Chemical Reactions(Chapter 13)

coefficients

reactants products

Student Learning Objectives

• Utilize chemical equations to determine the amounts of reactants, products, and energy change.

• Characterize acids and bases.

• Describe reactions involving oxygen.

How is a chemical equation balanced?

A chemical reaction is the process of forming new substances.

Atoms are combined to form molecules.Molecules separate into atoms.

The chemical equation shows definite proportions of each substance. (same numbers of atoms on each side)

Al + Br2 Al2Br6

Practice

1) Pb + PbO2 + H2SO4 PbSO4 + H2O

2) C3H8 + O2 CO2 + H2O

3) Al + HCL AlCl3 + H2

4) BaO + H2O Ba(OH)2

There are always 6.022 x 1023 atoms or molecules in 1 mole of a substance.

Avogadro’s Number

Hydrogen & Water

6.022 x 1023 particles mole

Practice

1) Calculate the amount of grams and number of particles in one mole.

a) Gold b) NaCl c) Cu(NO3)2

 

2) Calculate the number of moles, and the number of molecules, in 2 grams of carbon dioxide.

More Practice

3) How many grams of each reactant are needed and how many grams of product are produced in the reaction?

Al + Br2 Al2Br3

What are exothermic and endothermic reactions?

The amount of chemical potential energy may change during a chemical reaction.

ExothermicReaction

EndothermicReaction

Energy released Energy absorbedReactants →

higher PEProducts → higher

PE

Activation Energy

A spike of energy is needed to break existing bonds.

H2 + Cl2 2HCl

Bond StrengthBond Energy is the energy required to break 1 mole

of bonds. (Same energy released when bond forms)

The net bond energy indicates whether the process was exothermic or endothermic.

Bond Energy to Break = Bond Energy Released

Examples

1) N2 + H2 N2H4

2) H4C2 + O2 CO2 + H2O

What can cause reaction rates to vary?

1. Temperature

2. Concentration

3. Surface area

4. Catalysts

What is this?

Catalysts

A catalyst accelerates the reaction.

Provides different pathway

Lowers activation energyReappears as a product

Progression of ReactionWith catalyst

Practice

1) Why are there warnings on some medications not to break, crush, or chew the pill?

2) Why do you think some laundry soaps contain enzymes?

3) Why are photographic prints processed in the dark?

What is an acid? What is a base?

Acids contain H+ and H30+ ions, and are substances that donate hydrogen ions.

Bases contain hydroxide (OH−) ions, or are substances that accept a hydrogen ions.

Proton Donor Proton Acceptor

Examples

1) Baking soda (NaHCO3)

2) HCl + NaHCO3 H2CO3 + NaCl

Practice

Identify the acid substances and the base substances.

CH3CO2H + H2O CH3CO2- + H3O+

Strong Acids & Bases

Strong acids and bases completely ionize in water.

Some Strong Bases

LiOH lithium hydroxide

NaOH sodium hydroxide

KOH potassium hydroxide

Some Strong AcidsHNO3 nitric acid

HCl hydrochloric acid

H2SO4 sulfuric acid

The neutralization reaction of an acid and a base produces water and a salt.

Acid Base Water Salt

HCl  +  NaOH H2O  +  NaCl

HBr  +  KOH H2O  +  KBr

pH

A substance that has an equal number of acid and base ions is neutral. (pH balanced)

pH Solution1 to 6 Acidic

7 Neutral8 to 14 Basic

Substance pHBattery Acid 0.5

Stomach Acid 1.5 – 2.0Cola 2.5

Orange Juice 3.5Coffee 5.0Milk 6.5

Human Blood 7.34 – 7.45Hand Soap 9.0 – 10.0Ammonia 11.5

Bleach 12.5

Practice

1) If the water you drink has a pH of 6.8, is it acidic, basic, or neutral?

 

2) Blood has buffers to help control the natural pH your body needs. The amount of CO2 in the blood stream can change pH. What do we do naturally that can change the pH of blood?

What is “acid rain”?

Acid Rain is moisture that has a pH of less than 5.

 

Rain, snow, sleet, or fogRainwater normally has a

pH of 5.6 to 6.2Acids may combine with

dust (dry deposition)

Water naturally reacts with CO2 in the atmosphere, forming a weak acid.

water + carbon dioxide carbonic acid (H2CO3)

Strong Acids in Our Atmosphere

Human-produced exhaust emissions of sulfur and nitrogen oxides form strong acids.

sulfates + water sulfuric acid (H2SO4)nitrates + water nitric acid (HNO3)

Location Question

Where would you expect a higher level of acidity in the atmosphere, WY or CA? Why? What are the factors?

Image CreditSimon Fraser/Science

Photo Library

Trees killed by acid rain

Czech Republic

What is an oxidation-reduction reaction?

Redox reactions involve a transfer of electrons, and often the addition or subtraction of oxygen.

Oxidation ReductionReactant loses electrons Reactant gains electrons

Oxygen Added Oxygen Removed

Redox Reactions

Corrosion is a redox reaction between a metal and oxygen (O2).

Combustion is a redox reaction between a non-metal and oxygen (O2).

Redox Examples

Corrosion is a redox reaction between a metal and oxygen (O2).

Combustion is a redox reaction between a non-metal and oxygen (O2).

Practice

1) Which element is oxidized and which is reduced?

a) HgS + O2 Hg + SO2

b) SnO2 + 2C Sn + 2CO2

2) Is the iron oxidized or reduced in the formation of rust? 

4Fe(s) + 3O2(g) 2Fe2O3(s)

More Practice

3) Is a combustion reaction, like the combustion of methane, exothermic or endothermic?

 

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

How do batteries and fuel cells work?

Batteries have 2 diodes; one that is oxidized and one that is reduced.

Acidic solution reacts with metals

One metal is reduced (+ end)

One metal is oxidized (– end)

Reactions free electrons

Battery

Cathode (+)

Anode (–)

• Metal gains e−

• Metal is reduced

• Metal loses e−

• Metal is oxidized

Battery

Fuel Cells

A fuel cell uses a membrane to separate the reactants.

Hydrogen gas is oxidized.Oxygen gas is reduced.Product is H2O vapor.

Practice

1) Where do you store your batteries? Why?

2) What is the difference between a dry cell battery and an alkaline battery?

3) What is the primary difference between a battery and a fuel cell?

Practice: Where do you store your batteries? Why?

A fuel cell uses a membrane to separate the reactants.

Hydrogen gas is oxidized.Oxygen gas is reduced.Product is pure water vapor.

What is electrolysis?

Electrolysis is the use of electric current to cause chemical change.

Reverses chemical reactionsSplits compounds

Al2O3 + Energy Al2 + O2

What is electrolysis?

Electrolysis is the use of electric current to cause chemical change.

Reverses chemical reactionsSplits compounds

Example: Al2O3 + Energy Al2 + O2

Question: How do you recharge your cell phone battery?

Practice

1) Is electrolysis an exothermic or endothermic process?

2) Describe the chemical process of recharging your mobile phone device.

3) What other devices operate in the same way?

4) Can you think of other examples of electrolysis?