Spontaneity, Entropy/Enthalpy and Rates of Reactions

Endothermic:

• For an endothermic reaction, the system absorbs heat from the surroundings, surroundings get colder
• Thus the system gains energy
• This is an unfavorable change in energy because it is harder to do because it needs or takes in energy (loses entropy)
• Bonds break because energy must be absorbed to break attraction (must pull apart)
• Reactants absorb heat from surroundings when they react
• Reaction converts the absorbed heat into potential energy when products are formed
• H is positive because potential energy aka enthalpy increases when reactants convert into products (uphill)
• Products are less stable than reactants because the bonds within the product molecules are weaker than the bonds within the reactant molcules
• Energy added to reactants

Exothermic:

• For an exothermic reaction, heat is released, surroundings get hotter

• Thus the system loses energy

• This is a favorable change in energy

• Makes bonds because it enables attraction which is favored

• Reactants lose potential energy, lost potential energy is converted into heat which is released

• H is negative because potential energy decreases when reactants convert into products (downhill)

• Products are more stable than reactants because the bonds within the product molecules are stronger than the bonds within the reactant molecules

• Energy is added to products

  

To figure out whether a reaction is exothermic or endothermic:

1. Find the bond energy of bonds that are broken (bond energy is positive)
2. Find bond energy of bonds that are made (bond energy is negative)
3. Add both together
4. If positive number, then endothermic, if negative then exothermic

To find heat released/absorbed:

1. Find the moles of energy
2. Use mole to kj ratio to find moles of energy

Entropy (S):

• High entropy = lot of disorder/more molecules
• Increasing entropy is solid to liquid, liquid to gas, solid to gas
• Reactions prefer to proceed towards more energy and more entropy
• Favored change is increasing entropy or greater disorder (+S)
• Entropy increases in changes of state, dissolving solids into solution (solid -> aqueous solution), adding heat (increases kinetic, vibration, the rotational energy of molecules), and increases moles of gas in the reaction
• Find entropy by looking at changes in state (if same state look at moles of gas), look for dissolving solutions, or added heat to reactants

Enthalpy (H):

• The sum of the internal energy and pressure times volume
• We cannot measure the enthalpy of a system, but we can look at changes in enthalpy
• Change in potential energy from reactants to products
• Its is equivalent to the amount of heat released or absorbed by a system
• Favored change is an exothermic (-H) reaction because it loses the potential energy and bonds get more stable so you end up with stronger bonds
• Find enthalpy by where the reaction is endo or exo

Spontaneity:

• A process that occurs under a given set of conditions without outside intervention
• It is determined by enthalpy and entropy
• All spontaneous reactions increase the entropy of the universe
• Gibbs free energy (G) is another word for spontaneity
• Cases:

Gibbs free energy G=H-TS:

• Always want G to be negative so that the reaction is spontaneous

To find if the reaction is spontaneous:

1. Plug in H, T and S
2. Make sure all units are the same
3. If number is positive, not spontaneous, if number is negative, spontaneous

To find if reaction is spontaneous at lower or higher temperatures:

1. Write G=H-TS with positives and negatives
2. If a lower T is needed for G to be negative then lower
3. If higher T is needed for G to be negative than higher

To find what temp the reaction is spontaneous:

1. Set G to 0 (where is switches from +G to -G
2. Solve for t, T= H/S
3. Plug in H and S
4. Convert T to celsius
5. Depending on higher or lower temps, above or below that temp

Activation Energy/Collision Theory:

• Rates of reaction - how fast will a reaction occur, depends on the activation energy

• Activation energy is the amount of energy needed to start a reaction

• Lower temp means higher activiation energy

• The amount of energy needed to form the activated complex (go from reactants to activated complex)

• The activated complex is the high energy state that forms when transitioning from reactants to products

• Bonds are partially broken and partially made so very unstable

• Collision Theory is in order for a reaction to occur molecules must collide with enough energy to form the activated complex and collide with the correct orientation

• If you have higher activation energy, fewer molecules have enough energy to react

• If you have lower activation energy, more molecules react and a faster reaction

• Some reactions are more difficult to set correct orientation

  

  

Rate of Reactions:

To increase the rate of reaction:

• Increase the concentration of a reactant

• Molecules are more tightly packed

• Molecules collide more frequently

• Greater number of collisions will be effective because more frequent collisions

• Increase the temperature

• Molecules move faster on average so molecules collide more frequently and more forcefully

• Higher proportaion of molecules have enough energy to react and have more collisions

• The reaction is more likely to occur and occur faster

• Increase the surface area of the reactant

• When a reactant has a smaller size there is greated exposed surface area

• Thus molecules collide more frequently

• Dissolved solids react more quickly than undissolved solids

• Fine powders can react very quickly so they can be very dangerous if combustible

• Use a catalyst

• A catalyst is a chemical which speeds up a reaction without being consumed

• The overall reaction with a catalyst is the same but the steps of the reaction are different

• The catalyst allows for easier steps in the reaction so the overall activation energy is lower

• More molecules = more collisions = less energy needed to get to transitionaing state = lower activation energy

• Changes path of reaction

• Reaction happens faster because less energy is needed