Chemical Reactions and Enzymes 2.4 and 2.5

Objectives Describe how bonds break and reform during chemical reactions. Explain why chemical reactions release or absorb energy. Explain the effect of a catalyst on activation energy. Describe how enzymes regulate chemical reactions.

Chemical reactions A chemical reaction is a process that changes substances into different substances by breaking and forming chemical bonds. Hydrogen + oxygen  Water

Parts of a chemical reaction The elements or compounds that are changed during a chemical reaction are known as reactants. They are at the left side of the equations The elements or compounds produced by a chemical reaction are known as products. They are at the right side of the equation Reactants and products are separated by an arrow

Energy is needed All chemical reactions need energy to be completed Energy is needed to break chemical bonds in the reactants, this is called bond energy As new bonds form in the products, energy is released

Activation energy Before you roll a bowling ball, you must put in some energy to start the ball rolling! A chemical reaction must get a boost of energy before the reaction can start Hence, activation energy is the minimum amount of energy required to initiate a chemical reaction.

Exothermic reaction A chemical reaction that releases more energy than it absorbs. The products have a lower bond energy than the reactants. 2Na + Cl2  2NaCl + energy ( energy here)

Endothermic reaction ( energy here) A chemical reaction that absorbs more energy than it releases. The products have a higher bond energy than the reactants. 2H2O + energy  2H2 + O2 ( energy here)

Catalysts Some chemical reactions that make life possible are too slow or have activation energies. These chemical reactions are made possible by catalysts A catalyst is a substance that decreases the activation energy needed to start a chemical reaction and therefore increases the rate of the chemical reaction.

Enzymes Enzymes speed up chemical reactions that take place in cells.

Enzymes are involved in almost every process in organisms. Enzymes are used for breaking down food, are an important part of our immune system, and all enzymes are proteins.

Enzymes can be affected by pH values Changes in temperature A change in temperature or pH changes the shape and function of enzymes thus damaging them.

The enzyme-substrate complex Enzymes provide a site where reactants can be brought together to react, reducing the energy needed for reaction. The reactants of enzyme-catalyzed reactions are known as substrates.