Chapter 9-10 - Enthalpy&Reaction rates and equalibrium Flashcards
Enthalpy Change (ΔH) =
{when using hess’ law for ΔHf} ONLY
∑(Bond enthalpies of products) - ∑(Bond enthalpies of reactants)
Heat energy (q)
q = mcΔT
[q in J] [m in g] [c for water=4.18] [change in temp so C or K]
Define standard enthalpy change of formation (ΔfHo)
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
Define standard enthalpy change of combustion (ΔcHo)
The enthalpy change when 1 mole of a substance is burned completely in oxygen, under standard conditions.
Define standard enthalpy change of neutralisation (ΔneutHo)
The enthalpy change when an acid reacts with an alkali to form 1 mole of water, under standard conditions.
Standard conditions:
pressure of 100kPa & temp. of 298K
Define Average Bond Enthalpy:
The energy required to break 1 mole of a specified type of bond in a gaseous molecule, averaged over many different compounds.
Exothermic reactions
- Making bonds
- giving out energy
- ΔH is negative
Endothermic reactions
- Breaking bonds
- taking in energy
- ΔH is positive
Define Activation Energy
The minimum energy required for a reaction to occur
Hess’ Law
Total ΔH of a reaction is always the same regardless of the route taken
How does temperature affect the rate of reaction?
Increasing temp. [in terms of collision frequency]
- particles move faster…
- ∴ collide more frequently…
- ∴ more successful collisions / second…
- ∴ rate of reaction increases.
Increasing temp. [in terms of activation energy]
- particles have more energy…
- ∴ more particles exceed the activation energy…
- ∴ a greater proportion of the collisions are successful…
- ∴ rate of reaction increases.
Boltzmann distribution and temperature
At higher temperatures:
- same amount of molecules ∴ same area under curve
- average energy of molecules increases
- small proportion molecules still have low energy but more have a higher energy
- peak of graph is lower and shifted to the right [most probable energy of particles]
-higher end point as graph was shifted to right
- higher proportion of particles have enough energy to react
How does Pressure/concentration affect the rate of reaction?
- increasing pressure/ conc. makes the particles closer together…
- ∴ particles collide more frequently…
- ∴ more successful collisions / second…
- ∴ rate of reaction increases.
Define Catalyst
A substance that changes the rate of a chemical reaction without undergoing permanent changes itself.
Homogeneous Catalyst
- Same physical state as reactants
- reacts with reactants to form an intermediate which then breaks down to form products and regenerates catalyst.
Heterogeneous Catalyst
- different physical state from reactants [usually solid]
- reactant molecules adsorbed {weakly bonded} onto surface of catalyst, reaction takes place, products leave surface via desorption.
Boltzmann distribution and catalysts:
Activation energy lowered ∴ greater proportion of particles exceed activation energy
Define Dynamic Equilibrium
- exists in closed system
- when forward and reverse reactions both occur at same rate
- no overall change in concentrations of the reactants or products
Le Chatelier’s Principle (not a required definition)
When a system in dynamic equilibrium is subjected to change, the equilibrium position will shift to minimise the change.
Increasing the temp. effect on position of equilibrium
- shifts to the endothermic reaction (+ΔH)
- in order to oppose change and decrease temp.
Increasing the pressure effect on position of equilibrium
- shifts to the side with least gas molecules
- in order to oppose change and decrease pressure
Increasing the conc. of a reactant effect on position of equilibrium
- shifts to the product side and vice versa
- in order to oppose change and decrease conc. of reactant
Effect of catalyst on equilibrium position
- do not change the equilibrium position
- but the reaction occurs more quickly (equal increase on both sides)
