Chemical Reactions

 Atoms interact in chemical reactions: Chemical reaction: produces new substances by changing the way in which atoms are arranged. Bonds between atoms are broken New bonds are formed

 Physical changes: Change in the state of a substance Physical changes only **substance is the same**  Chemical changes: Changes in the chemical make up of the substance Creates a new substance

 Reactants: The substances present at the beginning of a chemical reaction Bonds between the reactants are broken during a chemical reaction  Products: The substances formed by a chemical reaction New bonds are formed  CH 4 + O 2 CO 2 + H 2 O productsreactants

 Color change  Formation of Precipitate Precipitate: a solid product formed when chemicals in two liquids react  Formation of a Gas  Temperature Change Flame from burning May need thermometer to notice

 Synthesis reaction: Forms a new compound by the combination of simpler reactants N NO 2  Decomposition reaction: A reactant breaks down into simpler products such as elements or compounds Like the reverse of a synthesis reaction 2H 2 0 2H 2 + O 2

 Combustion reaction: One reactant is always oxygen and another reactant usually contains carbon and hydrogen CH 4 + 2O 2 CO 2 + 2H 2 0 Methane + Oxygen Carbon dioxide + water The burning of methane is a combustion reaction

 Concentration: Measurement of the number of particles present in a certain volume The higher the concentration, the larger amount of particles available to react  Surface Area: The area available for reactants to produce a chemical reaction. Breaking apart a large area into smaller areas increases the overall surface area of the object Increase in surface area = increase in the rate of the reaction

 Temperature: By increasing temperature, you are adding energy to the reactants. The increase in energy increases the rate of the reaction  Catalyst: A substance that increases the rate of a chemical reaction but is not itself consumed in the reaction After the reaction, the catalyst remains unchanged Many reactions in your body would not take place without enzymes Example: enzymes

 Law of conservation of mass: In a chemical reaction atoms are neither created nor destroyed. All atoms present in the reactants are also present in the products

 To write a chemical equation you need to know: The reactants and the products in the reaction The atomic symbols and chemical formulas of the reactants and products in the reaction The direction of the reaction  The indicates the direction of the reaction

 Chemical equations must follow the law of conservation of mass so the same number of atoms must be present on both sides of the equation.  You can balance a chemical equation by changing the amounts of reactants or products represented.

 Coefficients: The number in front of the chemical formula Indicate how many molecules take part in the reaction  If there is no coefficient then only one molecule of that type takes place Only the coefficients can be changed in order to balance a chemical equation.  Look at page 211 for step by step instructions

 A balanced chemical equation shows that no matter how atoms are rearranged during a chemical reaction, the same number of atoms must be present before and after the reaction.  2NaN 3 2Na + 3N 2  2 molecules of sodium azide yields 2 molecules of sodium and 3 molecules of nitrogen when it goes through a decomposition reaction.

 Chemical reactions release or absorb energy bond energy: the energy associated with bonds (breaking bonds requires energy, forming bonds releases energy) Reactant molecules require energy to break the bonds Energy is released when bonds are formed in product molecules

 Exothermic reactions: A reaction in which energy is released More energy is released during the formation of the products than is needed to break the bonds in the reactants releasing energy during the reaction.  Endothermic reactions: A reaction in which energy is absorbed More energy is required to break the bonds in the reactants than is released when the products form so energy must be added to the reaction.

 Often produce an increase in temperature  The bond energies of the reactants are less than the bond energies of the products which leads to a release in heat  2Al + Fe 2 O 3 Al 2 O 3 +2Fe  Enough heat is generated by this reaction to melt the iron that is produced.

 All common combustion reactions are exothermic  Some chemical reactions release energy as light (glow sticks)  Exothermic chemical reactions occur in living things as heat or light Fireflies

 All endothermic reactions absorb energy  Often produce a decrease in temperature  The bond energies of the reactants are greater than the bond energies of the products  Energy is absorbed from the surroundings as heat = decrease in temperature

 Photosynthesis: Process by which plants absorb energy from sunlight to turn carbon dioxide and water into oxygen and glucose. The energy is stored in the glucose molecules 6CO 2 + 6H 2 0 C 6 H 12 O 6 + 6O 2

 In an exothermic reaction (releases energy), energy is found on the product side of the reaction Reactants Products + Energy  In an endothermic reaction (absorbs energy), energy is found on the reactant side of the reaction Reactant + Energy Products

 Living things require chemical reactions: Photosynthesis Respiration: the “combustion” of glucose to obtain energy Energy released by respiration is used for new cell growth, movement, or any other life function  Catalysts (enzymes) are used to speed up the process