3.2 Physical Chemistry Flashcards
What is enthalphy change?
heat energy transferred in a reaction at constant pressure. the units of ΔH are Kjmol^-1
Standard states
physical states under standard conditions
Standard Conditions
Pressure: 100kPa, Temperature: 298k
Enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements in their standard states
Enthalpy change of combustion
The enthalpy change for the complete combustion of one mole of a substance
Enthalpy change of neutralisation
The enthalpy change for the formation of 1 mol of water from neutralisation
Activation energy
the minimum energy required for a reaction to take place
How do you work out enthalpy change?
Work at Q (q=mcΔt)
convert to Kj
Enthlapy change = -Q/n (n is number of moles)
What is average bond enthlapy?
the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
Enthalpy change of reaction (don’t need to know def)
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
Exothermic
and it graph
gives out energy, ΔH is negative, bond making
reactants (less stable) higher then products (more stable)
Endothermic
and it graph
takes in energy, ΔH is positive, bond breaking
Reactants (more stable) lower then products (less stable)
How do you find out enthlapy changes in a lab?
- Find enthalpy of combustion of a flammable liquid, by burning it
- As fuel burns it heats water. Use q=mcΔt (4.18j g^-1k^-1 shc of water), -Q/n
- Ideally all the heat given out by fuel as it burns would be absorbed by the water, but in practice you always lose some heat
Diagram: (use calorimeter) combustion chamber, fuel inside, air flowing through, water with thermometer and stirrer on top being heated
What ways do the arrows go for Hess cycle when given enthalpy changes of combustion data?
Away
What ways do the arrows go for Hess cycle when given enthalpy changes of Formation data?
Towards
What are the limitations with Hess cycles?
- If using bond enthalpies in calculations – these are
averages, they vary in different compounds. - If measuring energy released in combustion –
incomplete combustion could occur, heat could be lost. - If calculating enthalpy of formation – different reactions could occur or additional products may form.
- As well as this non-standard conditions can always be a factor
How would you find the enthalphy change of a neutralisation reaction?
Combine known quantities of acid and alkali in insulated container and measure temperature change
How do you calculate enthalpy change with average bond enthalpies?
Total energy absorbed - total energy released
How does concentration and pressure effect rate of reaction?
If the concentration or pressure of a chemical is increased
this means there are more particles in the same volume
so there will be more frequent collisions
so the rate of reaction will increase
How does temperature effect rate of reaction?
If the temperature is increased
the number of collisions per second increases
as does the energy of the collisions
so the rate of reaction will increase
How does a catalyst effect rate of reaction?
Catalysts lower the activation energy by providing an
alternative path with a lower activation energy for the reaction without being used up.
Increasing the rate of reaction.
Homogeneous catalysts
Homogeneous catalysts are in the same phase as the reactants
Heterogeneous catalysts
Heterogeneous catalysts are in a different phase to the
reactants
Why are catalysts important to use?
They mean lower temperatures and less energy can be used with less waste products and a higher atom economy.
Reduces demand for combustion of fossil fuels, so reduction is CO2 emission
Why is a concentration against time graph curved?
reactions start off quickly because of the greater likelihood of collisions
reactions slow down with time as there are fewer reactants to collide
How do you investigate reaction rates?
- When the product made is a gas, its formation can be measured with mass balance
- When reaction start, start stop clock, take measurements at regular intervals
- Time, Mass table
- Reaction is finished when reading stops changing
Accurate method, but could release toxic gasses.
OR use gas syringe
Accurate method but vigorous reaction can blow up plunger in syringe
OR: change in pressure, change in colour, change in conductivity
Describe a boltzman distribution curve
Y axis=Number of molecules
X axis=Kinetic energy
Remember to label activation energy
How will an increase in temperature effect the Boltzmann distribution curve?
Particles will have more kinetic energy on average, so will move faster.
So curve pulls to the right
so greater proportion of molecules have at least the activation energy.
How will adding a catalyst effect the Boltzmann distribution?
Lowers activation energy mark (moves to left),
so greater proportion of molecules have at least the activation energy.
Dynamic Equilibrium
is when the rates of the forward and backwards reaction are the same in a closed system and the concentration does not change.
What happens when concentration increases on the position of equilibrium
If the concentration of a chemical is increased the
equilibrium shifts away from that chemical.
What happens when Pressure increases on the position of equilibrium
If the pressure increases the equilibrium shifts to the side with least moles of gas in the balanced equation.
What happens when temperature increases on the position of equilibrium
If the temperature is increased the equilibrium shifts in the direction that is endothermic.
What happens when a catalyst is added, to the position of equilibrium
Increases rate of forward and backward reactions
so no change to equilibrium
Why’s a compromise of conditions needed?
The yield of a reaction is important to prevent waste
but the rate of reaction is also important in terms of needing to make chemicals quickly so they can be sold.
Also costs and safety
What are the techniques and procedures used to investigate changes to the position of equilibrium for changes in concentration and temperature?
- By changing concentration of reactants/products or temperature of mixture then colour changes associated with reactants or products can be measured using colorimeter to see how equilibrium has shifted
- If an acid or alkali is in the reactants or products then changing concentration of reactants or products or temperature of mixture and taking a sample of mixture and doing a titration can allow you to find concentration of acid or alkali. This allows you to see how change affects position of equilibrium.
Whats the formula for Kc?
Kc = [C]c[D]d/
[A]a[B]b
aA + bB —> cC + dD
How can you estimate the position of equilibrium using the Kc value?
Kc’s much higher than 1 mean equilibrium lies far to the
right
Kc’s much lower than 1 mean equilibrium lies far to the left
How will Kc value change for exothermic reaction when increasing temperature?
For an exothermic reaction, increasing temperature means equilibrium shifts to the left so Kc decreases
What is another use of catalysts? Give example.
Change the properties of a product to make it more useful.
Poly(ethene) without catalyst: less dense, less rigid
With catalyst(Ziegler Natta): More dense, more rigid, higher melting point
What are catalytic convertors made up of?
Alloys of platinum, palladium, rhodium
What are the benefits of catalytic convertors?
Reduce pollution released into atmosphere by speeding up reaction
2CO + 2NO –> 2C02 + N2
