4. Chemical Reactions

Bonds break and form during chemical reactions

• Chemical reactions: reactions that change substances into substances by breaking and forming chemical bonds.

Reactants, Products, and Bond Energy

• Reactants: The substances that changed during a chemical reaction.

• Products: The substances made by a chemical reaction.

• Chemical equations show what happens during a reaction. The left has the reactants and the right has the products. The arrow shows the direction of the equation.

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

  • Bonds between different atoms have different bond energies.

• The energy released when a bond forms is the amount of energy that will break the bond.

Chemical Equilibrium

• Chemical equilibrium: When a reaction takes place at an equal rate in both directions. (reversible chemical reactions).

  • Equilibrium is reached when both the reactants and products are made at the same rate.

Chemical reactions release or absorb energy.

• All chemical reactions have changes in energy.

  • If energy is added to reactants, chemical bonds are broken.

  • Energy is released when new bonds form in the products.

• Activation energy: the amount of energy that needs to be absorbed for a chemical reaction to start.

  • Activation energy is absorbed by reactants.

• Exothermic chemical reaction: Reaction that releases more energy than it absorbs.

  • Energy that is released is given off as heat or light.

  • Think exo → out

• Endothermic chemical reaction: Reaction that absorbs more energy than it releases.

  • Energy that is absorbed can come from energy sources like the sun.

  • Think endo → in

5. Enzymes

A catalyst lowers activation energy

• Catalyst: a substance that decreases the activation energy needed to start a chemical reaction.

  • A catalyst will increase the speed of a chemical reaction as it gives it a “push”.

Enzymes allow chemical reactions to occur under tightly controlled conditions.

• Chemical reactions in organisms usually need catalysts.

• Enzymes: catalysts for chemical reactions in living things

  • Lower the activation energy and speed up chemical reaction.

  • Needed in most processes of organisms.

• Most enzymes are proteins.

  • Changes in temperature or pH can change an enzyme’s structure and affect how an enzyme functions.

• Substrates: The only reactants that a certain enzyme will act on.

  • Enzymes will bring the substrates together and act as a catalyst so the product can be formed easier.