va sol 6.4. we use our senses to detect chemical changes. example: toasting marshmallows over a...
TRANSCRIPT
VA SOL 6.4
We use our senses to detect chemical changes.
Example: toasting marshmallows over a campfire Logs change from a hard solid to soft pile of
ashes. Popping and hissing sounds as gases
produced by the reaction expand the wood Smell of the smoke Taste of the marshmallows after they are
toasted.
Chemical reactions involve 2 main kinds of observable changes.
1. changes in the properties of the materials involved.
2. change in energy
Changes in the properties occur when a new substance is formed:
A gas might be produced A color change might occur Solid may appear (called a precipitate )
Sometimes, physical changes give similar results.
Example: water changes physically when it changes state (solid to liquid to gas)
When water changes chemically, the molecules undergo a change.
Example: When an electric current is passed through water during electrolysis, two gases are produced: hydrogen gas (H
2) and oxygen
gas (O2)
One observable characteristic of a chemical reaction is the production of new materials with properties that are different from those of the starting materials.
Change in Energy: one common indication that energy has been absorbed or released is a change in temperature.
Endothermic reaction-type of reaction that absorbs energy (feels cold to touch)
example: baking soda reacting with vinegar, cold pack
Exothermic reaction- type of reaction that releases energy in the form of heat (feels hot to touch)
example: reaction of fuel and oxygen in airplane engine lifts the plane off the ground
How are endothermic reactions different from exothermic reactions?
When you use your senses to observe evidence of a chemical change, you are detecting the combined effect of countless small, invisible changes involving the rearrangement of atoms.
Chemical reactions occur when chemical bonds break or when new bonds form.
Types of chemical bonds determine the properties of a substance and whether or not a chemical reaction will occur in a specific set of circumstances.
Lets take an old house for example: If a bond is strong (ex. Glass) it is hard to
change. Chemical bonds in wood are not as strong and
react with other substances more easily. (This is why wood can rot over the years while glass shows no damage)
1. What are some examples of evidence for a chemical reaction?
2. What two kinds of energy changes can take place during a chemical reaction?
3. What happens to the chemical bonds in a substance during a chemical reaction?
4. When a solid forms as two solutions are mixed together, what is that solid called?
Why would it help to see a picture on the sign?
Symbols express a concept in a shorter form and help put chemical equations in a form that ALL chemists can understand.
“Hydrogen molecules react with oxygen molecules to form water molecules” is the same thing as saying H
2 + O → H
2O
A chemical equation is a shorter, easier way to show chemical reactions, using symbols instead of writing words.
Symbols = letters of alphabet (ex. H and O) Compounds/formula = words (H
2O)
Start with Symbols (O and H)
Put symbols together to make Formulas (H2O)
Put formulas together to make Chemical Equations ◦2H20 2H2 + O2
A formula does 2 things: Identifies the elements in a compound Shows the ratio of atoms that make up that
substance
Example: table sugar ( C12
H22
O11
)
Made up of 12 carbons, 22 hydrogens, and 11 Oxygen.
In Aluminum Chloride (AlCl3) there are ___
Chloride ions for every 1 aluminum ion.
Subscripts show the number of atoms of an element in a molecule or the ratio of elements in a compound. If a letter symbol in a chemical formula doesn't have a subscript the number 1 is understood.
Example: CO2
has ____ carbons and ___ oxygens
A chemical equation summarizes a reaction. It tells you:
The substances you start with (reactants) The substances you end up with (products)
Reactant + reactant → product + product
The number of reactants and products can vary.
“→” is read as “yields”
No matter how many reactants and products are involved, all of the atoms present at the start of the reaction are present at the end.
The total mass of the reactants must equal the total mass of the products. (Law of conservation of mass)
Think of this school and classes... Are all of your class sizes the same? Does the total amount of students in the
building change from period to period?
Chemical Reactions are classified into 3 groups: Synthesis-combining of 2 substances to make a
more complex one.Reactant + reactant → product
Decomposition-breaking down more complex componds into simpler products.
Reactant → product + product
Replacement-when one element replaces another in a compound, or when two elements in different compounds trade places.
2NH4NO3 → 2N2 + O2 + 4H2O
2 Al + Fe2O3 → Al2O3 + 2Fe
P4PO10+6H2O → 4H3PO4