8.2 Chemical Equations

law of conservation of mass

reactants

products

diatomic molecule

coefficient

8.2 Chemical Equations

Big Idea Ch8

1. In Chemical changes (reactions) atoms are rearranged, never made or destroyed

2. Mass stays the same in chemical changes (reactions)

3. Chemical reactions release energy or absorb energy to break and make chemical bonds

Is matter conserved in chemical reactions?

• Chemical equations show that in chemical reactions, atoms rearrange, but no atoms are gained or lost.


Conservation of Mass

• The law of conservation of mass The mass before a chemical reaction is equal to the mass after the reaction.


Chemical Equation

• Reactants react to produce Products






Quick check

Paper reacts with oxygen to produce carbon dioxide and ashes

WHAT ARE THE REACTANTS?

WHAT ARE THE PRODUCTS?

Is the mass of the paper and oxygen the same as the carbon dioxide, water, and ashes?

Balancing a Chemical Equation

• Atoms are neither gained nor lost in a reaction, so both sides of the equation must have the same number of atoms.


Reactants Products=

Balancing Chemical Equations

• “Tin + oxygen → tin oxide” reads as “tin plus oxygen produces tin oxide.”

To save space let’s use the chemical symbols


Sn + O2 SnO2

REACTANTSPRODUCTS

Balancing Equations• Use the symbols and formulas instead

of words.


Elements, Compounds, and Molecules (cont.)


Elements, Compounds, and Molecules (cont.)

• Some elements are diatomic, that is, they contain two atoms.


Equations for Common Chemical Reactions

Reaction of methane:

CH4 + O2 → CO2 + H2O balance hydrogen

CH4 + O2 → CO2 + 2H2O balance oxygen

CH4 + 2O2 → CO2 + 2H2O


Equations for Common Chemical Reactions (cont.)

Baking soda and vinegar:

NaHCO3 + HC2H3O2 → CO2 + H2O + NaC2H3O2


Using Parentheses with Formulas

• When counting atoms inside parentheses, the subscript multiplies all atoms inside.

Ca + H2O → Ca(OH)2 + H2

Ca + 2H2O → Ca(OH)2 + H2


Which is a diatomic molecule?

A O2

B H2O

C CH4

D N


What is potassium bromide (KBr)?

A an element

B a mixture

C a compound

D a diatomic molecule


In the following equation, which is a reactant?

Ca +2H2O → Ca(OH)2 + H2

A H2

B (OH)2

C Ca

D none of the above


8.3 Energy and Chemical Change

law of conservation of energy

exothermic process

endothermic process

Conservation of Energy

• The law of conservation of energy states that energy cannot be created or destroyed, simply changed from one form to another.



• Where is the energy from sunlight?

Net Energy

During a chemical reaction, reactant bonds are broken and product bonds are formed:

– Breaking bonds releases energy.

– Forming bonds absorbs energy.

– The total of energy in and energy out makes the entire reactions release or absorb energy


Net Release of Energy (cont.)

• An exothermic process releases energy.

• The products in an exothermic reaction have less energy than the reactants.


Net Absorbtion of Energy (cont.)

• An endothermic process absorbs energy.• The products in an endothermic reaction

have a higher energy than the reactants.


Chemical Bonds

• Molecules have chemical energy stored in their bonds.

• Energy is stored in the food that you eat and transferred to your cells.


• Where is the energy from sunlight?

• Which is endothermic or exothermic?

Products have a higher energy level than the reactants in an endothermic reaction.

A False

B True


____ bonds ____ energy.

A Breaking; releases

B Breaking; requires

C Forming; requires

D Ionic; release


The law of ____ states that energy is neither created nor destroyed in a chemical reaction.

A conservation of mass

B definite proportions

C conservation of matter

D conservation of energy


Chapter Assessment

California Standards Practice

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Which is a chemical change?

A copper turning green from exposure to air

B drying clothes

C ice melting

D none of the above

Which is a chemical property?

A boils at 100°C

B has a volume of 500 ml

C does not rust

D has a luster

Which does NOT describe an exothermic reaction?

A There is a net release of energy.

B The reactants have a higher energy than the products .

C The reactants are lower in energy than the products.

D The products are less stable than the reactants.

What is the coefficient of H2O?

Ca + 2H2O → Ca(OH)2 + H2

A 1

B 0

C 4

D 2

How many (OH) molecules are in the products?

Ca + 2H2O → Ca(OH)2 + H2

A 1

B 2

C 3

D 4

In a chemical reaction, which law states the products have the same mass as the reactants?

A Law of Conservation of Water

B Law of Conservation of Energy

C Law of Conservation of Mass

D Law of Conservation of Particles

SCI 5.b

Energy is ____ in exothermic reactions.

A absorbed

B created

C released

D stored

SCI 5.c

If an equation is balanced, the number of atoms in the products is ____ the number of atoms in the reactants.

A greater than

B less than

C equal to

D none of the above

SCI 3.b

Which of the following is a physical change?

A hydrogen burning

B iron rusting

C copper turning green when exposed to air

D none of the above

SCI X.X

Symbols represent ____ and ____ represent molecules.

A elements; formulas

B formulas; equations

C molecules; formulas

D elements; equations

SCI 3.f

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8.2 Chemical Equations