Unit 9, Chapter 29

Unit 9: The Atom
Chapter 29 Chemical Reactions
 29.1 Chemistry
 29.2 Chemical Bonds
 29.3 Chemical Reactions

Chapter 29 Objectives
1.
Classify matter as substances, homogeneous or heterogeneous mixtures, or solutions.
2.
Tell the difference between chemical change and physical change.
3.
Describe the types of chemical bonds and the role of electrons in forming bonds between atoms.
4.
Write and balance the chemical equation for a simple reaction.
5.
Explain how the terms acid, base, organic, and
solution relate to living systems.
6.
Describe the role of photosynthesis in maintaining life on Earth.

Chapter 29 Vocabulary Terms
 substance  element  carbohydrate
 mixture
 physical change
 compound
 valence electrons
 transition metal
 balanced equation
 reaction
 exothermic reaction
 refine
 heterogeneous mixture
 solute
 double bond
 solubility
 solvent
 pH
 covalent bond
 amino acid
 acid
 organic chemistry
 homogeneous mixture
 dissolve
 Solution
 polar molecule
 chemical bond
 alkali metal
 noble gas
 ion
 ionic bond
 hydrocarbon
 protein
 activation energy
 products
 reactants
 octane
 halogen
 photosynthesis
 endothermic reaction
 base
 chemical change
 cracking
 petroleum
 Lewis dot diagram
 fat

Key Question:
29.1 Chemistry
What techniques are used to separate heterogeneous mixtures?
*Students read Section 29.1
AFTER Investigation 29.1

29.1 Relationship between sciences
 Chemistry is the science of how atoms and elements create the world we experience.
 Our world contains millions of chemicals made from the basic elements and even more interactions between chemicals.

MATTER
MIXTURES SUBSTANCES
Homogeneous mixtures
Heterogeneous mixtures
Elements Compounds

29.1 Physical Changes
 The concept of temperature and changes of phase between solid, liquid, and gas are traditionally considered part of chemistry, as are the gas laws.
 These kinds of changes in matter are called physical changes , because matter changes physical form but one substance does not change into a completely different substance.

29.1 Chemical Changes
 Evidence of chemical change:
— bubbling (formation of gas)
— turning cloudy (formation of a new solid)
— temperature change (heat or light released)
— color change (formation of a new solid)

29.1 Solutions
 A solution is a mixture of two or more substances that is homogeneous at the molecular level.
 The particles in a solution exist as individual atoms, ions, or molecules.
 A solution is a mixture of solute dissolved in a solvent .
 Solubility describes the amount of solute that will dissolve in a given amount of solvent.

29.1 Solutions
 Several factors affect solubility:
— chemical nature of the solvent
— the volume of solute
— temperature
— solute particle size
 The solubility of solids in liquids usually rises with temperature.
 Powders dissolve quickly because they have a tremendous amount of surface area exposed to the solvent.

29.2 Chemical Bonds
Key Question:
Why do atoms form chemical bonds?
*Students read Section 29.2
AFTER Investigation 29.2

29.2 Chemical Bonds
 A chemical bond forms when atoms exchange or share electrons.
 Most of the properties of substances come from how they form chemical bonds with other substances.

29.2 Chemical Bonds
 Electrons that make bonds are called valence electrons.
 Not all electrons in an atom participate in making chemical bonds.
 Only the electrons in the highest unfilled energy level make bonds.

29.2 Chemical Bonds
 Molecules of the chemical benzene have six carbon atoms and six hydrogen atoms shaped in a ring.
 An atom can make one chemical bond for each valence electron.
 Bonds can also involve two or more valence electrons.

29.2 Chemical Bonds
 Elements with the same number of valence electrons are chemically similar.
 The same number of chemical bonds are made with the same elements.

29.2 Chemical Bonds
 Atoms are most stable when they have either 2 or 8 valence electrons.
 The Lewis dot diagram shows the element symbol surrounded by one to eight dots representing the valence electrons.

