Chemical Reactions. Chapter 9 chemical reaction is a process in which one or more substances change to make one or more new substances. NEW SUBSTANCE-

Chemical Reactions

Chapter 9

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

NEW SUBSTANCE-◦ chemical /physical properties that form in a

chemical reaction differ from those of the original substances.

Section 1 Forming New Substances

Chapter 9

Signs OF chemical reaction ◦ color change◦ formation of a gas or a solid◦ the release or absorption of energy.

Precipitate- solid that is produced as a result of a chemical reaction in solution


Chemical ReactionsChapter 9

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Chapter 9

SIGNALS OF ◦ they do not always guarantee that a reaction is

happening.◦ most important sign - formation of new

substances that have new properties.


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

For a chemical reaction to take place◦ Chemical bonds MUST BREAK

(in starting substance)◦ Atoms rearrange◦ new bonds form make new substances


Chapter 9

Energy◦ breaks chemical bonds ◦ As new bonds form energy is released

exothermic reaction- chemical reaction in which energy is released

can give off energy in several forms


Chapter 9

endothermic reaction- chemical reaction in which energy is taken in

energy taken in is absorbed from the surroundings.

Ex: Photosynthesis


Chapter 9

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

law of conservation of energy states that energy cannot be created or destroyed but can change from one form to another (TRANSFER)



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

2H2 + O2 2H2O What elements are present in water?

◦ Hydrogen and Oxygen How many atoms of each element are

shown in the reactants?◦ 4-H; 2-O

How many atoms of each element are shown in the products? 4-H; 2-O

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

Section 2 Chemical Formulas and EquationsChapter 9

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 formula- is a combination of chemical symbols and numbers to represent a substance.◦ shows how many atoms of each kind of

element are present in a molecule.

Subscript- is a number written below and to the right of a chemical symbol in a formula.



covalent compounds -written by using the prefixes in the names of the compounds.

ionic compounds- written by using enough of each ion so that the overall charge is 0.



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

Reactants-starting materials in a chemical reaction

Products-substances formed from a reaction are


writing a chemical equation◦ Must be balancing –◦ total number of atoms of each element in the

reactants must equal the total number of atoms of that element in the products

◦ #reactants=#products



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.



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.


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?

A chemical equation describes what happens in a chemical reaction.

The equation identifies ◦ reactants (starting materials) ◦ and products (resulting substance), ◦ the formulas of the participants,◦ the phases of the participants (solid, liquid, gas), ◦ and the amount of each substance.

Balancing a chemical equation◦ refers to establishing the mathematical

relationship between the quantity of reactants and products.

◦ quantities are expressed as grams or moles.

3 steps to the process: Write the unbalanced equation.

◦ Chemical formulas of reactants are listed on the lefthand side of the equation. ◦ Products are listed on the righthand side of the equation. ◦ Reactants and products are separated by putting an arrow between them to show the

direction of the reaction. Reactions at equilibrium will have arrows facing both directions. Balance the equation.

◦ Apply the Law of Conservation of Mass to get the same number of atoms of every element on each side of the equation. Tip: Start by balancing an element that appears in only one reactant and product.

◦ Once one element is balanced, proceed to balance another, and another, until all elements are balanced.

◦ Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas.

Indicate the states of matter of the reactants and products.◦ Use (g) for gaseous substances. ◦ Use (s) for solids. ◦ Use (l) for liquids. ◦ Use (aq) for species in solution in water. ◦ Write the state of matter immediately following the formula of the substance it describes.

Tin oxide is heated with hydrogen gas to form tin metal and water vapor. Write the balanced equation that describes this reaction.

Write the unbalanced equation. SnO2 + H2 → Sn + H2O

1.Count the atoms-◦ Look at the equation and see which elements are

not balanced.◦ there are two oxygen atoms on the lefthand side

of the equation and only one on the righthand side.

2. Balance them out- ◦ Correct by putting a coefficient of 2 in front of

water:◦ SnO2 + H2 → Sn + 2 H2O

2. Balance other elements-◦ This puts the hydrogen atoms out of balance. ◦ Now there are two hydrogen atoms on the left

and four hydrogen atoms on the right. ◦ To get four hydrogen atoms on the right, add a

coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if

we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=2 oxygen atoms.

SnO2 + 2 H2 → Sn + 2 H2O

The equation is now balanced. Be sure to double-check your math! Each side of the equation has 1 atom of Sn,

2 atoms of O, and 4 atoms of H. SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)

Indicate the physical states of the reactants and products. To do this, you need to be familiar with the properties of various compounds or you need to be told what the phases are for the chemicals in the reaction. Oxides are solids, hydrogen forms a diatomic gas, tin is a solid, and the term 'water vapor' indicates that water is in the gas phase:

SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g) This is the balanced equation for the reaction.

Balance equations How is the law of conservation of energy

true for both endothermic/exothermic reactions?

What happens to bonds in chemical reaction?

As a solution of water and sugar dries, a crystal forms on the end of a string that is suspended in the solution. Is this an example of a chemical reaction? Explain.

Energy is always 1)___________ in 2)___________..

Which are classified as 3)___________if energy is 4)________________ and stored in 5)____________such as in photosynthesis.

Which are classified as 6)___________ if energy is 7)____________ from 8)__________ such as in burning wood.

Chemical reaction◦ What is an example of one?◦ What happens to the bonds

Chemical equations◦ How to balance◦ Why must be balanced

Diagram ◦ Activation energy◦ Energy absorbed◦ Energy released◦ Exothermic/endothermic reaction

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Chemical Reactions. Chapter 9 chemical reaction is a process in which one or more substances change to make one or more new substances. NEW SUBSTANCE-