Chemical Reactions
description
Transcript of Chemical Reactions
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Chemical Reactions
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Section 1 Chemical Changes
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A. Describing chemical reactions- change of one or more substances into new substances
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1. Reactants are substances that combine or change
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2. New substances that are produced are called products.
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B. Conservation of Mass- a law which states that, in a chemical reaction, matter is not created or destroyed; it stays the same
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1. Antoine Lavoisier- experimented with mercury(II) oxide and heat
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2. Found mass of products (liquid mercury and oxygen gas) equaled mass of reactants
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C. Writing equations- a chemical equation uses chemical formulas and symbols to describe a chemical reaction and the product(s) it produces
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1. Chemical formula expresses the relationship between elements in the compounds and molecules they make up
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2. Coefficients- numbers which represent the number of units of each substance in a reaction
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3. Knowing coefficients of chemical reactions allows chemists to use the correct amounts of reactants to predict the amounts of products
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4. Subscripts- numbers which represent the number of atoms in a molecule of a particular element.
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5. Symbols used to show state of reactants: (s)solid, (aq) aqueous, (g)gas, (clear) liquid.
When a chemical has been dissolved in water, this is denoted by writing (aq) after the chemical name.
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D. Metals react with atmosphere in different ways.
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Section 2 Chemical Equations
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A. Checking for balance- law of conservation of mass requirement
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1. A balanced chemical reaction- both sides of equation have the same number of atoms of each element
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2. Choosing coefficients- becomes easier with practice; trial and error at first
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B. Writing balanced chemical reactions- a four step process
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1. Describe the reaction in words.
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2. Write the equation using formulas and symbols.
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3. Check for balance.
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4. Add coefficients where needed for balance.
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Section 3 Classifying Chemical Reactions
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A. Synthesis reaction- two or more substances form a new substance; A + B ---> C
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+ ->
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B. One substance breaks down into two or more substances in a decomposition reactionAB ---> A + B
The starting compound is ammonium dichromate. When heated, it begins to decompose into nitrogen gas, water vapor and powdered chromium (III) oxide. It looks like a volcano with ash being spread all over the place.
http://www.youtube.com/watch?v=jowikn6tsyY
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->
+
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C. Single-displacement reaction- one element replaces another one in a compound:A + BC ---> AC + B or D + BC ---> BD + C
A + BC --> AC + B
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+ ->
+
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D. A double-displacement reaction results if a precipitate, water, or a gas forms when two ionic compounds in solution are combined; AB + CD --> AD + CB
AB + CD --> AD + CB
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++ -
>
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Combustion- O2 as a reactant and CO2 and H2O as the product
MgCO3 + 2HCl --> MgCl2 + H2O + CO2
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Section 4 Chemical Reactions and Energy
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A. Chemical reactions involve energy exchange.
Reaction of sulfuric acid and sugar.
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1. Breaking chemical bonds requires energy.
Dr. Pyenta melted some potassium chlorate in a test tube over a Bunsen burner in a hood. Then he dropped in a sugar cube. The result was a considerable jet of flame that burned for 30 seconds or more.
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2. Forming chemical bonds releases energy.
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B. More energy out
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1. Exergonic reactions- energy required to break bonds is less than the energy released from new bonds; energy given off is usually light
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2. Exothermic reactions- energy given off in the form of heat
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C. More energy in
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1. Energonic reactions- more energy is required to break bonds than to form new ones; need energy for the reaction to occur
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2. If energy needed is heat; the reaction is endothermic.
vinegar & baking soda
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3. A catalyst speeds up a chemical reaction without itself being permanently changed.
Enzymes speed up chemical reactions
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4. An inhibitor prevents or slows a chemical reaction or interferes with a catalyst's action
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