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Regents Chapter 11 Chemical Reactions 1 Section 11.1 Describing Chemical Reactions • Objectives: – Describe how to write a word equation. – Describe how to write a skeleton equation. – Describe the steps for writing a balanced chemical equation. 2 Chemical Reactions • Chemical reactions occur everyday, everywhere. There are thousands of different reactions that occur Digestion Baking Cookies Photosynthesis Respiration Explosion of the Hindenburg 3 All chemical reactions… – Have two parts: • Reactants - the substances you start with • Products- the substances you end up with – The reactants turn into the products. – Reactants Products • The arrow is read as yields 4 Symbols used in Equations • • • • (s) after the formula = solid: AgCl(s) (g) after the formula = gas: CO2(g) (l) after the formula = liquid: H2O(l) (aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) salt water solution • ↑ used after a product indicates a gas has been produced: H2 ↑ • ↓ used after a product indicates a precipitate has been produced: PbI2 ↓ 5 Symbols used in equations • indicates a reversible reaction (more later) heat , • shows that heat or energy is supplied to the reaction 6 Balanced Chemical Equations • Before we begin to balance chemical equations lets talk about what the term balanced means by using an equation for the production of a bicycle – What are the basic components needed to manufacture a bicycle? frame + wheel + handlebar + pedal = bike – Our equation shows the reactants and products • What if we were responsible for ordering the parts to make the bike? Is this equation sufficient? 7 Balanced Chemical Equations • We need to know how many of each component is needed to manufacture the bike – 1 frame, 2 wheels, 1 handlebar, and 2 pedals • Using these symbols the formula for our bike would be FW2HP2 • The equation for the manufacture of the bike would become: F + W + H + P FW2HP2 – There is a problem with our equation • It is not a balanced equation – the number of parts do not equal the whole – There is not the same number of each part on the left as there is on the right • Our equation should be: F+ 2W + H + 2P FW2HP2 • This is now a balanced equation showing the same number 8 of each part on the left and on the right Balanced Chemical Equations • The same is true for a balanced chemical equation • Remember… – Atoms can not be created or destroyed in an ordinary reaction – the atoms are merely rearranged by the breaking and forming of bonds – In any chemical change, mass is conserved • The atoms in the products are the same atoms that were in the reactants – they are just rearranged – Law of Conservation of Mass • A balanced equation has the same number of atoms of each element on both sides of the equation. 9 Balanced Chemical Equations • Use coefficients IN FRONT of the entire formulas to balance the equation so that it obeys the law of conservation of mass. However, the coefficient of 1 is omitted from the front of the formula. • Carbon burns in the presence of oxygen to produce carbon monoxide The skeleton or base equation: REMEMBER – diatomic molecules 2C + O2 2CO 1. Count the number of atoms of each element on both sides of the equation This is a balance C = 2 C=2 10 equation O=2 O=2 Rules for Balancing: 1. 2. 3. 4. 5. 6. 7. Assemble the correct formulas for all the reactants and products Use + in between more than one reactant and product if necessary Use → between reactants and products Count the number of atoms of each type appearing on both sides Balance the elements one at a time by adding coefficients where needed (the numbers in front) - save balancing the H and O until last! (Hint - save O until the very last) RECHECK to make sure equation is balanced (equal amount of each element on both sides) Check to make sure all coefficients are in the 11 lowest whole number ratio • Never change a subscript to balance an equation. – If you change the formula you are describing a different reaction. – H2O is a different compound than H2O2 • Never put a coefficient in the middle of a formula 2NaCl is okay, but Na2Cl is not. 12 Balance the Following Chemical Equation • When hydrogen and oxygen are mixed, a spark will initiate a rapid reaction. The product of the reaction is water. Step 1: Write the proper equation - correct formulas