Module 3.2 - Physical Chemistry Flashcards
Define Enthalpy, H
The thermal energy stored in a chemical system
What is a system?
The actual chemical reaction (atoms and bonds involved)
How do you work out enthalpy change, ∆H?
Enthalpy change = enthalpy of products - enthalpy of reactants
∆H = H(products) - H(reactants)
Describe an exothermic reaction
Heat loss in system - negative enthalpy change.
How is a exothermic reaction shown on a enthalpy profile diagram?
The enthalpy line for the reactants will be higher than the enthalpy line for the products.
Describe an endothermic reaction
Heat loss in a system - positive enthalpy change
How is an endothermic reaction shown on an enthalpy profile diagram?
The enthalpy line for the reactants will be lower than the enthalpy line for the products.
Define activation energy, Ea
The minimum energy required to start a chemical reaction by breaking bonds in the reactants.
Where is the activation energy on an enthalpy profile diagram?
The difference between the enthalpy line of the reactants and the peak of the energy barrier
What are the standard conditions?
- 100kPa - 100,000Pa
* 273K
What is a standard state?
The most stable form
Define enthalpy change of formation, ∆fH. Include an equation.
Energy change associated with 1 MOLE of a compound formed from its constituent elements in their standard state under standard conditions.
H2(g) + 1/2O2(g) —> H2O(l)
∆fH = -286kJ mol^-1
Define enthalpy change of combustion, ∆cH. Include an equation.
Energy change when 1 mole of a substance is completely combusted.
C2H6(g) + 3 1/2O2 —> 2CO2(g) + 3H2O(l)
∆cH = -1560
Define enthalpy change of neutralisation, ∆neutH. Include an equation.
Energy change from the formation of one mole of water from a neutralisation reaction.
What is the calorimetry equation?
q = mc∆T
q - heat exchanged to the surroundings (J)
m - mass of substance heated/cooled (g)
c - specific heat capacity of substance (Jg^-1 K^-1)
∆T - change in temperature (K)
Describe what type of enthalpy change is breaking bonds?
Energy is needed to break bonds so the change in the system is endothermic as there is a heat gain.
Describe what type of enthalpy change is making bonds?
When bonds form, the same quantity of energy is released to the surroundings - therefore it is exothermic as there is a heat loss in the system.
Define average bond enthalpy
Mean energy needed for one mole of a given type of gaseous bonds to undergo homolytic fission.
How do you work out average bond enthalpies, ∆H?
∆H = ∑(enthalpies of reactants) - ∑(enthalpies of products)
What is Hess’ law?
The enthalpy change in a chemical reaction is independent of the route it takes.
Draw the enthalpy cycle of the enthalpy change of reaction from the enthalpy change of combustion
Reactants ——A——> Products
Arrow down to combustion products
Reactants to combustion products = C
Products to combustion products = B
A + B = C
A = C - B
Draw the enthalpy cycle of the enthalpy change of reaction from the enthalpy change of formation
Reactants —————> Products
Arrows up to reactants/products
Elements to reactant = C
Elements to products = B
B = C + A
B - C = A
How do you work out the rate of reaction?
Change in concentration ÷ time
What are the units of rate of reaction?
Mol dm^-3 s^-1
Explain if and how temperature:
- Increases frequency of collisions
- Increases energy
- Increase n.o of successful collision
- More energy, move around faster, more likely to collide
- The increase in temperature gives them more energy
- More particles with required activation energy, more likely for collisions to be successful
Explain if and how pressure:
- Increases frequency of collisions
- Increases energy
- Increase n.o of successful collision
- Same n.o of molecules in a smaller volumes, more likely to collide
- n/a
- n/a
Explain if and how concentration:
- Increases frequency of collisions
- Increases energy
- Increase n.o of successful collision
- More molecules in the same volume, molecules closer together so more likely to collide, more frequent.
