Chemical Equations and Changes

Unequal Sharing (Polar Covalent Bond) Why do you think the two Hydrogen atoms share equally, but the Hydrogen and fluorine do not? Bonded hydrogen atoms showing equal sharing of electrons Hydrogen and fluorine bond with an unequal sharing of electrons The unequal sharing of electrons between two atoms that gives rise to negative and positive regions of electric charge Results from an atom’s electronegativity – the ability to attract electrons to itself Electron Cloud

Physical Change a change in shape, size, color, or state a change without a change in chemical composition a change that is reversible Examples tearing paper cutting your hair change in state Physical changes do not indicate a chemical reaction has taken place

Chemical Change a change in which a substance becomes another substance having different properties a change that is not reversible using ordinary physical means Changes that usually cause, heat, sound, light, odor, fizzing/foaming, color changes You usually need more than one of the above characteristics to be considered a chemical change! Examples mixing vinegar & baking soda burning a piece of wood soured milk A chemical analysis is the only 100% way to know a chemical change has occurred.

Chemical Equation Shorthand form for writing what reactants are used and what products are formed in a chemical reaction Sometimes shows whether energy is produced or absorbed Examples: 2H 2 + O 2  2H 2 O CH 4 + 2O 2  CO 2 + 2H 2 O C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + energy

Components of a Chemical Equation 2H 2 + O 2 2H 2 O  (Reactants) (Products) (Yield) Chemical Formulas Chemical Formula Subscripts Coefficient Subscript

Energy and Chemical Reactions Exothermic Reaction A chemical reaction in which energy is released. The products have greater bond energy than the reactants C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + energy (respiration) Endothermic Reaction A chemical reaction in which energy is absorbed. The products have lower bond energies than the reactants 6CO 2 + 6H 2 O + energy  C 6 H 12 O 6 + 6O 2 (photosynthesis) Can you think of other reactions where energy is gained or released?

Rates of Chemical Reactions Temperature – a measure of the average kinetic energy of the particles in a sample of matter – Ex. Increasing the temperature when cooking Surface area – amount of material that comes into contact with the reactants – Ex. Cutting a potato into smaller pieces when cooking Concentration – amount of substance per volume – Ex. Turning the valve on a gas stove to increase the concentration of methane molecules Catalysts (enzymes) – organic substances that help speed up chemical reactions, but are not consumed in the reaction The rates at which chemical reactions can take place are based on the interaction (collisions) between the different particles. These rates can be impacted by the following:

Law of Conservation of Mass In a chemical reaction, atoms are neither created nor destroyed All atoms present in the reactants are also present in the products Chemical equations must account for/show the conservation of mass  balancing equations H 2 + O 2 H 2 O  In its present form, does this chemical equation show a conservation of mass? How would you balance this equation to show the conservation of mass? 22 Reactants H  2 O  2 Products 4242 H  2 O 

Hints For Balancing Equations Count the atoms – List the number of atoms of each element to see which elements must be balanced Use a coefficient to add atoms to one side of the equation – Start with the reactant or product that has the greatest number of different elements Add a coefficient to another reactant or product – Make sure that the coefficients in your balanced equation are the smallest whole numbers possible (they should have no common factor other than one) Tutorial on Balancing Equations