chemical reactions: is the process by which one or more substances change to produce one or more...
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•Chemical Reactions: is the process by which one or more substances change to produce one or more different substances.
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Chemical Equation: a shorter, easier way to show a chemical reaction.
2H + O H2O
The arrow means “yields” or “makes”
Reactant – substance that enters into a chemical reaction (these are on the left of the arrow)
Product – the new substance (these are on the right of the arrow)
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Ways to Identify a Chemical Reaction
1. Change in color
Examples: a)Rust on
iron b)bleach on
blue jeans
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2. Forms a gas (you can see bubbles)
Example: vinegar and baking soda mixed together
forms bubbles.
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3. Change in energyExothermic: reactions in which heat or light are released (container will feel hot)
Endothermic: reactions in which heat is absorbed (when heat is absorbed, the container will feel cold.)
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4. Produces a precipitate
Example: Lead nitrate + potassium iodine yields lead iodide, which sinks to the bottom of the beaker.
The solid that appears in a solution during a chemical reaction is called a precipitate
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A chemical reaction…• Is indicated by the formation of new
substances with properties that are different from those of the starting substances.
• Ex: Chlorine is a green, toxic gas and Sodium is a highly reactive, shiny metal. When they chemically combine to form Sodium Chloride, they form ordinary table salt with properties very different from the reactants (Chlorine & Sodium).
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• If a substance releases bubbles, it may be undergoing a chemical reaction, but it is not for sure…
• A boiling pot of water releases bubbles, is that a chemical reaction?
• Sometimes, we may need to look for more evidence of a chemical change occuring.
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Writing chemical formulas
• Elements are represented by symbols, right?
• Well, when we put elements together to form compounds, we can use a formula to represent the compound
• Ex: NaCl is the formula for sodium chloride
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Subscripts• The subscripts show how many atoms
are in a chemical formula.
• H2O The 2 in this formula is a subscript, (written below the line) that tells us there are 2 Hydrogen atoms in this formula. How many oxygen atoms are in a water molecule?
How many Iron atoms are in rust, Fe2O3?
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Law of conservation of mass
• When a reaction occurs, atoms are not destroyed, they are just rearranged.
• The law of conservation of mass says the amount of matter does not change in a reaction, and the total mass of the reactants must equal the total mass of the products.
• To demonstrate this law, we balance equations to show that atoms don’t
disappear or appear, they are simply rearranged.
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Four types of Chemical
Reactions
Four types of Chemical
Reactions
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1. Synthesis• 2 or more simple substances
combine to form a new complex substance
– Example: 2H + O H2O
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2. Decomposition• A complex substance breaks down
into 2 or more simpler substances.
– Example: H20 2H + O
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3. Single Replacement• An uncombined element replaces
an element that is part of a compound.
– Example: Zn + CuSO4 ZnSO4 + Cu
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4. Double Replacement
• Different atoms in two different compounds replace each other.
Example: NaCl + 2AgNO3 2AgCl + NaNO3