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Chapter 9 Chemical Reactions

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Section 1 Forming New Substances

Section 2 Chemical Formulas and Equations

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Section 1 Forming New SubstancesChapter 9

Bellringer

The formation of water from its elements may be represented as follows:

2H2 + O2 2H2O

What elements are present in water? How many atoms of each element are shown in the reactants? How many atoms of each element are shown in the products?

The representation 2H2 + O2 2H2O is called a chemical equation. Why do you think that the numbers of atoms do not change? Write your answers in your Science Journal.


Chapter 9

What You Will Learn

• Four signs that indicate that a chemical reaction may be taking place are a change in color, the formation of a gas, the formation of a precipitate, and a change in energy.

• Chemical reactions produce new substances whose chemical and physical properties differ from the properties of the original substances.

• In a chemical reaction, chemical bonds break and atoms rearrange.

• Chemical reactions absorb or release energy.



Chapter 9

Chemical Reactions

• A chemical reaction is a process in which one or more substances change to make one or more new substances.

• The chemical and physical properties of the new substances that form in a chemical reaction differ from those of the original substances.



Chapter 9

Chemical Reactions, continued

• Signs that indicate a chemical reaction may be taking place are a color change, formation of a gas or a solid, and the release or absorption of energy.

• A solid that is produced as a result of a chemical reaction in solution is called a precipitate.



Chemical ReactionsChapter 9

Precipitate


Chapter 9

Chemical Reactions, continued

• Even though the signs that indicate a chemical reaction is taking place are good signals of chemical reactions, they do not always guarantee that a reaction is happening.

• The most important sign that a chemical reaction has taken place is the formation of new substances that have new properties.



Chapter 9

Bonds: Holding Molecules Together

• A chemical bond is a force that holds two atoms together in a molecule.

• For a chemical reaction to take place, the chemical bonds in the starting substances must break. The atoms then rearrange, and new bonds form to make new substances.



Chapter 9

Reactions and Energy

• Energy is needed to break chemical bonds in the starting substances. As new bonds form in the final substances, energy is released.

• A chemical reaction in which energy is released is called an exothermic reaction. Exothermic reactions can give off energy in several forms.



Chapter 9

Reactions and Energy, continued

• A chemical reaction in which energy is taken in is called an endothermic reaction.

• The energy taken in during an endothermic reaction is absorbed from the surroundings.

• Photosynthesis is an example of an endothermic process.



Chapter 9

Reactions and Energy, continued

• Neither mass nor energy can be created or destroyed in chemical reactions.

• The law of conservation of energy states that energy cannot be created or destroyed but can change from one form to another.




Law of Conservation of Energy


Section 2 Chemical Formulas and EquationsChapter 9

Bellringer

Look at the following formulas for chemical compounds, and identify the elements in each formula.

NaCl KBr C12H22O11

NH3 SiF4 Fe(NO3)3

H2O2 MgSO4

What is the name and symbol of each element? How many atom of each element are present in each compound?

Write your answers in your Science Journal.



What You Will Learn

• Chemical formulas are a simple way to describe which elements are in a chemical substance.

• Chemical equations are a concise way to write how atoms are rearranged in a chemical reaction.

• A balanced chemical equation shows the law of conservation of mass.



Chemical Formulas

• A chemical formula is a combination of chemical symbols and numbers to represent a substance.

• A chemical formula shows how many atoms of each kind of element are present in a molecule.

• A subscript is a number written below and to the right of a chemical symbol in a formula.



Chemical Formulas, continued

• The formulas for many covalent compounds can be written by using the prefixes in the names of the compounds.

• The formulas for ionic compounds are written by using enough of each ion so that the overall charge is 0.



Chemical Equations

• A chemical equation uses chemical formulas, chemical symbols, and coefficients to describe a reaction.

• The starting materials in a chemical reaction are reactants. The substances formed from a reaction are products.



Chemical Equations, continued

• A chemical equation that has a wrong chemical symbol or formula will not describe the reaction correctly.

• When writing a chemical equation, the total number of atoms of each element in the reactants must equal the total number of atoms of that element in the products. This process is called balancing the equation.



Chemical Equation




• The law of conservation of mass states that mass cannot be created or destroyed in ordinary chemical and physical changes.

• This law means that the total mass of the reactants is the same as the total mass of the products.



Law of Conservation of Mass




• A coefficient is a number that is placed in front of a chemical symbol or formula.

• Coefficients are used to balance an equation.

• For an equation to be balanced, all atoms must be counted.




• To count all the atoms in an equation, multiply the subscript of each element in the formula by the formula’s coefficient.

• To balance an equation, change the coefficients—not the subscripts.

• Changing the subscripts in the formula of a compound changes the compound.


Chapter 9 Chemical Reactions

Use the terms below to complete the concept map on the next slide.

products

reactants

chemical equations

coefficients

chemical reactions

chemical formulas

Concept Map



Concept Map

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