Chemical Reactions Chemical Reactions A Chemical reaction a

Chemical Reactions

Chemical Reactions A 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). + 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 + O 2 CO 2

2. Synthesis Reaction ◦ Two or more substances combine to form another substance ◦ Follows this form: A + B AB ◦ Ex: H 2 + O 2 H 2 O 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: H 20 H 2 + O 2 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 + Ag. NO 3 Cu(NO 3) + 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(NO 3) + K 2 SO 4 Ba. SO 4 + KNO 3 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)