Review Chapter Two: Chemical Reactions

Pages 42 - 71

Chemical Reactions Chemical reactions are not just something that scientists study in a laboratory. Chemical changes or reactions are an essential part of nature ( past, present and future). Examples:Lighting initiates many chemical reactions in the atmosphere including formation of nitrogen compounds that provide nutrients for plants on earthPhotosynthesis

Chemical Reactions Continued

Metabolism Decomposition of dead plants and animals The cycle of life is a cycle of chemical reactionsStudying chemical reactions, enables construction generalizations, laws and, eventually theoretical structure of compoundsInitially, theories of the structure of matter attempt to explain the known chemical properties of a substance

Science and Technology in Society

The science of chemistry goes hand in hand with the technology of chemistry Chemists make use of many technologies, from test tubes to computersYou can acquire specialized knowledge and skills for understanding STS issues by studying science. Ex. A discussion of Global Warming

Deciding how to use science and technology to benefit society is extremely complex. Most STS issues can be discussed from many different points of views, or perspectives.

Five Types of STS Perspectives

Scientific perspective – leads to researching and explaining

Technological perspective – the concern with development and use of machines, instruments, and process that have a social purpose

Ecological perspective – considers relationships between living organisms and the environment

Continued Economic perspective – focuses on the production, distribution, and consumption of wealth

Political perspective – involves governments, vote-getting actions or arguments based on an ideology

Questions Section 2.1 Questions 1-3 on page 45

Changes in Matter

Changes In Matter

The explanation of natural events is one of the aims of science. Careful observation, leads to the formation of a concept or theory, which followed by testing and evaluating the ideas involved. Thisdefines the basic process scientists use to increase understanding of the types of change surrounding us.

Continued Changes of matter can be explained at three levels:

1. Macroscopic – visible to the naked eye2. Microscopic – too small to see without a

microscope3. Molecular – smallest entities of a substance

Types of Changes in Matter 1. Physical Change

Any changes where the fundamental entities remain unchanged at a molecular level, such as the phase changes of evaporation and melting.There is no change in the chemical formulaExamples would be dissolving of a substance, changes in physical structure that change only the shape of appearancePhysical Change uses a small amount of energy

Continued 2. Chemical Change

Involves some kind of change in the chemical bonds between the atoms and/or ions of a substanceThere is a change in the chemical formula, at least one substance is formed with physical and chemical properties different from the original matter Chemical Change involves a larger amount of energy

Continued 3. Nuclear Change

Involves changes within the nucleus, which created entirely new atomic entities These entities are represented by formulas that show new atomic symbols which are different from those of the original matter Nuclear Change involves an extremely large amount of energy

Table 1, on page 47

The Kinetic Molecular Theory Kinetic Molecular Theory – the idea

that the smallest entities of a substance – atoms, ions or molecules - are always in continuous motions, colliding with each other and objects in their paths

How and Why Chemical Reactions Occur To explain chemical reactions we can expand on the idea of Kinetic Molecular Theory. According to this theory the entities of a substance are in continuous, random motion. This motion inevitably means results in the collision between entities. If the collision has certain orientation and sufficient energy, the components of the entities will rearrange to form a new entities.

Figure 3, on page 47

Chemical Reactions

Recall the chemical reactions produce new substances. How do we know if a chemical reaction has occurred? Certain characteristic evidence is associated with chemical reactions. (Table 2 on page 48)

Chemical Reactions

Conservation of Mass in Chemical Changes

Experimenters have found that the total mass of matter presented after a chemical change is always the same as the total mass of matter prior to the change, this is known as the law of Conservation of Mass. If chemical change is thought of as a rearrangement of entities at the molecular level, then it is simple to argue that the mass must be constant.

Communicating Chemical Reactions

A balanced chemical equation is one in which total number of each kind of atom or ion in the reactants is equal to the total number of the same kind of atom or ion in the products If more than one molecule is involved, then the number is called a coefficientA substance’s state of matter is given in parentheses after the chemical formula

Figure 5, on page 49

Summary, on page 49

Homework

Section 2.2, Questions 1-10 on page 50

Section 2.3: Balancing Chemical Reaction Equations

Chemical amount – is the SI quantity for the number of entities in a substance. It is measured in units of moles (SI symbol, mol) . Modern methods of estimating this number of entities have led to the number 6.02x10²³. This value is called Avogadro’s number, named after the Italian chemist, Amedeo Avogadro.

