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

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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

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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

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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.

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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

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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!