Chapter 7 Chemical Reactions

Transcription

1 Chapter 7 Chemical Reactions Chemical Equation --> is a representation of a chemical reaction in which the reactants and products are expressed as formulas Reactants --> substances that undergo change Products --> new substance formed as a result of that change Reactants Product 1

2 Word Equation = Carbon + Oxygen Carbon Dioxide Chemical Equation = C + O 2 CO 2 Arrow ( ) means reacts to form or yields 2

3 Law of Conservation of Mass - states that mass is neither created nor destroyed in a chemical reaction Balancing Equations In order to show that mass is conserved during a reaction, a chemical equation must be balanced Coefficients - numbers that appear before the formula use these to balance a chemical equation in an unbalanced equation the coefficients are understood to be 1 as you balance equations, you should never change the subscripts a formula 3

4 How to Balance 1. count the number of atoms of each element on each side of the equation N2H4 + O2 N2 + H2O Reactants Products N = N = H = H = O = O = 4

5 2. Change one or more coefficients until the equation is balanced N2H4 + O2 N2 + H2O Reactants Products N = N = H = H = O = O = 5

6 7.2 -Classifying Reactions 1. Synthesis is a reaction in which 2 or more substances react to form a single substance reactants - elements or compounds products - always compounds A + B AB 2Na + Cl 2 2 NaCl 6

7 2. Decomposition reaction in which a compound breaks down into 2 or more simpler substances reactants - compounds products - elements or compounds AB A + B CaCO 3 CaO + CO 2 7

8 3. Single Replacement reaction in which one element takes the place of another element in a compound reactants - element + compound product - element + compound A + BC B + AC Cu + 2AgNO 3 2Ag + Cu(NO 3 ) 2 8

9 4. Double Replacement - 2 different compounds exchange positive ions (cations) and form 2 new compounds reactants - 2 compounds products - 2 compounds AB + CD AD + CB Pb(NO 3 ) 2 + 2KI PbI 2 + 2KNO 3 9

10 5. Combustion substance reacts rapidly with oxygen, often producing heat and light CH 4 + 2O 2 CO 2 + 2H 2 O some combustion reactions are also synthesis reactions EX: 2H 2 + O 2 2H 2 O 10

11 7.3 - Chemical Bonds and Energy Chemical Energy --> is the energy stored in the chemical bonds of a substance Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products Breaking Bonds - requires energy Forming Bonds - releases energy 11

12 Exothermic Reactions - a chemical reaction that releases energy energy released as the products form is greater than the energy required to break the bonds in the reactants 12

13 Endothermic Reactions - a chemical reaction that absorbs energy from its surroundings more energy is required to break the bonds in the reactants than is released by the formation of the products 13

14 Conservation of Energy --> the total amount of energy before the reaction equals the amount of energy after the reaction Reaction Rates --> the rate at which reactants change into products over time (Tell you how fast a reaction is going!) 14

15 7.4 - Reaction Rates Factors Affecting Reaction Rates chemical reactions occur because reactant particles collide increase the collisions, increase reaction rate decrease the collisions, decrease reaction rate 15

16 Factors Affecting Reaction Rates 1. Temperature increase the temperature of a substance causes its particle to move faster and are more likely to collide and react 2. Surface Area increase surface area increases the exposure of reactant particles the greater the exposure the more collisions and more reactions will occur 16

17 3. Stirring increase the exposure of reactants, which increases the collision and increases the reaction rate 4. Concentration increase the number of reacting particles that are present increases the number of collisions, and increases the reaction rate for gases, concentration changes with pressure. The greater the pressure the greater the concentration and the faster the reaction rate 17

18 5. Catalysts is a substance that affects the reaction rate without being used up in the reaction a catalyst lowers the amount of energy required for an effective collision between reacting particles (one way to do this is by providing a surface on which the reacting particles come together) 18

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