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10 Cards in this Set
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CHAPTER 6: Chemical Reactions; An introduction
Evidence of a chemical reaction. |
Chemical reactions often give a visual signal (e.g. color change, solid formed, a flame, bubbles, etc.) that a reaction has occured.
However, reactions are not always visible (e.g. change in temperature as heat is produced or absorbed.) |
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CHEMICAL EQUATIONS:
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Chemical reactions involve a rearrangement of the ways in which the atoms are grouped.
Chemical equations reactions (chemicals present before the reaction) are shown to the left of an arrow and the products (chemicals formed by reaction) are shown to the right of the arrow. In a chemical reaction, atoms are neither created or destroyed. All atoms present in the reactants must be accounted for among the products. |
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Example of Chemical equation:
Balancing the Chemical equation for the above reaction: |
Reactants -> Products (the arrow signifies "yields" or "produces".
Methane + Oxygen -> Carbon Dioxide + Water ---------------------------------------------(reaction is not yet balanced) The products contain the same atoms as the reactants but the atoms are associated in different ways. Balancing the equation for the above reaction: --------------------------------------------- (the total number of each atom is not the same on both sides of the equation.) *Add co-efficients to balance. To balance the chemical equation, we must have the same total number of each type of atom on both sides of hte equation. ------------------------------------------------------------------------------------------ (equation is balanced) |
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Important chemical reaction information:
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The chemical equation for a reaction provides us with two important types of information.
1. The identities of the reactants and products. 2. The relative numbers of each. (s) solid (l) liquid (g) gas (aq) aqueous solution (can be dissolved in water). |
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Example of chemical reation:
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When solid potassium reacts with liquid water, the products are hydrogen gas and potassium hydroxide; the latter remains dissolved in the water.
The unbalanced equation is: --------------------------------------------- The hydrogen gas then reacts with the oxygen gas in the air, producing gaseous water and a flame. The unbalanced equation for this second reaction is: --------------------------------------------- |
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Chemical equations: Recognizing Reactants and Products (unbalanced equations).
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a. Solid mercury (II) oxide decomposes to produce liquid mercury metal and gaseous oxygen.
--------------------------------------------- (oxygen is a diatomic molecule under normal conditions). It is not balanced. b. Sold carbon reacts with gaseous oxygen to form gaseous carbon dioxide. --------------------------------------------- This is a balanced equation. c. Solid Zinc is added to an aqueous solution containing dissolved hydrogen chloride to produce gaseous hydrogen that bubbles out of the solution and zinc chloride that remains dissolved in the water. --------------------------------------------- This equation is not balanced. |
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Self check exercises: Identify the reactants and products and write the unbalanced equation (including symbols for states) for each of the following chemical reactions.
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a. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas.
--------------------------------------------- b. Solid ammonium dichromate decomposes to solid chromium (III) oxide, gaseous nitrogen and gaseous water. --------------------------------------------- c. Gaseous ammonia reacts with gaseous nitrogen monoxide and gaseous water. --------------------------------------------- *all are unbalanced. |
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Balancing Chemical Equations:
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*An unbalanced chemical equation is not an accurate representation for a reaction, since atoms are conserved in a chemical reaction. Atoms are neither created nor destroyed.
*The identities of the reactants and products of a reaction are determined through experiments. *The fomulas of the compounds cannot be changed in balancing a chemical equation. *You can change only the coefficients (numbers) in front of the compounds, but never subscripts nor the atoms. *Most chemical reactions can be balanced by inspection (trial and error.) |
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EX: balanced chemical equation
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Balance the following equation:
--------------------------------------------- --------------------------------------------- --------------------------------------------- --------------------------------------------- --------------------------------------------- --------------------------------------------- This is a balanced chemical equation. *Each chemical equation has many possible balanced equations by multiplying or dividing all parts of the original balanced equation by any number. EX:______________________ *The best balanced equation is the one with the smallest integers (whole numbers), co efficients, for the balanced equation. This type of balanced equation is the only one that will be accepted for credit on exams. *Example: Balance the equation for the reaction of liquid ethanol, C2H5OH, with oxygen gas to form gaseous carbon dixodie and water. 1. Write the unbalanced equation for the reaction from the description given in the problem. --------------------------------------------- 2. Start with the most complicated molecule (contains the most elements) and begin by considering the products that contain carbon (C). --------------------------------------------- (both sides have 2 C atoms) 3. Then consider hydrogen (H) and balance the products containing hydrogen. --------------------------------------------- (both sides have 6 H atoms) 4. Finally count the oxygen (O) atoms. --------------------------------------------- (both sides have 7 O atoms) Rewrite with the coefficient 1: --------------------------------------------- Check: --------------------------------------------- |
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How to write and balance equations:
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Step 1: Read the description of the chemical reaction. What are the reactants, the products and their states? Write the appropriate formulas.
Step 2: Write the unbalanced equation that summarizes the information from Step 1. Step 3: Balance the equation by inspection, starting with the most complicated molecule. Proceed element by element to determine what coefficients are necessary so that the same number of each type of atom appears on both the reactant side and the product side. Do not change the idetities (formulas) of any of the reactants or products. Step 4: Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. (Note that an "atom" may be present in an element, a compound or an ion.) Also check to see that the coefficients used at the smallest integers that give the balanced equation. This can be done by determining whether all coefficients can be divided by the same integer to give a set of smaller integer coefficients. |
