Understanding Chemical Reactions

Chapter 9.1p 300 - 309

Changes is Matter

• Physical Changes– Does not produce

new substances.– Might have

different physical properties

– Changes in states of matter: solid, liquid, gas

• Chemical Changes– One or more

substances change into new substances

– Have different physical and chemical properties.

– Cooking is an example of chemical change.

Chemical Reactions

• Chemical changes are called chemical reactions

• A chemical reaction is a process in which atoms of one or more substances rearrange to form one or more new substances.

Signs of a Chemical Reaction

• Changes in the physical properties of color, state of matter, and odor are all signs that a chemical reaction might have occurred

• A change is energy is another sign.

Signs of a Chemical Reaction

• Change in Properties– Color– Odor– Formation of bubbles– Formation of a

precipitate

• Change in Energy– Warming or cooling– Release of light

* A precipitate is a solid that forms from a reaction between two liquids

What happens in a chemical reaction?

• Atoms rearrange to form new substances– There are 2 types of substances: Elements and

Compounds– Substances have a fixed arrangement of atoms– A drop of water each water molecule will have one

oxygen atom bonded to two hydrogen atoms– If this arrangement changes the substance is no

longer water and a different substance has formed– The new substance will have different chemical and

physical properties– This is what happens during a chemical reaction

Chemical Reactions

• Bonds break and bonds form– Atoms rearrange when chemical bonds

between the atoms break– Particles are moving in all substances, even

solids– These particles collide with one another.– If particles collide with enough energy, the

bonds between the atoms can break, separate, rearrange, and form new bonds.

Chemical Equations

• A chemical equation is a description of a reaction using element symbols and chemical formulas

• Element symbols represent elements

• Chemical formulas represent compounds

Elemental Symbols

• Each element can exist as just one atom– C = carbon Cu = Copper

• Some elements exist in nature as diatomic molecules – two atoms of the same element bonded together.– This is shown by a subscript 2– H2 = hydrogen O2 = oxygen

Chemical Formulas

• A chemical formulas uses elements’ symbols and subscripts to describe the number of atoms in a compound

• If an element does not have a subscript, the compound contains only one atom of that element

Balanced: 2Na + Cl2 => 2NaCl

Writing Chemical Equations

• A chemical equation included both the substances that react and the substances that are formed in a chemical equation.

• The starting substances in a chemical reaction are the reactants

• The substances produced by the chemical reaction are products

• The arrow shows the direction of the reaction• Reactants are written left of the arrow and

products are written to the right

Conservation of Mass

• French chemist Antoine Lavoisier found that the total mass of the reactants always equaled the total mass of the products.

• This is the law of conservation of mass, which states that the total mass of reactants before the reaction must equal the total mass of products after the chemical equation.

Atoms are conserved

• Mass is conserved because atoms are conserved

• Bonds break and new bonds form during a chemical reaction

• However, atoms are neither created, nor destroyed

Is an equation balanced?

• An equation is written so that the number of atoms of each element is the same, or balanced, on each side of the arrow

Conservation of mass

Reactants Products

Conservation of atoms

Make it balanced!

• Add Coefficients!• A coefficient is a number placed in front of an element

symbol or chemical formula in an equation.• Only coefficients can be changed when balancing an

equation.• Changing subscripts changes the identities of the

substances that are in the reaction.

Reactants

Elements

Products

H

O

2

2

2

12

44 22

4 H 2 O 4 H, 2 O

Balancing a Chemical Equation

1. Write the unbalanced equation.

2. Count atom of each element in the reactants and in the products.

3. Add coefficients to balance the atoms.

4. Write the balanced chemical equation.

Balance

• ____ N2 + ____ H2 → ____ NH3

• ____ Al + ____ HCl → ____ AlCl3 + ____ H2 (g)

Lighting a Candle

• Lab