Chapter 7 Chemistry in Action. Chemical Reactions A chemical reaction is a chemical change resulting...

Chapter 7

Chemistry in Action

Chemical Reactions

• A chemical reaction is a chemical change resulting from a collision of atoms or molecules.

• The original substances are reactants• The substances produced by the reaction are

called products

for example: carbon can collide with oxygen and make carbon dioxide

Chemical Equation: C + 2 O = CO2

Conservation of Mass

In any chemical reaction, atoms are conserved…

That is, the same number of atoms used in the reaction is the same number of atoms in the products.

This is conservation of mass (or matter)…Matter is not created or destroyed, it just changes form.

Energy in Chemical Reactions

• Energy Changes:Some reactions “need” energy to occur

and therefore absorb heat during a reaction – these are endothermic reactions

Reactants + energy → products

Some reactions “give” energy in a reaction and therefore give off heat – these are exothermic reactions

Reactant → products + energy

Energy in Chemical Reactions

Heat and other natural processes in a system always tend toward less usable energy and greater disorder…

This is known as the second law of thermodynamics

When you eat something, only about 55% of energy is actually used…the rest is converted into heat and “lost” to your body

This energy cannot be used again….

Energy in Chemical Reactions

Entropy:The amount of decay or disorder in a

system

According to the 2nd law of thermodynamics, entropy always tends to increase; a decrease in entropy in one place requires a greater increase of entropy somewhere else.

Factors Affecting Rates of Reactions

• Much of practical chemistry involves identifying the reaction characteristics of different substances so that reactions between them may be conducted safely and efficiently in the laboratory and industry!

Factors Affecting Rates of Reactions

1. Temperature – an increase in temperature will usually speed up a chemical reaction.

This is because the rising temperature causes the atoms to move more quickly.

Factors Affecting Rates of Reactions

2. Concentration – Increased concentration increases the rate of reaction.

This is because, the more atoms that are squeezed into a limited space, the more likely they are to react with each other.

Factors Affecting Rates of Reactions

3. Surface Area – increasing the surface area of the reactants increases the speed of the reaction.

This is because there is more area over which the molecules can collide and react.

Example: Grinding a substance into a powder allows it to burn more easily!

Factors Affecting Rates of Reactions

4. Catalysts – A catalyst is a substance that alters the rate of a chemical reaction without being permanently changed in the reaction.

Catalysts change the activation energy of the reaction!!!

Your body has thousands of catalysts called enzymes that God created to keep your body running smoothly!

Types of Chemical Reactions

There are four basic categories of chemical reactions:

1. Combination Reactions

2. Decomposition Reactions

3. Single-replacement Reactions

4. Double-replacement Reactions

Combination Reactions

• Chemical reactions which combine two or more substances to form a third substance.

• Also called synthesis reactions.

A + B → AB

Example:

C + O2 → CO2

Decomposition Reactions

• Complex compounds break down into different substances.

AB → A + B

Example:

CaCO3 → CaO + CO2

Single-Replacement Reactions

• One element is replaced by another element.

A + BC → AC + B

Example:

2 NaBr + Cl2 → 2 NaCl + Br2

Double-Replacement Reactions

• Two compounds swap ingredients.• Two compounds react to form two new

compounds.

AC + BD → AD + BC

Example:

AgNO3 + NaCl → NaNO3 + AgCl

Reversing a Reaction

• Some chemical reactions are reversible.

• These reactions can be made to go back the other way.

Salts

• A salt is a general term that refers to an ionic compound.

• When salts dissolve in water, they “dissociate” (physically separate) in component ions.

• So in water, NaCl is really Na+ ions and Cl- ions.

Acids and Bases

Acids produce hydrogen ions (H+) in solution…

A acid has a pH of 1 to 6

Bases produce hyroxide ions (OH-) in solution

A base has a pH of 8 to 14

Something neutral has a pH of 7…examples are milk and salts

Human blood is a weak base…it has a pH of 7.3

pH scale

The pH Scale

Organic Chemistry

• Organic chemistry has to do with any compound containing carbon.

• Organic compounds include: fuels, foodstuff, paper products, cosmetics, plastics, soaps, fabrics, and paints.

Carbon is Special!

• More compounds are formed with carbon than all other elements combined!

• Why?1. Carbon forms four covalent bonds.2. Carbon can bond to form several

different shapes.1. Rings2. Chains3. Three dimensional shapes

Carbon is Special!

3. Carbon atoms may form single, double, or triple bonds.

4. Carbon may form single and double bonds with the atoms of many other elements.

5. Carbon may form compounds that contain different structural arrangements and combinations with the same molecular formula.

isomers: carbon compounds having the same molecular formula but different structural formulas.

Hydrocarbons

• Compounds that contain only hydrogen and carbon are called hydrocarbons.

• Most industrial compounds are hydrocarbons.

Naming Hydrocarbons

• Hydrocarbons are named according to the number of carbons in the molecules and the arrangement.

• The first part is the prefix…it is based on the number of carbon atoms.

Naming Hydrocarbons

• Alkanes: Hydrocarbons with one bond are called alkanes.– Example: propane, petroleum jelly

• Alkenes: Hydrocarbons with one or more double bond are called alkenes.– Example: beta carotene (vitamin A), olive oil

Naming Hydrocarbons

• Alkynes: Hydrocarbons with one or more triple bond are classified as alkynes.– Example: used in manufacture of plastics

• Cyclic hydrocarbons: ring shaped carbon molecules.– Example: cyclohexane

• Aromatic hydrocarbons: ring shaped, but there are no double or triple bonds.– Example: benzene, TNT, mothballs

Substituted Hydrocarbons

• Hydrocarbons that have one or more hydrogen atoms replaced by different atoms or groups of atoms are called substituted hydrocarbons.

• The substituted group is called a functional group.

• Substituted hydrocarbons include: haloalkanes, alcohols, caboxylic acid, and esters.

Haloalkanes

• The hydrogen atoms are replaced by one of the halogens (chlorine, fluorine, bromine, iodine, or astatine)

• Example: freon and methane

Alcohols

• One or more of the hydrogens are replaced by an OH group.

• Example: methanol made from methane.

Carboxylic Acids

• One or more hydrogen atom replaced by a carboxyl group (COOH)

• Example: acetic acid

Esters

• The insertion of an ester group.

• An ester group is –COO-

• Most have pleasant odors and are used for perfumes and flavor enhancers.

Soaps

• Soaps are a useful organic compound in that it can dissolve non-polar substances.

Polymers

• Polymers are synthetic materials and have been useful in plastics and synthetic fibers.

• They are long molecular chains that link many smaller molecules together.

• Common polymers are polyesters, polystyrene, and teflon.