3.2 Physical Chemistry

Question 1

What is enthalphy change?

Answer 1

heat energy transferred in a reaction at constant pressure. the units of ΔH are Kjmol^-1

Question 2

Standard states

Answer 2

physical states under standard conditions

Question 3

Standard Conditions

Answer 3

Pressure: 100kPa, Temperature: 298k

Question 4

Enthalpy change of formation

Answer 4

The enthalpy change when one mole of a compound is formed from its elements in their standard states

Question 5

Enthalpy change of combustion

Answer 5

The enthalpy change for the complete combustion of one mole of a substance

Question 6

Enthalpy change of neutralisation

Answer 6

The enthalpy change for the formation of 1 mol of water from neutralisation

Question 7

Activation energy

Answer 7

the minimum energy required for a reaction to take place

Question 8

How do you work out enthalpy change?

Answer 8

Work at Q (q=mcΔt)
convert to Kj
Enthlapy change = -Q/n (n is number of moles)

Question 9

What is average bond enthlapy?

Answer 9

the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

Question 10

Enthalpy change of reaction (don’t need to know def)

Answer 10

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

Question 11

Exothermic

and it graph

Answer 11

gives out energy, ΔH is negative, bond making

reactants (less stable) higher then products (more stable)

Question 12

Endothermic

and it graph

Answer 12

takes in energy, ΔH is positive, bond breaking

Reactants (more stable) lower then products (less stable)

Question 13

How do you find out enthlapy changes in a lab?

Answer 13

1. Find enthalpy of combustion of a flammable liquid, by burning it
2. As fuel burns it heats water. Use q=mcΔt (4.18j g^-1k^-1 shc of water), -Q/n
3. 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

Question 14

What ways do the arrows go for Hess cycle when given enthalpy changes of combustion data?

Answer 14

Away

Question 15

What ways do the arrows go for Hess cycle when given enthalpy changes of Formation data?

Answer 15

Towards

Question 16

What are the limitations with Hess cycles?

Answer 16

1. If using bond enthalpies in calculations – these are
   averages, they vary in different compounds.
2. If measuring energy released in combustion –
   incomplete combustion could occur, heat could be lost.
3. If calculating enthalpy of formation – different reactions could occur or additional products may form.
4. As well as this non-standard conditions can always be a factor

Question 17

How would you find the enthalphy change of a neutralisation reaction?

Answer 17

Combine known quantities of acid and alkali in insulated container and measure temperature change

Question 18

How do you calculate enthalpy change with average bond enthalpies?

Answer 18

Total energy absorbed - total energy released

Question 19

How does concentration and pressure effect rate of reaction?

Answer 19

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

Question 20

How does temperature effect rate of reaction?

Answer 20

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

Question 21

How does a catalyst effect rate of reaction?

Answer 21

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.

Question 22

Homogeneous catalysts

Answer 22

Homogeneous catalysts are in the same phase as the reactants

Question 23

Heterogeneous catalysts

Answer 23

Heterogeneous catalysts are in a different phase to the

reactants

Question 24

Why are catalysts important to use?

Answer 24

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

Question 25

Why is a concentration against time graph curved?

Answer 25

reactions start off quickly because of the greater likelihood of collisions reactions slow down with time as there are fewer reactants to collide

Question 26

How do you investigate reaction rates?

Answer 26

1. When the product made is a gas, its formation can be measured with mass balance 2. When reaction start, start stop clock, take measurements at regular intervals 3. Time, Mass table 4. 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

Question 27

Describe a boltzman distribution curve

Answer 27

Y axis=Number of molecules X axis=Kinetic energy Remember to label activation energy

Question 28

How will an increase in temperature effect the Boltzmann distribution curve?

Answer 28

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.

Question 29

How will adding a catalyst effect the Boltzmann distribution?

Answer 29

Lowers activation energy mark (moves to left), | so greater proportion of molecules have at least the activation energy.

Question 30

Dynamic Equilibrium

Answer 30

is when the rates of the forward and backwards reaction are the same in a closed system and the concentration does not change.

Question 31

What happens when concentration increases on the position of equilibrium

Answer 31

If the concentration of a chemical is increased the | equilibrium shifts away from that chemical.

Question 32

What happens when Pressure increases on the position of equilibrium

Answer 32

If the pressure increases the equilibrium shifts to the side with least moles of gas in the balanced equation.

Question 33

What happens when temperature increases on the position of equilibrium

Answer 33

If the temperature is increased the equilibrium shifts in the direction that is endothermic.

Question 34

What happens when a catalyst is added, to the position of equilibrium

Answer 34

Increases rate of forward and backward reactions | so no change to equilibrium

Question 35

Why's a compromise of conditions needed?

Answer 35

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

Question 36

What are the techniques and procedures used to investigate changes to the position of equilibrium for changes in concentration and temperature?

Answer 36

- 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.

Question 37

Whats the formula for Kc?

Answer 37

Kc = [C]c[D]d/ [A]a[B]b aA + bB ---> cC + dD

Question 38

How can you estimate the position of equilibrium using the Kc value?

Answer 38

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

Question 39

How will Kc value change for exothermic reaction when increasing temperature?

Answer 39

For an exothermic reaction, increasing temperature means equilibrium shifts to the left so Kc decreases

Question 40

What is another use of catalysts? Give example.

Answer 40

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

Question 41

What are catalytic convertors made up of?

Answer 41

Alloys of platinum, palladium, rhodium

Question 42

What are the benefits of catalytic convertors?

Answer 42

Reduce pollution released into atmosphere by speeding up reaction 2CO + 2NO --> 2C02 + N2