Post on 22-Dec-2015
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Chemical Reactions
• Fireworks are a result of chemical reactions
Chemical Reactions
• Chemical Reactions form new substances by breaking and making chemical bonds
• Chemical reactions change the way the atoms are arranged
Difference between Physical and Chemical Changes• Physical
• A change in the state
• Solid to liquid• Liquid to gas• Gas to liquid
• Atoms/molecules of the substances do not change
• Chemical• Atoms/molecules
broken down or combined to form new substances
• Properties of new substances are different than properties of the substances that make them up
Reactants and Products
• Reactants: present at the beginning of the reaction
• Products: are the substances formed by the chemical reaction
• Example burning natural gas
• CH4 + 2O2 > CO2 +2H2O• Reactants > Products
Evidence of Chemical Reactions
• Color Change• Formation of Precipitate (solid
formed from two liquids)• Formation of gas• Temperature change:
endothermic (temp down) exothermic (temp up)
• Change in smell
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Classification of Chemical Reactions
• Synthesis: a new compound is formed by the combination of simpler reactants
• Smog formed when nitrogen and oxygen combine
• N2 +2O2 > 2 NO2
oceanworld.tamu.edu
Classification of Chemical Reactions• Decomposition: A
reactant breaks down into simpler products (reverse of synthesis)
• Water can be decomposed into hydrogen and oxygen
• 2 H2O > 2H2 +O2
www.blewbury.co.uk
Classification of Chemical Reactions
• Combustion: One reactant is always oxygen and another reactant often contains carbon and hydrogen
• Burning of methane
• CH4 + 2O2 > CO2 +2H2O
earthguide.ucsd.edu
The Rates of Chemical Reactions can vary
• Concentration: measures the number of particles present in a certain volume.
• A high concentration of reactants means there is a large number of particles that can collide and react
• Turning the valve on a gas stove increases the methane molecules that can combine with oxygen and results in a bigger flame and faster combustion reaction
The Rates of Chemical Reactions can vary
• Surface Area: the exposed surface of a substance.
• To increase reaction break a large piece of the material so that there is more surface area of the material
• Increase surface area=Increase reaction rate
Temperature
• Rate of reaction can be increased by an increase in temperature which increases how fast the particles are moving and increases the number of collisions
• Remember the cold and hot water distribution of food coloring
Catalyst
• Catalyst: is a substance that increases the rate of chemical reaction but is not consumed in the reaction
• Enzymes are catalysts that are used by living things to cause a chemical reaction. Resulting in a new product being made and the catalyst not being changed.
The masses of reactants and products are equal
• Law of conservation of mass: states that in a chemical reaction atoms are neither created nor destroyed. (Antoine Lavoisier)
• All atoms present in the reactants are also present in the products
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Chemical Equations• Reactants > Products• Balancing Chemical
Equations• Use conservation of mass
to balance: atoms can not be created or destroyed.
• Reactants = productsmooni.fccj.org
Chemical equations• CH4 + O2 > CO2 + H2O• Balanced? No• Reactants: C=1, H=4, O=2• Products: C=1, H=2, O=3• H and O not equal
• Use coefficients to balance equations• CH4 + 2O2 > CO2 + 2H2O• C=1, H=4, O=4 > C=1, H= 4, O=4• Equation is now balanced!
Chemical Reactions Involve Energy Changes• It takes energy to break
chemical bonds. This energy is called bond energy.
students.ed.uiuc.edu
Exothermic Reactions• Exothermic
reaction: energy is released and temp goes up…more energy is released when products are formed.
• Space shuttle on take off: white clouds of water vapor are formed
www.sanchezcircuit.com
Exothermic Reactions
• Exothermic reactions release energy!• All common combustion reactions are
exothermic• Fireflies release energy in the form of
light as an exothermic reaction
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Endothermic reactions• Endothermic reactions: produce a
decrease in temperature because the bond energies of the reactants are greater than the bond energies of the products
• All endothermic reactions absorb energy!
• Alka Seltzer and water…temperature goes down.
Endothermic Reactions
• Photosynthesis (general formula)
• 6CO2 +6H2O > C6H12O6 +6O2
• plants absorb energy from sunlight to turn carbon dioxide and water into oxygen and glucose (sugar).
• Energy is stored in the glucose molecule to be used when needed.
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Life and Industry depend on Chemical Reactions
• Living things require chemical reactions• Respiration: living things get energy from glucose by
respiration. (combustion of glucose)
• Photosynthesis• 6CO2 +6H2O + energy> C6H12O6 +6O2
• Respiration• C6H12O6 +6O2 > 6CO2 +6H2O + energy
• Respiration is the opposite of photosynthesis!