29.2 Periodic Table and Valence
The periodic table arranges elements from left to right by the number of valence electrons.

29.2 Chemical Bonds
 Most chemical bonds fall into two categories, depending on whether the valence electrons are transferred or shared.
 Electrons in an ionic bond are effectively transferred from one atom to another.
 In a covalent bond the electrons are shared between atoms.

29.2 Ionic bonds
 The ionic bonds in a salt crystal (NaCl) come from electrical attraction between:
— negative chloride ions (Cl )
— positive sodium ions (Na + )

29.2 Covalent bonds
 A diamond crystal is made of pure carbon connected by a strong network of covalent bonds.
 The hardness of diamonds is due to the fact that four covalent bonds must be broken to move each carbon atom.

29.2 pH
 When substances are dissolved in water they divide into two categories called acids and bases.
 An acid creates a sour taste and can dissolve reactive metals like zinc. Vinegar and lemon juice are examples of acids.
 A base creates a bitter taste and tends to feel slippery. Ammonia is an example of a base.
 The pH is scale that measures how acidic or basic a solution is.

29.2 Acids
 Acid molecules include ionic bonds.
 When an acid dissolves in water, the ionic bond breaks to create two ions- one is a hydronium ion.
 The strong chemical reactivity of the hydronium ion
( H
3
O + )is what results in the properties of acids.

29.2 Bases
 Bases have a strong attraction for hydrogen ions
(H + ).
 When a base such as ammonia (NH
3
) dissolves in water, the molecules of ammonia take a hydrogen ion (proton) from water molecules and become ammonium ions (NH
4
+ ).
 The pH is a measure of the concentration of H
3
O +
(acid) ions in a solution.
 The higher the pH, the stronger the base.

29.2 Organic Chemistry
 Organic chemistry is the chemistry of carbon and its compounds.
 The mass of your body is mostly oxygen, carbon, and hydrogen.
 Proteins, amino acids, fats and carbohydrates are types of carbon molecules found in the body.

29.3 Chemical Reactions
Key Question:
How can you predict the yield of a chemical reaction?
*Students read Section 29.3
AFTER Investigation 29.3

29.3 Chemical Reactions
 In chemical reactions you start with reactants that are combined into products .
 The reactants and products may include atoms , molecules , and energy.

29.3 Chemical Reactions
 The formation of rust from oxygen and iron is an example of a common chemical reaction.

29.3 Chemical Reactions
 Chemical reactions proceed in two stages.
 First, the chemical bonds must be broken between the atoms in the reactants.
 In the second stage, new bonds form between atoms to make the products of the reaction.

29.3 Activation energy
 The energy needed to break chemical bonds in the reactants is called the activation energy of the reaction.
 A spark supplies the activation energy to start the reaction between hydrogen and oxygen.

29.3 Activation energy
 A chemical equation is balanced when the number of each type of atom is the same in reactants and products.

29.3 Mass in chemical reactions
 A balanced chemical equation is like a recipe.
 The mass of the products must equal the mass of the reactants.
 Counting molecules is not practical so we usually need the mass of reactants and products of a chemical reaction.

29.3 Calculating mass
1.
Calculate the mass of each molecule or atom.

29.3 Calculating mass
2.
Use a balanced equation to calculate mass of reactants or products.

29.3 Photosynthesis
 The energy that supports life on Earth starts with a reaction that takes energy from sunlight and stores it as chemical bonds in molecules of glucose.
 This reaction is called photosynthesis .

29.3 Photosynthesis
 Nearly all energy in living things can be traced to this important reaction.
 Energy and oxygen are two useful products of photosynthesis.

29.3 Photosynthesis
 A third crucial function of photosynthesis is to remove carbon dioxide from the atmosphere.
 If too much carbon dioxide is present, the
Earth cannot cool itself by radiating energy into space.

Application: Energy from Gasoline