Chapter 11 Matter and Change 11.1 Describing Chemical Reactions 11.2 Types of Chemical Reactions

11.1 Chemical Reactions >

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Chapter 11Matter and Change

11.1 Describing Chemical Reactions

11.2 Types of Chemical Reactions11.3 Reactions in Aqueous Solution



Introduction to Chemical Equations


Parts of a Chemical Formula:

2 Al2(SO4)3

CoefficientElement Symbols Subscripts

Coefficient: Number of moles of molecules or formula units of that compound; Symbols: Elements in the compound;Subscripts: Proportion of atoms in one mole or one molecule or f.u. of the compound. Remember that these can NEVER change in a formula!!




Interpreting Chemical Equations• Chemical equations describe actual chemical

reactions• Starting substances are called reactants, and

are on the left side of the arrow• Ending substances are called products, and

are on the right side of the arrow• Recall that all chemical equations must

demonstrate the Law of Conservation of Matter by being balanced!! (Chapter 2)




Diatomic Elements--exist naturally as two atom molecules:

Fluorine: F2 Hydrogen: H2

Chlorine: Cl2 Oxygen: O2

Bromine: Br2 Nitrogen : N2

Iodine: I2

Fondly known as “HOBrFINCl” Be sure to use these diatomic symbols when

writing chemical equations with these individual elements, or equations will never balance!



Interpret DataInterpret Data

Symbols Used in Chemical Equations

Symbol Explanation

+ “And”; Separates two reactants or two products

→ “Yields,” “changes,” “becomes”; separates reactants from products

Use in place of → for reversible reactions

(s), (l), (g)Designates a reactant or product in the solid state, liquid state, or gaseous state; placed as a subscript after the formula

(aq) Designates an aqueous solution; the substance is dissolved in water; placed as a subscript after the formula

Indicates that heat or electricity is supplied to the reaction

A formula written above the “yields” sign indicates its use as a catalyst (in this example, platinum).

Δ

elec

Pt




How could you describe the rusting of iron?

• You could say, “Iron reacts with oxygen to produce iron(III) oxide (rust).”

• It is quicker to identify the reactants and product by means of a word equation.

Skeleton (Word) Equations

Iron + oxygen → iron (III) oxide




The production of a new substance, a gas, is evidence of a chemical change.

• Two new substances are produced in this reaction, oxygen gas and liquid water.

Skeleton (Word) Equations

Hydrogen peroxide → water + oxygen gas

• You could describe this reaction by saying “hydrogen peroxide decomposes to form water and oxygen gas.”



Chemical Equations


In many chemical reactions, a catalyst is added to the reaction mixture.

• A catalyst is a substance that speeds up the reaction but is not used up in the reaction.

• A catalyst is neither a reactant nor a product, so its formula is written above the arrow in a chemical equation.

H2O2 (aq) H2O (l) + O2 (g)

MnO2




Translating word equations to skeleton equations:• Word equations are translated into “skeleton

equations” by using chemical formulas and common reaction symbols.

• Skeleton equations are not balanced.

• Draw a box around names of chemicals and circle descriptions of physical states.

• The verb phrase will determine the placement of the reaction arrow.

• Write formulae/symbols for all chemicals (Ch. 9!!)• Write symbols for physical state as subscripts after

formulae.




Examples1. Liquid hydrogen peroxide (H2O2) decomposes to

form water vapor and oxygen gas in the presence of the catalyst manganese (IV) oxide.

2. Solid calcium carbide (CaC2) reacts with water to

form ethyne (C2H2) gas and aqueous calcium hydroxide.




3. Ethyne gas reacts with oxygen in the air in the presence of a flame to produce carbon dioxide gas and water vapor.

4. Aqueous solutions of lead (II) nitrate and sodium iodide react to form solid lead (II) iodide and aqueous sodium nitrate.



We can use everyday words to describe chemical reactions. What is the advantage of using an equation?

A chemical equation for a reaction is easier to read quickly.

It shows all of the relevant information: quantities of reactants and products, the direction of the reaction, and any catalysts needed



A chemical reaction is described by a balanced equation in which each side of the equation has the same number of atoms of each element and mass is conserved.

Balancing Chemical Equations

• As reactants are converted to products, the bonds holding the atoms together are broken, and new bonds are formed.

• The atoms themselves are neither created nor destroyed; they are merely rearranged.

• Recall from Chapter 2: In any chemical change, the TOTAL MASS remains the same!!





1. Be sure all formulas/symbols are correct before attempting to balance the equation!

2. Subscripts are “Off- Limits”!!

3. If it is helpful to draw a picture of the molecules, do so!!

3. Adjust coefficients in front of formulas/symbols only!

4. The number and type of atoms on each side of reaction must balance!

5. Coefficients used must be in the lowest ratio possible!!



C(s)Carbon

+ O2(g)Oxygen

CO2(g)Carbon dioxide

Reactants1 carbon atom, 2 oxygen atoms

Product1 carbon atom, 2 oxygen atoms


Example #1: Carbon burns in the presence of oxygen to produce carbon dioxide.

• This equation is balanced—the number of each type of atom is the same on both sides of the equation.

• You do not need to change the coefficients.




Example #2: When hydrogen and oxygen are mixed, the product is water.

• The formulas for all the reactants and the product are correct, but this equation is not balanced.

– As written, the equation does not obey the law of conservation of mass (check the atom counts!! )




When hydrogen and oxygen are mixed, the product is water.

• If you put the coefficient 2 in front of H2O, oxygen will be balanced.

– Now twice as many hydrogen atoms are in the product as are in the reactants.




When hydrogen and oxygen are mixed, the product is water.

• To correct this equation, put the coefficient 2 in front of H2.

– The equation is now balanced.




Hints for Balancing Reactions:

Balance by listing chemicals individually & counting as needed;

Balance using multiples of two where possible;

Utilize least common multiples where possible;

Balance polyatomic ions as a whole unit rather than as individual atoms;

Use fractions as appropriate to balance mathematically, and then multiply the whole reaction by the factor that clears the fraction.

Make sure final equation uses lowest possible ratio of coefficients.



•Practice Problems:_____H2O2 _______H2O + _______O2

___CaC2 + ___H2O ____C2H2 +___Ca(OH)2

__ C2H2 + ___O2 ____CO2 + ___H2O



___Pb(NO3)2 + ____NaI ____NaNO3 + ____PbI2

___Al + ___HCl ___AlCl3 + ____H2

___C10H22 + ___O2 ___CO2 + ____H2O

Practice Problems:



_____HBr _____H2 + _____Br2

___Mg + _____H3PO4 ___Mg3(PO4)2 + ___H2

Practice Problems:



Balance the following equation.

C3H8 (g) + O2 (g) → CO2 (g) + H2O (l)

C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (l)



BIG IDEABIG IDEA

• The law of conservation of mass states that mass is neither created nor destroyed.

• In order to show that mass is conserved during a reaction, a chemical equation must be balanced.

Chemical Reactions



END OF 11.1

Chapter 11 Matter and Change 11.1 Describing Chemical Reactions 11.2 Types of Chemical Reactions

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Transcript of Chapter 11 Matter and Change 11.1 Describing Chemical Reactions 11.2 Types of Chemical Reactions