mod 4 Flashcards
Energy is absorbed to ___
Energy is released to ___
break bonds (endothermic)
create bonds (exothermic)
What is dissolution?
when a solute dissolves in a solvent to form a solution using 3 processes
1. solute-solute interactions are broken (endo needs energy)
2. solvent-solvent interactions are broken (endo needs energy)
3. solute-solvent interactions are formed (exo releases energy)
What do solute-solvent interactions do?
called salvation or hydration when the water/solvent creates concentric rings/hydration spheres around ions and ion-dipole forces bond them together
What is a salt that endothermically dissolves?
NaOH
What is a salt that exothermically dissolves?
KCl
What is the molar heat of a solution?
heat released or absorbed upon the dissolution of a substance measured in J or kJ per mole
molar heat in endo = q/n
molar heat in exo = -q/n
(q = energy/heat released or absorbed in J or kJ)
(n = moles of substance)
What is enthalpy (H)?
is the amount of stored heat within a substance. only CHANGE of enthalpy can be measured using
change in enthalpy = sum of change in enthalpy of products - sum of change in enthalpy of reactants
What are the two types of enthalpies?
exothermic means change in H < 0
endothermic means change in H > 0
Manipulating enthalpy equations:
if you multiply or divide or change ratios of the coeff’s in a chemical equation, you must do the same to the enthalpy
How can heat released be measured in a calorimetry experiment?
q = m x c x change in T
(q = energy released in J)
(m = mass of solvent/water in g) (can use density and mL to get mass)
(c = specific heat capacity, e.g in water it is 4.18g/K)
(T = temperature in kelvin)
What factors affect calorimetry experiments?
some heat can be lost via conduction
we assume reactions are complete combustions
What is Hess’s Law?
he states that the enthalpy change accompanying a chemical change is independent of the route by which it occurs
change in H = change in H1 + change in H2 + change in H3
What is enthalpy of formation?
is the change of the formation of product (coeff should be 1) in its STP
Which elements have 0kJ at STP?
H2
N2
F2
O2
I2
Cl2
Br2
How can you manipulate thermochemical equations?
- multiply equations = multiply change in H
- reverse equations = flip change in H sign
- add equations = add change in H
What are the two types of catalysts?
Homogenous and Heterogenous
What are homogenous catalysts?
are in the same phase/state as reactants (in l, aq or g)
1. the catalyst reactions with one or more of the reactants to create a temporary intermediate
2. the intermediate reacts with the other reactants
3. it then decomposes to form the products and regenerates the catalyst (NOT consumed)
- If a substance is on both reactant and product side, it is a catalyst
- if a substance is formed and then gone it is an intermediate
What is a heterogenous catalyst?
are in different phases/states to the reactants (in s, l, aq or g)
1. reactants absorb/stick to the catalyst at a specific active site which causes their reactions to speed up (happens on the catalyst surface)
2. the products then desorb
- an intermediate may or may not be created
What is bond enthalpy?
refers to the amount of energy needed to break 1 mole of a certain type of bond into its constituent gaseous atoms (endo)
- higher the bond energy, the more bonds needed to be broken
- there is a table of bond energy
How to calculate bond enthalpy?
- write the chemical equation
- draw table
- (bond-breaking and bond forming on y-axis)
- (no/type of bonds and energy on x-axis) - change in bond enthalpy = change in bond breaking + change in bond forming
What is the Haber process?
process that involves the production of ammonia from its constituent gas elements
1/2N2(g)+3/2H2(g) == NH3(g)
- heterogenous Fe3O4(s) is a catalyst
What is entropy?
the amount of randomness of disorder in a substance
- absolute entropy can be measured
- change in entropy = sum of products entropy - sum of reactant entropy
What is the standard molar entropy? (S)
the entropy of one mole of a substance at STP in unit J
What is the second law of thermodynamics?
states that a SPONTANEOUS reaction will occur naturally without an ongoing supply of heat
Entropy trends:
- increase in MM = increase in entropy
- entropy of (g)>(l)>(aq)>(s)
- increase entropy with increase in chemical complexity
Entropy trends:
- increase in MM = increase in entropy
- entropy of (g)>(l)>(aq)>(s)
- increase entropy with increase in chemical complexity
What are the drivers of reaction?
- entropy drive (forward = S>0, reverse = S<0)
- enthalpy drive (forward = H<0, reverse = H>0)
What is Gibbs free energy?
the change in gibbs free energy (change in G) is the maximum amount of work that can be performed during a chemical reaction (in J)
- only change can be measured
- if G<0 spontaneous
- if G>0 non-spontaneous
- if G=0 equilibrium
How to measure change in G?
- change in G under STP = (change in H) - T(change in S)
- change in G = (change in H) - (change in S)
- change in G = (sum of change in products G) - (sum of change in reactants G)
Low activation energy required =
spontaneous reaction
What is a state function?
the values which depend on the state of the substance, like temperature, pressure or the amount or type of the substance (e.g Hess’ law says enthalpy is independent of the pathway)
What is a path function?
properties or quantities whose values depend on the transition of a system from the initial state to the final state (pathway it takes) (e.g heat and work)
Photosynthesis:
Respiration:
6CO2 + 6H2O → C6H12O6 + 6O2 (endo)
C6H12O6 + 6O2 → 6CO2 + 6H2O (exo)
What is the first law of thermodynamics?
energy cannot be created or destroyed; it can only be converted from one form to another.
When does a reaction occur (entropy)?
change is system entropy + change in surroundings entropy = change in universe entropy >0
- reaction only occurs when change in universe entropy >0
What affects entropy?
increased molecular weight, temperature, complexity
decreased pressure/concentration