for reactants and products Reactants – hydrogen and oxygen Product - water H 2 + O2 H 2O Step 2: Count the number of atoms of each type appearing on both sides H= 2 O= 2 H= 2 Step 3: Balance atoms one at a time by adding coefficients 2 H2 + O2 Placing a coefficient of 2 in front of H2 rebalances the hydrogen 2 H2O O= 1 Placing a coefficient of 2 in front of water balances the O but unbalances the 13 hydrogen Balanced Chemical Equation • This is a properly balanced chemical equation 2H2 + O2 2H2O • There is the same number and kinds of each element in the reactants and products – 4 hydrogen and 2 oxygen atoms in the reactants – 4 hydrogen and 2 oxygen atoms in the products 14 Practice Examples/ Problems • _AgNO _Cu(NO _Ag 2 1 1 3 + _Cu 3)2 + 2 • _Mg + _N 3 1 2 1_Mg3N2 4 + _O 5 2 _P 1 4O10 • _P • _Na + _H 2 2 2O _H 1 2 + _NaOH 2 • _CH _O2 _CO _H2O 1 1 4 + 2 2 + 2 15 Section 11.2 Types of Chemical Reactions • Objectives: –Describe the five general types of reactions. –Predict the products of the five general types of reactions. 16 Types of Reactions • There are millions of reactions. • We can’t remember them all, but luckily they will fall into several categories. • We will learn 5 major types. • We recognize reaction type by the reactants 17 Types of Reactions 1. Synthesis Reaction: A+B AB 2. Decomposition Reaction: AB A + B 3. Single Replacement: A + BC B + AC 4. Double Replacement: AB + CD AC + BD 5. Combustion Requires oxygen (O2) as a reactant and H2O and 18 either CO or CO as products Synthesis Reactions • Combination reactions • Combine = put together • Two or more substances to form a single new substance (a compound) • 2Ca +O2 2CaO • SO3 + H2O H2SO4 • We can predict the products if the reactants are two elements – Example Mg + N2 Mg3N2 19 Decomposition Reactions • decompose = fall apart • Single compound breaks down into two or more simpler products – 1 reactant 2 products electricity • 2NaCl • CaCO3 2Na + Cl2 CaO + CO2 – Note that energy (heat, sunlight, electricity, etc.) is usually required electricity 2H2O(l) 2 H2(g) + O2(g) http://www.youtube.com/watch?v=m_21KafrwRc&feature=related (3:18 min) 20 http://www.youtube.com/watch?v=__gHPblNumM&feature=related (6:23 min) Single Replacement • One element replaces another in a compound • Reactants must be an element and a compound. • Products will be a different element and a different compound. K + NaCl KCl + Na Zn + 2HCl ZnCl2 + H2 21 Double Replacement • Reactants are always two compounds and products are always two different compounds • Involves the exchange of positive ions between 2 compounds • Two things replace each other. – Reactants must be two ionic compounds. • NaOH + FeCl3 ?????? – The positive ions change place. • NaOH + FeCl3 Fe+3 OH- + Na+1 Cl-1 • 3NaOH + FeCl3 Fe(OH)3 + 3NaCl • http://www.youtube.com/watch?v=opY3FLrPTa4 (1:30 min) 22 Combustion • Means add oxygen – Normally, a compound composed of only C, H, (and maybe O) is reacted with oxygen – usually called burning – Often releases energy in the form of heat and light – If the combustion is complete, the products will be CO2 and H2O. – If the combustion is incomplete, the products will be CO and H2O. – http://www.youtube.com/watch?v=q6eIqNpdvlw&NR=1 (1:32 min) 23 SUMMARY: an equation... • Describes a reaction • Must be balanced in order to follow the Law of Conservation of Mass • Can only be balanced by changing the coefficients (NEVER SUBSCRIPTS!) • Has special symbols to indicate physical state, if heat or energy is required, etc. • Come in 5 major types. • We can tell what type of reaction is occurring by looking at the reactants. • http://www.youtube.com/watch?v=i-HHvx1VC_8&NR=1 (1:29 min) 24 Identifying Reactions • Synthesis – Reactants will be 2 elements – Product will be 1 compound • Decomposition – Reactant will be 1 compound – Products will be 2 elements • Single replacement – Reactants will be 1 element and 1 compound – Products will be a different element and a different compound (if the reaction occurs) • Double replacement – Reactants will be 2 compounds – Products will be 2 different compounds • Combustion – One of the reactants will be oxygen – Water will always be one of the products – the other product will either be CO or CO2 – http://www.youtube.com/watch?v=tE4668aarck 1:36 min – http://www.youtube.com/watch?v=ul4xRy8hcsQ (5:12 min) 25