Continued

A mole is the unit of the chemical amount of the substance with the number of the entities corresponding to Avogadro’s number For example:

- one mole of sodium is 6.02x10²³ Na atoms - one mole of chlorine is 6.02x10²³ Cl molecules

Mole ratio

Note that the coefficients in front of each substance is equal to the mole ratio. For example the ratio for the equation on the previous slide is (1:3:2) A complete translation of the chemical equation for the formation of ammonia is: “ One mole of nitrogen gas and three moles of hydrogen gas react to form two moles of ammonia gas.” This translation includes all the symbols in the equation, including the coefficients and the states of matter

Balancing Chemical Equations Step one : write the chemical formula for each reactant and product, including the state of matter for each one.Step Two: Try balancing the atom or ion present in the greatest number. Find the lowest common multiple to obtain coefficients to balance this particular atom or ion.Step Three: Repeat step two to balance each atom or ion. Step four: Check final reaction equation to ensure that all atoms and ions are balanced.

Sample problem on page 52

Homework

Section 2.3 Questions 1-6 on pages 53-54

Section 2.4: Chemical Amount The molar mass, M, of a substance is the mass of one mole of the substance. The unit for molar mass is grams per mole (g/mol) Each substance has a different has a different molar mass, which can be calculated as follows: 1. Write the correct chemical formula for the substance. 2. Determine the chemical amount (in moles) of each atom ( or monoatomic ion) in one mole of the chemical

Continued 3. Use the atomic molar mass from the periodic table and the chemical amounts (in moles) to determine the mass of one mole of the chemical.4. Communicate the mass of one mole as the molar mass in units of grams per mole, precise to two decimal places: for example 78.50g/mol

Molar Mass When doing calculations involving molar masses, remember that the relevant SI quantities and unites are : Mass, m, in grams, g Chemical amount, n, in moles, mol Molar mass, M, in grams per mole, g/mol

Molar Mass, sample problem on page 56

Mass – Amount Conversion communication example on page 56

Mass – Amount Conversion communication example on page 57

Homework

Section 2.4 Questions 1-7 on page 57

Section 2.5: Classifying Chemical Reactions

Formation Reactions

A formation reaction is the reaction of two or more elements to form either an ionic compound (metal + non metal) or a molecular compound (two or more nonmetals) Word Equation: Magnesium + oxygen magnesium oxideChemical Equation: 2 Mg(g) + O 2 MgO (s)

Simple Decomposition Reactions

A simple decomposition reaction is the break down of a compound into elements, the opposite of a formation reactionWord Equation: Water Hydrogen + Oxygen Chemical Equation: 2H O (l) H (g) + O (g)

Combustion Reaction A combustion reaction is the burning of a substance with sufficient amount of oxygen available to produce the most common oxides of the elements making up the substance that is burned.

Homework Section 2.5 Questions 1-6 on pages 59

Section 2.6: Chemical Reactions in Solutions

The remaining two reactions types are single replacement and double replacement reactions, which usually occur in aqueous solutions.

A solution is a homogeneous mixture of a solute (the substance dissolved) and a solvent (the substance, usually liquid that does the dissolving.

Chemical Reactions in Solutions Solubility of a substance is the maximum quantity of the substance which will dissolve in a solvent at a given temperature. When very soluble substances are formed as products in a single or a double replacement reaction, the maximum quantity of solute that can dissolve is rarely reached; thus, the new substance remains in the solution, and an (aq) notion is appropriate.

Chemical Reactions in Solutions When these substances are formed in a chemical reaction, the maximum quantity that can dissolve is usually reached and most of this substance settles to the bottom as a solid. Solid substances formed from this reaction are known as precipitates.

Solubility Chart A solubility chart outlines solubility generalizations for a large number of ionic compounds formed as productions in chemical reactions: Table 1, on page 61

Solubility Chart A major purpose of this chart is to predict the state of matter for ionic compounds formed as products of chemical reactions in a solutions. Solubility evidence is listed in two categories:

1. Very soluble (aq), for example Sodium Chloride 2. Slightly soluble (s), for example Calcium Carbonate

Sample Problem 2.4

Communication Example

Single Replacement Reactions A single replacement reaction is the reaction of an element with a compound to produce a new element and an ionic compound. This reaction usually occurs in aqueous solutions.Ex.

Double Replacement Reactions A double replacement reaction can occur between two ionic compounds in a solution. In the reaction, the ions, by analogy “change partners” to form the products. If one of the products is slightly soluble it may form a precipitate. As the term implies precipitation is a double replacement reaction in which a solid substance forms.

Double Replacement Reaction

Double Replacement Reaction

In another kind of double replacement reaction, an acid reacts with a base, producing water and a ionic compound. This kind of double replacement reaction is known as neutralization reaction. Ex.

Summary on page 63

Continued

Homework

Section 2.6 Questions, 1-7 on page 64Chapter 2 Review 1-12