CHEM&121 Exam 3: Chapter 6
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Units of Energy
- Calories (cal) and Joules (J)
- 1 cal = 4.184 J
- 1 kcal = 1000 cal
- 1 kJ = 1000 J
- 1 kcal = 4.184 kJ
Potential and Kinetic Energy
- Potential: energy that is STORED (no movement)/a compound with LOWER potential energy is more stable than a compound with HIGHER energy.
- Kinetic: energy of MOTION
Bond Breaking and Forming
- Breaking a bond always REQUIRES an input of energy, which means the reaction is endothermic.
- Forming a bond always RELEASES energy, which means the reaction is exothermic.
- Energy is ABSORBED
- The heat of reaction (or enthalpy change, ΔH) is positive + (i.e. +58 kcal/mole)
- The bonds broken in the reactants are stronger than the bonds formed in the products.
- The products are higher in energy than the reactants.
- Energy is RELEASED
- The heat of reaction (or enthalpy change, ΔH) is negative - (i.e. -58 kcal/mole)
- The bonds formed in the products are stronger than the bonds formed in the reactants.
- The products are lower in energy than the reactants.
- Activation Energy Ea: the difference between the reactants and the transition state, and the minimum amount of energy for the reaction to occur. If high, the reaction is slow beccause few molecules have enough energy to cross the energy barrier. If low, the reaction is fast and many molecules can cross. Determines rate of reaction.
- Enthalpy of Reaction ΔH: Determines whether products or reactants are lower in energy. Reactions are favored when negative and products are lower in energy, making them more stable than the reactants.
- In this diagram, the products are lower in energy which makes the reaction NEGATIVE and EXOTHERMIC.
Concentration and Temperature Affect on Reaction Rate
- INCREASING the concentration of the reactants INCREASES the number of collisions, so the reaction rate increases.
- INCREASING temperature INCREASES the reaction rate (increases # of collisions and the average kinetic energy).
- A catalyst is something that speeds up the rate of reaction.
- It does not change the reactant or product,
- Catalyzed Reaction has a LOWER Ea than the uncatalized which makes the reaction FASTER.
- Equilibrium: Rate of forward and reverse reaction are equal, which means the concentration of all species do not change.
- This means the reaction is reversible (it can go from reactants to products, or products to reactants).
The Equilibrium Constant
- The equilibrium constant is known as K (the number of moles in a given volume).
- K>1 when the products are LARGER than the reactants, and equilibrium goes to the right.
- K<1 when the reactants are LARGER than the products, and equilibrium favors to the left.
- K=1 Both the reactants and products are at equilibrium.
- The products of a reaction are favored when K>1, and enthalpy is negative. Equilibrium favors products when they're lower in energy than reactants.
Le Chatelier's Principle
- If a chemical system at equilibrium is disturbed or stressed, the system will react in the direction that counteracts the disturbance or relieves the stress.
- Concentration Changes
- Adding reactant → Equilibrium goes right: products favored
- Adding product ← Equilibrium goes left: reactants favored
- Removing product → Equilibrium goes right: products favored
- Temperature Changes
- When temp is increased, the reaction that removes heat is favored (i.e. increased heat reactants → products favored/endothermic)
- When temp is decreased, the reaction that adds heat is favored (i.e. increased heat to products, reverse reaction ← reactants favored/exothermic)
- Pressure Changes
- Increasing pressure favors the side with fewer moles.
- Decreasing pressure favors the side that has more moles.
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