3.2 Physical Chemistry Flashcards
What is an endothermic reaction?
In an endothermic reaction
enthalpy of products is greater than enthalpy of reactants - enthalpy change is positive
heat is gained in the chemical system and lost from the surroundings
temperature decreases
What is activation energy?
The minimum energy required to start a reaction by breaking bonds in the reactants
What are the standard conditions of enthalpy changes?
101 kPa
298K
What are standard states?
Physical states under standard conditions
What is enthalpy change of reaction?
Enthalpy change associated with a given reaction
What is enthalpy change of formation?
The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
What is enthalpy change of combustion?
The energy change that takes place when 1 mole of a substance is completely combusted
What is enthalpy change of neutralisation?
The energy change associated with the formation of 1 mole of water from a neutralisation reaction
What is represented by the formula Q=mcDeltaT?
Q is the heat exchanged by the surroundings, usually expressed in J
m is the mass of the substance heated or cooled, usually expressed in g
c is the specific heat capacity of the substance heated or cooled, expressed in J g-1 K-1
Delta T is the temperature change, measured in Kelvin
Explain the term average bond enthalpy.
The mean energy needed to break 1 mole of bonds in gaseous molecules
What is the three-step process for chemical reactions?
- reactants bonds are broken - this process takes in energy so it is endothermic
- atoms rearrange to form products
- product bonds are formed - this process releases energy so it is exothermic
How can the enthalpy change for a reaction be calculated?
Sum of bond enthalpies of reactants - Sum of bond enthalpies of products
What is an exothermic reaction?
In an exothermic reaction,
enthalpy of products smaller than enthalpy of reactants - enthalpy change is negative
heat loss from chemical system and heat gained by surroundings
temperature increases
What is the enthalpy cycle for combustion?
reactants ———————–>products
| /
| /
|_____[combustion products]_____/
—-> = |__ - /__
|__ = —-> + /__
What is the enthalpy cycle for formation?
reactants ———————–> products
| /
| /
|___________ elements ____________/
What is the formula for the rate of reaction in terms of concentration?
Change in concentration
Rate = —————————
Time
rate is fastest at the start
rate slows down as reactant concentrations decrease
What are the factors affecting the rate of a chemical reaction?
Increased:
temperature - more molecules with energy > activation energy so more frequent successful collisions
pressure - more crowded so more collisions likely to occur
concentration - more molecules occupied in same volume so closer together so collisions more frequent
surface area
adding a catalyst
What is a catalyst?
A substance that increases the rate of reaction without being used up in the overall reaction
How dos a catalyst lower activation energy?
A catalyst lowers activation energy by allowing a reaction to proceed via a different route with lower activation energy, as shown by enthalpy profile diagrams
What is a homogenous catalyst?
When a catalyst for a reaction is in the same phase as the reactants
What is a heterogenous catalyst?
When the catalyst for a reaction is in a different phase from the reactants
What are the benefits of catalysts?
Reduce costs
Less fossil fuels burnt to generate required energy so lower carbon dioxide emissions
Increased sustainability
What are some risks associated with catalysts?
Could contain toxic substances
What is the Boltzmann distribution?
The distribution of energies of molecules at a particular temperature
What are important features of the Boltzmann distribution?
Area under curve is equal to the total number of molecules within the sample
No molecules in the system with zero energy
No maximum energy for a molecule
Only molecules with energy greater than activation energy are able to react
What is the effect of temperature on reaction rate in terms of Boltzmann distribution?
At higher temperatures, the kinetic energy of all the molecules increases
The Boltzmann distribution flattens and shifts to the right
The number of molecules in the system does not change so the area under the curve remains the same
A greater proportion of molecules exceeds activation energy so reaction rate increases
What is the effect of a catalyst on reaction rate in terms of Boltzmann distribution?
Catalysts lower activation energy
By lowering activation energy, more molecules surpass the activation energy barrier
More molecules will overcome the new lower activation energy, giving more successful collisions and reaction rate increases
When is a chemical state in dynamic equilibrium?
A dynamic equilibrium exists in
a closed system
when the rate of the forward reaction is equal to the rate of the reverse reaction
and the concentrations of the reactants and products don’t change
How can the position of a dynamic equilibrium be altered?
By changing:
concentrations of reactants or products
pressure in reactions involving gases
temperature
What is Le Chatelier’s Principle?
When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise that change
What is the effect of concentration on equilibrium?
Increasing concentration of a reactant causes equilibrium to shift to the right hand side, forming more products
Vice versa
What is the effect of pressure on equilibrium?
Increasing total pressure of the system shifts equilibrium to the side with fewer moles
Vice versa
What is the effect of temperature on equilibrium?
When forward reaction is exothermic and reverse reaction is endothermic
increasing the temperature will shift equilibrium to the left as it minimises the change by taking in heat (moves in endothermic direction)
Vice versa
What is the effect of a catalyst on equilibrium?
A catalyst does not alter the position of equilibrium
It speeds up the rate of forward and reverse reactions in an equilibrium equally so there is an unchanged position in equilibrium
Why do chemists need to balance equilibrium and yield?
Chemists strive to achieve the highest yield possible
This must be balanced against the optimum position of equilibrium which allows industrial processes to be cheap and energy efficient
What is the equilibrium constant?
aA + bB =cC + dD
[C]c [D]d Kc = -----------
[A]a [B]b
Kc value of 1 - position of equilibrium halfway between reactants and products
Kc greater than 1 - position of equilibrium is to the right (favours products)
Kc less than 1 - position of equilibrium is to the left (favours reactants)
