12/17/2014. Introduction to Chemical Reactions. The table below shows symbols commonly used in chemical equations:

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1 A chemical reaction is the process by which substances transform into new substances having chemical and physical properties that differ from those of the original substances. This lesson will include the first three things from our topics list: Introduction to chemical equations 5 Simple types of equations Balancing chemical reactions When a chemical reaction happens, a CHANGE CHEMICAL occurs. Look at the following options and tell which ones were chemical reactions: Write REACTION or NO REACTION Boiling Water: Breaking a Beaker: Salt is dissolved in water: Burning a match: Rustin Iron: Putting and Acid and Base together: Introduction to Chemical Reactions Chemical Reactions are represented by chemical equations. These consist of symbols give specific information about the chemical change that is taking place. In general, chemical reactions can be represented as follows: REACTANTS PRODUCTS Reactants are present before the reaction occurs Products are the chemicals formed by the reaction (present after the reaction occurs) The arrow ( ) signifies the direction of the change An example of a balanced chemical equation is: The table below shows symbols commonly used in chemical equations: 2Al (s) + 3H 2 SO 4 (aq) ----> Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) This equation shows exactly what reacts and what is produced. State symbols are used to identify the physical state of all of the reactants and products. Coefficients tell the proportions of each ingredient and product relative to each other. 1

2 Evidence for Chemical Reactions There are several clear indications that a chemical change (as opposed to a physical change has taken place. Probably the most obvious is a color change as seen in the rust forming here Two other indications of a chemical reaction are: Formation of a gas or formation of a precipitate (a solid that forms during an aqueous phase reaction) One further indication of a chemical change is: Finally, some chemical changes are accompanied by a change in ph. The release or absorption of energy. Energy can be released in the form of heat, light and/or sound. We will learn about ph when we study Acid/Base chemistry in the spring. 5 ways to know a chemical change occurred: 5 simple types of equations 1. Color Change 2. Formation of a Gas 3. Formation of a Precipitate 4. Release or Absorption of Energy 5. Change in ph The five general types of chemical reactions are: Synthesis (a.k.a. Combination) Decomposition Single Replacement Double Replacement, and Combustion reactions It will be important for you to be able to classify chemical equations into these different categories. QUICK VIDEO! 2

3 1. Synthesis Reactions A Synthesis Reaction Up Close and Personal! The Formation of Magnesium Oxide As the name suggests, synthesis (or combination) reactions involve the formation of new compounds from separate parts. Simple reactants combine to form a more complex product. 2Mg(s) + O2 (g) 2MgO (s) General Formula: A + B AB Example: C(s) + O 2 (g) CO 2 (g) ONE PRODUCT ONLY! 2. Decomposition Reactions Another decomposition example: In a decomposition reaction, a single compound breaks down into two or more simpler compounds or into pure elements. Usually, energy in the form of heat, light, or electricity is required for decomposition to take place. General Formula: AB A + B QUICK VIDEO DEMO H 2 O 2 (aq) H 2 O (l) + O 2 (g) Observations? Example: H 2 O(l) H 2 (g) + O 2 (g) electricity ONE REACTANT ONLY! Decomposition 3. Single Replacement Reactions Only one reactant Usually needs input of energy Can make elements or compounds If binary compound Make 2 elements If 3 or more A single replacement reaction involves one element knocking another element out of a compound. This only happens when A is more Active than the B. (It s higher on the Activity Series!) General Formula: A + BC AC + B Metal carbonate metal oxide + CO 2 Metal chlorate metal chloride + O 2 Metal hydroxide metal oxide + H 2 O metal metal nonmetal DIFFERENT PURE ELEMENTS ON EACH SIDE OF THE EQUATION! 3

4 A Single Replacement Reaction Up Close and Personal! In this single replacement reaction, the alkali metal sodium displaces hydrogen from water and forms a sodium hydroxide solution. 2Na(s) + 2H 2 O (l) 2NaOH (aq) + H 2 CONDITIONS FOR A DOUBLE REPLACEMENT REACTION TO OCCUR: Double replacement reactions usually take place in WATER (aqueous solution). They can occur only if there is a driving force involving the formation of something new! The driving force for these reactions can be either: formation of a precipitate (MOST COMMON!) formation of a gas, or formation of water. 4. Double Replacement Reactions Example: Double Replacement Reaction A double replacement reaction involves the exchange of positive ions between two compounds. General Formula: AB + CD AD + CB SWITCHING PARTNERS! USE SOLUBILITY RULES AND TABLE TO DETERMINE IF A SOLID FORMED! Note the formation of a solid precipitate, barium carbonate 4. Double Replacement Reactions 5. Combustion Reactions SPECIAL CASE DOUBLE REPLACEMENT REACTION ACID BASE REACTION General Formula: HB + A(OH) AB +H 2 O REACTANTS HAVE H + and OH - PRODUCTS ARE A SALT AND WATER A combustion reaction is a chemical change in which an element or compound reacts with oxygen, often producing ENERGY in the form of heat and light! General Formula: C x H y + O 2 CO 2 + H 2 O OXYGEN IS ALWAYS A REACTANT! CO 2 AND H 2 O ARE ALWAYS PRODUCTS! 4

