physical change or 2) chemical change

A. ____ a burning candleB. ____ melting ice

C. ____ toasting a marshmallow D. ____ cutting a pizza E. ____ polishing silver

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Solution E1

Classify each of the following as a 1) physical change or 2) chemical change

A. __2__ a burning candleB. __1_ melting ice

C. __2__ toasting a marshmallow D. __1__ cutting a pizza E. __2__ polishing silver

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A process in which at least one new substance is produced as a result of chemical change.

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A Chemical Reaction Reactants

Products

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Word Equations The simplest way to represent a

reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change.

Reactants are placed to the left of the arrow, and products are placed to the right.

Plus signs are used to separate reactants and also to separate products.

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Word Equations The compound in vinegar that is

involved in the reaction is acetic acid, and baking soda is sodium hydrogen carbonate.

The equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products.

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Chemical Equations Symbols in the parentheses are put after

formulas to indicate the state of the substance. Solids, liquids, gases, and water (aqueous)

solutions are indicated by the symbols (s), (l), (g), and (aq).

Now the equation tells us that mixing an aqueous solution of acetic acid (vinegar) with solid sodium hydrogen carbonate (baking soda) results in the formation of an aqueous solution of sodium acetate, liquid water, and carbon dioxide gas.

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Energy and Chemical Equations Noticeable amounts of energy are often released or

absorbed during a chemical reaction. Some reactions absorb energy. If energy is

absorbed, the reaction is known as an Endothermic reaction.

For a reaction that absorbs energy, the word energy is sometimes written along with the reactants in the chemical equation.

For example, the equation for the reaction in which water breaks down into hydrogen and oxygen shows that energy must be added to the reaction.

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Exothermic reactions. Reactions that release heat energy are called

exothermic reactions. When writing a chemical equation for a reaction

that produces energy, the word energy is sometimes written along with the products.

Some of this energy is in the form of light. You may have also noticed that the word energy is not always written in the equation.

It is used only if it is important to know whether energy is released or absorbed.

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Balancing Chemical Equations The mass of the products is always the same as

the mass of the reactants that react to form them. The law of conservation of mass: Matter is

neither created nor destroyed during a chemical reaction.

The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products.

For example, consider the equation that represents breaking down carbonic acid into water and carbon dioxide.

Because of the principle of the conservation of matter,

an equation must be balanced.

It must have the same number of atoms of the same kind on both sides.

Lavoisier, 1788

Chemical Equations

When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may not change the subscripts.Changing the subscripts changes the

compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Balancing Equations

Subscripts vs. Coefficients

The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

Chemical Equations4 Al(s) + 3 O2(g)

---> 2 Al2O3(s)This equation means

4 Al atoms + 3 O2 molecules ---produces--->

2 molecules of Al2O3

There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if: The numbers of atoms on both sides of the

equation are now balanced. The coefficients are in the lowest possible whole

number ratios. (reduced)

Steps to Balancing Equations

Some Suggestions to Help YouTake one element at a time, working left to

right except for H and O. Save H for next to last, and O until last.

IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

(Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Balancing Equations

___ H2(g) + ___ O2(g) ---> ___ H2O(l)2 2What Happened to the Other Oxygen Atom?????This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

Balancing Equations

___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)2 3

Balancing Equations

____C3H8(g) + _____ O2(g) ---->

_____CO2(g) + _____ H2O(g)

____B4H10(g) + _____ O2(g) ---->

___ B2O3(g) + _____ H2O(g)

Balancing EquationsSodium phosphate + iron (III) oxide

sodium oxide + iron (III) phosphate

Na3PO4 + Fe2O3 ---->

Na2O + FePO4

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Good luck