Chemical Reactions

Chapter 2: Section 2.4

Objectives

• SWBAT describe how bonds break and reform during chemical reactions.

• SWBAT explain why chemical reactions release or absorb energy.

Life runs on chemical reactions

• Think of chemical reactions we have talked about in class or about which you know. You can discuss this with your tablemate.

• Once you have come up with a chemical reaction “you like,” jot down what you know about it (or them). What are the chemicals or compounds and what is the product of the reaction?

Fundamentals of Chemical Reactions

• Chemical reactions change substances into different ones by breaking and forming chemical bonds.

• Examples of reactions we have talked about: plant cells making cellulose by linking simple sugars (glucose) together; plant and animal cells break down sugars to get usable energy; all cells bond amino acids together to get protein molecules.

Reactants and Products

• Reactants are changed during the chemical reaction.– Your respiration has as two of its reactants glucose

and oxygen.

• And products are made by the chemical reaction.– In this case the glucose and oxygen chemical

reaction produces carbon dioxide, water and useable energy (ATP).

Fundamentals of Chemical Reactions

• A reaction is at chemical equilibrium when reactants and products form at the same rate.

• Do you remember Carbonic Acid? Ocean acidification?– When we add more carbon dioxide to the

equation, we increase the availability of one of the reactants. If it is the limiting reactant (as CO2 is) then more product will result (in this case carbonic acid) until equilibrium is re-established.

CO2 + H2O H2CO3What is this reaction?

Bond Energy

• Bond energy is the amount of energy that will break a bond between two atoms.

• Bonds between different types of atoms have different bond energies.– Energy is added to break bonds.– Energy is released when bonds form.

• So, the energy needed to break the bonds in an oxygen molecule is less than those needed to break bonds in a glucose molecule.

Example: burning methane

• CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[l] + 891 kJ• This is the formula that characterizes the

burning of methane (natural gas). • A spark (energy added) breaks the oxygen’s

and methane’s bonds. The result is the creation of carbon dioxide, water, and a large amount of heat.

Bond Energy

• Bond energies differ greatly.• For our purposes, it is necessary to realize that

of the two types of bonding we have talked about (covalent and hydrogen), covalent bonds have higher bond energies.– They share electrons and it takes more energy to

break them apart.

Chemical Reactions and Energy• Chemical reactions can release or absorb energy.• Activation energy is the amount of energy that

needs to be absorbed to start a chemical reaction.

Exothermic• Exothermic reactions release more energy

than they absorb.– Reactants have higher bond energies than products.– Excess energy is released by the reaction.

http://www.youtube.com/watch?v=iP6CRZdDu6o

Endothermic• Endothermic reactions absorb more energy

than they release.– Reactants have lower bond energies than products.– Energy is absorbed by the reaction to make up the

difference.

http://www.youtube.com/watch?v=GQkJI-Nq3Os

Entrance Ticket

• What is a chemical reaction (give an example with the definition)?

• What is an endothermic reaction? What is an exothermic reaction? How do they differ?