Chemistry - Chapter 4 Notes



Chemistry - Chapter 4 Notes

I. Chemical Equations

A. A chemical reaction is a process where new substances with different properties are formed.

B. Example - 2 moles of hydrogen gas combines with 1 mole of oxygen gas to form 2 moles of water.

C. A chemical equation is a shorthand way of describing a chemical reaction.

D. Example - 2 H2 + O2 --------- 2 H2O

E. Parts of an equation.

1. Reactants - The substances you start with in a reaction.

2. Products - The substances that are formed in the reaction.

3. Coefficients - The large numbers in front of the substances that indicate the number of moles of the substance.

4. Yields sign - Means to form or to produce.

2 H2 + O2 ----------- 2 H2O

F. A catalyst is a substance that speeds up a reaction but is not used up in the reaction. Catalysts are written above the yields sign in an equation if they are present.

G. The state of matter a substance is in can be shown in the equation by using the following letters.

s designates a solid.

l designates a liquid.

g designates a gas.

aq designates a aqueous solution, the substance is dissolved in water.

II. Balancing Equations

A. You must have the same number of atoms of each element in the reactants and the products. Remember, in chemical reactions matter can neither be created or destroyed.

B. Steps

1. Write out the unbalanced equation making sure the correct elements and compounds are present. Be very sure the formulas are balanced.

2.Write the symbols of the elements present on both sides of the yields sign under the equation.

3. Write the number of atoms of each element present beside the symbol you wrote down under the equation.

4. Change only the coefficients in the equation until you get the same number of atoms of each element for both the reactants and the products.

C. Example - Zinc reacts with hydrochloric acid to form hydrogen gas an zinc chloride.

Zn + HCl -------- ZnCl2 + H2

Zn Zn

H H

Cl Cl

D. Example - Calcium acetate reacts with potassium sulfate to produce calcium sulfate and potassium acetate.

E. Example - Magnesium reacts with tin IV chloride to form magnesium chloride and tin.

III. Types of Chemical Reactions.

A. Combination - Two or more substances combine to form a new substance.

H2 + Cl2 ------- 2 HCl

B. Decomposition - One substance breaks into two or more simpler substances.

2 H2O --------- 2 H2 + O2

C. Single Replacement - One element displaces another in a compound. Positive ions replace positive ions, negative ions replace negative ions.

Na + KCl ----------- NaCl + K

D. Double Replacement - The positive and negative portions of two compounds are interchanged.

BaO + 2LiCl ------------ BaCl2 + Li2O

E. Combustion – An element or compound reacts with oxygen.

4 K + O2 ------------ 2K2O

E. Examples

2AgNO3 + FeCl2 --------- Fe(NO3)2 + 2AgCl

H2SO4 ------------ H2O + SO3

4 Al + 3 O2 ----------------- 2Al2O3

IV. Net Ionic Equations

A. An equation for a reaction in solution that shows only those particles that are directly involved in the chemical change.

B. In many reactions in water (aqueous reactions) compounds break apart or dissociate, forming free ions. In aqueous reactions some the ions formed do not take part in the chemical reaction, they are called spectator ions. Since these ions are not part of the reaction in an equation they make what is actually happening harder to see. For this reason, we drop the spectator ions from the equation creating a net ionic equation. Net ionic equations must be balanced like all other equations.

C. Change the following equation into a net ionic equation.

2Al + 3 SO4-2 + 6 H+ -------------- 2 Al+3 + 3 H2 + 3 SO4-2

2 Al + 6 H+ ------------- 2 Al+3 + 3 H2

* Read in Prentice Hall Chemistry pages 321 thru 344.

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