Chemical Reactions Ch 9

Chemical reactions Chemical Reaction (RXN): when a substance undergoes chemical changes, forming a new substance Involve a rearrangement of atoms and CANNOT be reversed The new substance will have DIFFERENT chemical and physical properties than the original

Chemical reactions There are some signs that provide clues a chemical reaction is happening Bubbles (formation of gas) Form a solid – precipitate Color change – (bleach on fabric) Change in energy (NRG) – gain or loss

Chemical equations Reactants: starting materials in a reaction Products: ending materials in a reaction The products of a reaction are made up of the same number and type of atoms as were present in the reactants

Chemical Equations Chemical reactions are represented by chemical equations Reactants  Products The arrow stands for yield

Chemical equations Due to the Law of Conservation of Mass, the amount of atoms in the products must be EQUAL to those of the reactants This is achieved by balancing the chemical equation - changing ratios of molecules/atoms so both sides have the lowest possible same whole number and type of atoms

Balancing equations A coefficient is a number that shows the relative amount of a compound in a reaction You CAN change the coefficient in front of any atom/molecule You CANNOT change the subscript (this would change the formula of the molecules)

Balancing equations Count the number of atoms for each reactant Count the number of atoms for each product Use coefficients to balance the equation

C6 H12 O6 + 6O2 6C +  O2 6H2 O Carbon - 6 Carbon - 6 Hydrogen - 12 Hydrogen - 12 Oxygen - 18 Oxygen - 18

_____H2 + _____ O2  ____ H2O ____ H2 + ____ N2  ____ NH3 Let’s Practice! _____H2 + _____ O2  ____ H2O ____ H2 + ____ N2  ____ NH3

Chemical reactions Chemical reactions involve breaking old bonds followed by the making of new ones Breaking bonds REQUIRES energy Forming bonds RELEASES energy Energy stored in bonds = chemical energy Activation Energy: the minimum amount of energy needed to start a chemical reaction

Chemical reactions Exothermic Reactions: reactions that RELEASE energy Energy in the bonds is greater than in the products

Chemical reactions Endothermic Reactions: reactions that ABSORB energy during a reaction More energy needed to break the bonds than is released Example: photosynthesis

Types of reactions Patterns can be used to identify kinds of chemical reactions and to predict the products

Types of reactions Synthesis: 2 or more substances combined to form a new compound A + B  AB Decomposition: substances are broken apart AB  A + B

Types of reactions Single Displacement: elements trade places, usually forms a precipitate AX + B  BX + A Double Displacement: two compounds exchange ions AX + BY  AY + BX

Types of equations Combustion: must contain oxygen in the reaction Water is also common in combustion Redox: transfer of electrons (free radicals) Substances accept e- = reduced Substances give e- = oxidized

Types of reactions S8 + O2  SO2 6CO2 + 6H2O  C6H12O6 + 6O2 KClO3  KCl + O2 Zn + HCl  ZnCl2 + H2

Reaction rates Reaction Rate: how quickly the concentration of products increases, or how quickly the concentration of reactants decrease Reaction rate can be changed by changing the concentration or temperature of the reactants.

Reaction rates Molecules must collide in order to have a reaction Increasing temperature makes molecules move faster, creating more forceful collisions Increasing concentration puts more molecules in a given volume, making collisions more probable

Reaction rates A third way to increase reaction rate is to add a catalyst Catalyst: Any substance that increases the rate of a chemical reaction by lowering its activation energy It does NOT change what is made or how much, just how fast

entropy Entropy: a measure of random activity in a system Energy and temperature are directly proportional If you increase the temperature of a system, entropy increases (particles move faster) If you decrease temperature, entropy decreases (particles slow down)

Entropy As a gas expands in a system, entropy increases. This one is also easy to visualize. If an atom has more space to bounce around, it will bounce more. Gases and plasmas have large amounts of entropy when compared to liquids and solids.

Gas laws Properties of Gases: fluids with particles that move in all directions expand to fill their containers, have low densities, and are compressible Gas Laws describe how the behavior of gases is affected by pressure, volume, and pressure. The laws will help you understand and predict the behavior of gases in specific situations

Boyle’s law For a fixed amount of gas at a constant temperature, the volume of a gas increases as the pressure decreases, and vice versa.

Gay-Lussac’s Law The pressure of a gas increases as the temperature increases, if the volume of the gas does not change.

Charles’s law For a fixed amount of gas at a constant pressure, the volume of the gas increases as the gas’s temperature increases.