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)
enthalpy change (ΔH)=
{when using average bond enthalpies}
ΔHr= Bonds broken - Bonds made
ΔHc is always…
negative (exothermic)
ΔHf and ΔHr can be…
exothermic or endothermic
Define enthalpy change of reaction
The enthalpy change when the number of moles of reactants as specified in the balanced equation react together under standard conditions
Explain the bonding in a C=C double bond. Use the orbital overlap model
- C=C bonds are formed by the sideways overlap of p orbitals from 2 carbon atoms
- This results in a C=C bond formed above and below the plane of the molecule.
