Chemical Reactions

Chemical Reactions Chemical reaction – a process that changes one set of chemicals into another set of chemicals; involves the breaking and reforming of chemical bonds. Reactants - chemicals that undergo a change (left side of equation). Products - chemicals that are the result of a change (right side of equation). A + B ---------> C + D

Energy in Chemical Reactions Energy is stored within chemical bonds. When bonds are broken, energy is released. All living organisms must have a source of energy to carry out chemical reactions! Two types or reactions deal with the energy stored in chemical bonds: Endergonic reactions Exergonic reactions

Endergonic Reactions Endergonic reactions – reactions that absorb energy. Need a source of energy (usu. electricity) to trigger the reaction (don’t occur spontaneously). Reactions tend to feel cold. Ex: electroplating, aluminum metal obtained from its ore When the energy absorbed is thermal we call it an endothermic reaction.

Exergonic Reactions Exergonic reactions – reactions that release energy. Energy is released as heat, light, or gas. Can occur spontaneously. Often feel warm. Ex: glowsticks, heat packs If energy given off is thermal- we call it an exothermic reaction

5 Types of Reactions 1. Combustion A substance reacts with oxygen to produce energy in the form of heat and light. Burning Ex: C + O2  CO2

2. Synthesis Reaction Two or more substances combine to form another substance Follows this form: A + B  AB Ex: H2 + O2  H2O Hydrogen burns in oxygen to form water

3. Decomposition Reaction Reverse of syntheis: one substance breaks down into two or more substances Most require the use of heat, light, or electricity Follows this form: AB  A + B Ex: H20  H2 + O2 An electric current passed through water produces hydrogen and oxygen

4. Single Displacement One element replaces another element in a compound Follows this form: A + BC  AC + B Ex: Cu + AgNO3  Cu(NO3) + Ag Copper wire is put into a solution of silver nitrate. Copper is a more active metal than silver, it repaces the silver, forming a blue copper (II) nitrate solution. Silver is not soluble, and forms on the wire.

5. Double Displacement The positive ion of one compound replaces the positive ion of the other to form two new compounds. Takes place if a precipitate, water, or a gas forms when two ionic compounds in solution are combined. Precipitate- an insoluble compound that comes out of solution Follows this form: AB + CD  AD + CB Ex: Ba(NO3) + K2SO4  BaSO4 + KNO3 Reaction of barium nitrate with potassium sulfate – a precipitate (barium sulfate) forms.

Oxidation-Reduction Reactions (Redox Reactions) Oxidation- the loss of electrons Reduction- the gain of electrons Often results in corrosion of metal (rusting)