Chemical Reactions Chapter 24. Free powerpoint template: 2 Clues that a Chemical Reaction is...

Chemical Reactions

Chapter 24

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Clues that a Chemical Reaction is Occurring•Evolution of heat or light•Formation of a gas•Formation of a precipitate•Color change•Some reactions have more than one clue!!


Chemical Reactions•Defined: a change in which one or more substances are converted into new substances

•Reactants= substances that react

•Products= new substances formed


•Chemical Equations are written in the following format:

• A + B --> C + D (reactants) (products)


Characteristics of Chemical Equations•1)The

equation must contain the correct formulas for both the reactants and products!!

• Remember that some elements exist as diatomic molecules: HOFBrINCl


Characteristics of Chemical Equations•Other exceptions- phosphorus

is P4 and sulfur is S8.

•All other elements have no subscripts because they do not exist as molecular compounds in nature.


Characteristics of Chemical Equations• 2) Law of conservation of mass• Antoine Lavoisier, a French

chemist, developed the law of conservation of mass in his work with mercury.


•Law of conservation of mass states that the mass of the reactants equals the mass of the products.

•Lavoisier determined this by noting the increase in mass of a metal after being heated in the presence of oxygen.


Significance of the Law of Conservation of Mass• The same number of atoms must

be on each side of a balanced and correct chemical equation.

• To equalize or balance equations, coefficients are placed in front of a formula to specify the relative number of moles of that substance.


Writing Chemical Equations•A chemical

equation is a way to describe a chemical rxn using chemical formulas and other symbols.

•This shorthand method makes it easier for chemists to communicate.


Symbols Used in Writing Chemical Equations• --> means

produces• + plus• (s) solid• (l) liquid• (g) gas• (aq) aqueous

• Heat- reactants are heated

• Light- reactants are exposed to light

• Double arrow means reversible rxn


Unit Managers• Numbers to the left

of the chemical formulas are called coefficients, which represent the number of moles of each substance that are taking part in the reaction.


Balancing Equations

• A balanced equation has the same number of atoms on each side of the equation.

• Treat polyatomic ions as one unit.


Let’s do an Example

• HgO(s) heat--> Hg(l)+ O2

(g)

•How can we balance the equation?

2 2


Choosing Coefficients

•Mostly a trial-and-error process, but it gets easier with practice.

•Let’s try another example:•Iron metal plus oxygen

produces iron(II) oxide


• Fe(s) + O2(g) --> FeO(s)

•This is the equation that you should have

before it is balanced with coefficients.

•2 Fe(s) + O2(g) --> 2 FeO(s)

•This is the equation that you should have

once it is balanced.


Classifying Chemical Reactions•5 main categories:•Synthesis Rxns•Combustion Rxns•Decomposition Rxns•Single Replacement Rxns•Double Replacement Rxns


Synthesis Reactions

•Aka: composition reactions•Two or more substances combine to form a new substance.

•Example: hydrogen burns in oxygen to form water.


General scheme of a synthesis reaction• A + X --> AX• There are a few basic types of

synthesis reactions: • 1) reaction of metals with oxygen

to produce oxides

• 2 Mg(s) +O2(g) --> 2MgO(s)


Types of synthesis reactions• 2) reactions of metals with sulfur

to produce sulfides

• 16 Rb(s) + S8(s) --> 8Rb2S(s)

• Non-metals can also combine with oxygen and sulfides to form compounds.


Types of Synthesis Reactions•3) Reaction of metals with

halogens (group 17 nonmetals)•Sr(s) + Br2(l) --> SrBr2(s)

•4) Synthesis reactions with oxides

•These oxides react with water to form metal hydroxides.

•CaO(s) + H2O(l) --> Ca(OH)2(s)


Combustion Reactions• Type of synthesis reaction• Characterized by burning.• Occurs when a substance reacts

with oxygen to produce energy in the form of light and heat.


Example of combustion reaction• Propane + oxygen --> carbon

dioxide + water

• C3H8(g) + 5O2(g) --> 3CO2(g) + 4 H2O(g)

Propane combustion!!!!


Decomposition Reactions• One substance

breaks down or decomposes into two or more substances.

• Many require the use of heat, light or electricity.


General scheme of Decomposition reactions• AX --> A + X• Examples:• 1) Decomposition of binary compounds• Electrolysis of water is an excellent

example. Water is split into hydrogen and oxygen by adding an electric current.

• See if you can write the balanced equation for the electrolysis reaction described above!


Types of Decomposition reactions• 1) Decomp of metal carbonates

produces metal oxide and carbon dioxide gas

• CaCO3(s) --> CaO(s) +CO2(g)

• 2) Decomp of metal hydroxides (except group 1 metals) yield metal oxides and water

• Ca(OH)2(s) --> CaO(s) + H2O(g)


Types of Decomposition Reactions• 3) Decomp of metal chlorates to

produce a metal chloride and oxygen

• 4) Decomp of some acids into nonmetal oxides and water

€

2KClO3(s)

heat,MnO2( s ) ⏐ → ⏐ ⏐ ⏐ 2KCl(s) + 3O2(g )

€

H2CO3(aq ) →CO2(g ) + H2O(l )


Single Displacement Reactions• When one element

replaces another in a compound.

