3.2 Flashcards
Enthalpy = ?
Measure of heat energy in a chemical system
🔼H = H(products) - H(reactants)
Energy is conserved
Exothermic
🔼H = NEGATIVE (exo)
Heat to surroundings
Temp of surroundings increases
Endothermic
🔼H = POSITIVE (endo)
Heat from surroundings
Temp of surroundings decreases
0 degrees = how many K?
0 degrees = 273 K
Ideal gas equation
q=mc🔼T
could be inaccurate due to heat loss to surroundings, incomplete reaction/combustion, or not standard conditions
Activation energy = ?
The minimum energy required for a reaction to take place
Ea
Standard conditions = ?
25 degrees C
100kPa (1atmosphere)
1moldm-3
Average bond enthalpy = ?
The energy required to break 1 mole of bonds in a gaseous molecule
- energy required to break bonds
- bond enthalpies always ENDOTHERMIC = 🔼H+ve
Bond breaking
Endothermic
🔼H+ve
Energy required to break bonds is greater that the energy released making them
Bond forming
Exothermic
🔼H-ve
Energy released making bonds is greater than energy required breaking them
Hess’ law = ?
States that enthalpy changes and be determined indirectly
Route1=Route2
A+B=C
Rate of reaction = ?
Rate = change in concentration ➗ time
Affected by: • concentration/pressure • temperature • SA of solid reactants • presence of a catalyst
Ineffective / effective collisions
Ineffective: No reactions (right particles didn’t collide)
Effective:
Reaction (right particles collide)
How to follow the progress of a reaction
Monitor removal of reactant
Follow formation of the product
Gas: volume produces, or loss of mass of reactants
definition of a CATALYST
A substance that changed the rate of a chemical reaction without undergoing any permanent change itself
- not used up
- provides surface for reaction
- regenerated at the end
Provides an alternative route for a reaction with a LOWER ACTIVATION ENERGY
Homogenous catalyst = ?
SAME physical state as reactants
Heterogenous catalyst = ?
DIFF physical state from the reactants (absorption and desorption)
The Boltzmann distribution
After Ea = small proportion of molecules have more than the Ea which is enough energy to react
- no molecules have 0 energy
- area under curve = total no. molecules
- no maximum energy of a molecule
Boltzmann distribution; increasing temperature
- lower peak
* more molecules have the Ea (higher rate of reaction)
Boltzmann distribution; addition of a catalyst
- same curve as normal
- lower Ea
- more molecules have a new lower Ea
Dynamic equilibrium
Rate of forward = rate of backward
Dynamic = both taking place
Closed system = isolated from surroundings
Le Chatelier’s principal
States that if a equilibrium system is subjected to external change, the system readjusts itself to minimise the effect of said change
Equilibrium; changing the temp
Increasing temp:
Will change to favour the ENDO reaction (🔼H+ve)
Decreasing temp:
Will change to favour EXO reaction (🔼H-ve)
Equilibrium constant Kc = ?
Kc =
[C]c [D]d (products)
➗
[A]a [B]b (reactants)
When
aA + bB <> cC + dD
Kc values
Greater than 1 = positioned towards the products
Less than 1 = positioned towards the reactants