- n/a
- n/a
Explain if and how surface area:
- Increases frequency of collisions
- Increases energy
- Increase n.o of successful collision
- More reactants particles exposed, increased chance of collision
- n/a
- n/a
Explain if and how catalyst:
- Increases frequency of collisions
- Increases energy
- Increase n.o of successful collision
- n/a
- n/a
- Lowers the activation energy required, more particles are likely to have the new activation energy.
What are the 5 factors that affect the rate of reaction?
- Temperature
- Pressure
- Concentration
- Surface area
- Catalyst
What does the gradient of concentration-time graph represent?
The rate of reaction
What is a catalyst?
A substance that increases the rate of reaction without being used up. It lowers the activation energy by providing an alternate reaction pathway. The peak of the enthalpy profile diagram will be lower.
What is a homogenous catalyst?
A catalyst that is in the same phase (state) as the reactants
What is a heterogeneous catalyst?
A catalyst that is in a different phase (state) as the reactants.
What the two reasons of importance of catalysts?`
- Lowers energy demands, lower costs, less fossil fuels used, lower CO2 emissions
- Catalytic converters, reduces toxic emissions, prevents photochemical smog, improves air quality.
What is the Boltzmann Distribution?
The distribution of energies of molecules at a particular temperature.
Describe the difference between a Boltzmann distribution curve with one at 180K and the other at 400K
The curve at 180K will have a higher peak at a lower energy point as most particles will have a lower energy than the particles with 400K. However, the curve at 400K will have a lower peak a higher energy point as most particles will have a higher energy.
How is a catalyst shown on a Boltzmann Distribution curve?
A dotted line at a lower energy point than the activation energy point.
What does the area under a Boltzmann Distribution curve represent?
The total number of particles
Describe dynamic equilibrium
> exists in a closed system
when the rate of the forward reaction = the rate of the backward reaction
CONCENTRATION of reactants and products don’t change
How does the concentration of reactants affect equilibrium?
> Increase: shift to right
> Decrease: shift to left
How does the concentration of products affect equilibrium?
> Increase: shift to left
> Decrease: shift to right
How does an increase in pressure affect equilibrium?
Shifts to side of fewer moles to get rid of the extra pressure
How does an decrease in pressure affect equilibrium?
Shifts to the side of more moles to increase to optimum pressure
How does an increase in temperature affect equilibrium?
Shift in the endothermic direction to absorb the extra heat.
How does an decrease in temperature affect equilibrium?
Shifts in the exothermic direction to release more heat.
How does a catalyst affect equilibrium?
No effect
Why is a compromise important to the chemical industry between equilibrium and rate of reaction?
Allows for an optimum yield where the industrial process is affordable and energy-efficient.
Explain a catalyst’s effect on equilibrium
A catalyst increases the rate of reaction of both the forward and backward reaction by the same amount resulting in an unchanged position of equilibrium
How do you find the equilibrium constant, Kc, of the following?
aA + bB <=> cC + dD
[C]^c x [D]^d
Kc = ———————
[A]^a x [B]^b
What does Le Chatelier’s principle state?
When a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise the change.
What happens to the equilibrium when Kc < 1?
The equilibrium shifts to the LHS as there are more reactants than products and the equilibrium favours the reactants.
What happens to the equilibrium when Kc ≈ 1?
The equilibrium stays halfway as there are the same amount of products as reactants.
What happens to the equilibrium when Kc > 1?
The equilibrium shifts to the RHS as there more products than reactants and the equilibrium favours the products
Write an expression to find the Kc of the following. Include one for the units
N2 + 3H2 —> 2NH3
[NH3]^2
Kc = ——————
[N2] x [H2]^3
(mol dm^-3)^2 Units = ————————
(mol dm^-3)(mol dm^-3)^3
= mol^-2 dm^6
Define enthalpy change of reaction, ∆rH. Include an equation.
Energy change associated with a given reaction.
CuCO3(s) —> CuO(s) + CO2(g)
∆rH = +1146kJ mol^-1