• Many of these reactions take place in solution.

• General scheme:A + BC --> AC + B orY + BC --> BY + C

• Silver nitrate and copper metal form copper(II) nitrate and silver metal.


Tips for single replacement reactions• When predicting

products, remember that metals will always be first in a chemical formula or they will be by themselves.

• Also remember that nonmetals or polyatomic ions will be second in a chemical formula, NEVER first!


Types of single replacement rxns• 1) Replacement of one metal by

another

• 2) Replacement of hydrogen in water by a metal to produce metal hydroxides and hydrogen gas

€

2Al(s) + 3Pb(NO3)2(aq ) → 3Pb(s) + 2Al(NO3)3(aq )

€

2Na(s) + 2H2O(l ) → 2NaOH(aq ) + H2(g )


Types of single replacement rxns• 3) Replacement of hydrogen in an

acid by a metal to produce a salt and hydrogen gas

• 4) Replacement of halogens

€

Mg(s) + 2HCl(aq ) →H2(g ) + MgCl2(aq )

€

Cl2(g ) + 2KBr(aq ) → 2KCl(aq ) + Br2( l )


Double Displacement Reactions• The positive ion

of one compound replaces the positive ion from the other compound, forming two new compounds.


General Scheme of Double replacement rxns

•AX + BY --> AY + BX• Types of Double Replacement

Rxns:• 1) Formation of a precipitate

€

2KI(aq ) + Pb(NO3)2(aq ) → PbI2(s) + 2KNO3(aq )


More double replacement rxns• 2) Formation of a gas

• 3) Formation of water

€

FeS(s) + 2HCl(aq ) →H2S(g ) + FeCls(aq )

€

HCl(aq ) + NaOH(aq ) → NaCl(aq ) + H2O(l )


Acids• These were recognized as a distinct class of

chemicals due to the following properties:• 1) Aqueous solutions have a sour taste. • 2) Acids change the color of acid-base

indicators, like litmus paper or pH paper.• 3) Some acids react with metals to form

hydrogen gas.• 4) Acids react with bases to produce salts and

water.• 5) Some acids conduct electric current.


Bases• These are the properties of bases:• 1) Aqueous solutions of bases taste bitter.• 2) Bases change the color of acid-base

indicators.• 3) Dilute aqueous solutions of bases feel

slippery.• 4) Bases react with acids to produce salts

and water.

• 5) Bases conduct electric current.


Acid-Base Reactions• When strong acids and bases react,

the products are ALWAYS a salt and water!

• An example of this is hydrochloric acid and sodium hydroxide (from double replacement reactions example).

• Acids and bases will be a primary focus in SPAN/AP chemistry.


Acid and Base Strength• The strength of an acid or base

depends on how well the ions separate in aqueous solution.

• The more they separate, the stronger they are.

• The pH scale indicates how strong the acid or base is by indicating how much hydrogen ion is in the solution.


The pH scale

• This scale usually runs from 0-14.• A ranking of 0-6 is acidic, 7 is

neutral and 8-14 is basic.


Oxidation• Review of rules for assigning oxidation

numbers.• 1) The oxidation number of any uncombined

element is zero. ie: Zn• 2) The oxidation number of a monatomic ion

equals the charge on the ion… Group 1=+1 and so on…

• 3) Oxidation number of H is +1 in most cases, unless it is combined with a metal…then it is -1.

• 4) The sum of the oxidation numbers of all atoms in a neutral compound is zero.


Oxidation Reactions• Defined as reactions in which ions or atoms

of an element experience an increase in oxidation state.

• Example:

• In oxidation reactions, electrons are lost so the oxidation state becomes greater (more positive).€

Na0 → Na+1 + e−


Factors that Affect the Rates of Chemical Reactions• 1) Temperature- an increase in

temperature may cause the rate of the reaction to speed up and a decrease in temperature may cause a reaction to slow down

• Why? Increasing the temperature causes the kinetic energy of the particles to increase, which

• increases the collisions of the particles


Factors that Affect the Rates of Chemical Reactions• 2) Surface area- the rate of

reaction depends on the area of contact for each substance

• An increase in the surface area will increase the rate of heterogeneous reactions- which are reactions that involve reactants in two different phases. ie: solids and liquids


Reduction Reactions• Defined as reactions in which the ions

or atoms of an element experience a decrease in oxidation state.

• Example:

• In reduction reactions, electrons are gained so the oxidation number decreases

(becomes more negative).€

Cl20 + 2e− → 2Cl−1

Chemical Reactions Chapter 24. Free powerpoint template: 2 Clues that a Chemical Reaction is...

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