5 An example of combustion PRACTICE! Classify the following chemical reactions according to type of reaction: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) (your Bunsen burners) Predict the products: C 3 H 8 + O 2 C 2 H 5 OH + O 2 C 6 H 14 + O 2 1. C 2 H 4 (g) + 3O 2 (g) 2CO 2 (g)+ 2H 2 O(g) answer: combustion 2. 2K (s) + Cl 2 (g) 2KCl (s) answer: synthesis 3. Fe (s) + H 2 SO 4 (aq) FeSO 4 (aq) + H 2 (g) answer: single replacement 4. KBr (aq) + AgNO 3 (aq) AgBr (s) + KNO 3 (aq) answer: double replacement PRACTICE! (continued) Classify the following chemical reactions according to type of reaction: 5. Al(OH) 3 (s) Al 2 O 3 (s) + H 2 O (l) answer: decomposition 6. Li (s) + O 2 (aq) Li 2 O (s) answer: synthesis Balancing Chemical Equations Of course, it s very important to be able to be able to predict the products of a chemical reactions. Equally important is the ability to estimate how much reactant will be needed to form certain quantity of product. The first step to knowing how much of each reactant to use is to be able to balance chemical equations. 7. Zn(s) + AgNO 3 (aq) Ag(s) + Zn(NO 3 ) 2 (aq) answer: single replacement Balancing Chemical Equations (ctd) Balancing a chemical equation depends mostly upon the simple idea called Conservation of Mass. Basically, every atom that enters into a chemical reaction must wind up in one of the products of the reaction. atoms on the reactant side is the Greek capital letter sigma representing a sum = atoms on the product side Atoms are not created or destroyed, they just change places! 3 Sure-Fire Steps to Correctly Balancing a Chemical Equation 1. Write an unbalanced equation first. Make sure that all of the compounds are correctly written so that they are electrically neutral. 2. Balance all of atoms starting with the most complicated molecule or the most uncommon element. (i.e. don t start with water, or O). Coefficients can be added in front of any compound but never change compound formulas! A good rule for deciding what to start with is Metals, Ions, Nonmetals, Oxygen then Hydrogen (MINOH) 3. Check quantities of all atom of reactants and products. reduce all coefficients to lowest terms If there are fractional coefficients, multiply by the denominator to leave whole number coefficients only 5

6 EXAMPLE 1: Starting with an unbalanced equation Example 1 (ct d) NH 3 + O 2 NO + H 2 O Since the only element other than H and O already balances, you can really start anywhere. An important note: trial and error is the method usually. Just keep fiddling with the coefficients until you get something that works! Then be sure to reduce all of the coefficients to lowest terms. 2 NH 3 + 5/2 O 2 2 NO + 3 H 2 O Notice that there are 3 H atoms on the reactant side and 2 H atoms on the product side. If we assume for a moment that 3 H 2 O are produced, how many NH3 molecules would be needed on the left? Now the N atoms need to be balanced......and that just leaves oxygen. Only problem is there are 5 O atoms total now on the left, so a coefficient of 5/2 would be needed on the right to balance (Remember, oxygen is only available as diatomic molecules.) Example 1 (ct d) Example 2 According to the 3 rd rule for balancing equations from a couple of slides ago, fractional coefficients are not allowed. So, now for the final step. Multiply all coefficients by 2 so that all coefficients will be whole numbers. The final result? 4_ NH 3 + 5_ O 2 4_ NO + _6 H 2 O Let s try to balance the following equation: MnO 2 (s) + 2 Al(s) Mn(s) + Al 2 O 3 (s) Both Al and Mn might be considered to be less common than O. Since Mn is already balanced, let s start with Al. A quick glance at the oxygen situation shows that we re going to end up with a fractional coefficient again. Let s try using 4 Al atoms on the left and see what happens Example 2 (ct d) MnO 3 2 (s) + _4_ Al(s) 3 Mn(s) + Al 2 2 O 3 (s) Now we can account for these aluminum on the product side... Giving us the need for 6 O atoms on the reactant side......and we can finish up by balancing off the Mn on the product side... Voila! 6