Unusual Reactions L.O: To understand what happens in some unusual chemical reactions.
Chapter 9.1: Understand Chemical Reactions
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Transcript of Chapter 9.1: Understand Chemical Reactions
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