Physical Chemistry

Question 1

Enthalpy:

Answer 1

Enthalpy, H, is the thermal energy stored in a chemical system

Question 2

The law of conservation of energy:

Answer 2

The law of conservation of energy states that energy cannot be created or destroyed, only moved from one place to another

Question 3

Enthalpy change:

Answer 3

Question 4

Exothermic reactions:

Answer 4

Question 5

Endothermic reactions:

Answer 5

Question 6

Activation energy:

Answer 6

Activation energy is the minimum energy required to start a reaction by breaking bonds in the reactants

Question 7

Standard conditions:

Answer 7

Question 8

Enthalpy change of reaction:

Answer 8

Question 9

Enthalpy change of formation:

Answer 9

Question 10

Enthalpy change of combustion:

Answer 10

The enthalpy change of combustion is always exothermic (negative)

Question 11

Enthalpy change of neutralisation:

Answer 11

Question 12

Calorimetry:

Answer 12

Calorimetry is the quantitative study of energy in a chemical reaction

Question 13

Calorimetry equation:

Answer 13

Question 14

Specific heat capacity:

Answer 14

The specific heat capacity is the energy required to raise the temperature of 1 gram of a substance by 1 Kelvin

Question 15

Experiment: A coffee cup calorimeter

Answer 15

The reaction vessel can be the insulated coffee cup held in a beaker. You could use a polystyrene lid with a hole for the thermometer to reduce heat loss to the surroundings (minimise the biggest experimental error)

• Add a measured mass of the first liquid reactant. Take the temperature every minute until it is stable. This usually takes around 4 minutes
• At 5 minutes, add the second reactant. Do not take or record the temperature for the fifth minute
• Monitor the temperature of the reaction mixture every minute for a further 5 minutes
• Plot a graph to infer the maximum temperature change generated by the reaction (extrapolation)

Question 16

Experiment: Copper calorimetry

Answer 16

When a fuel is combusted, the heat energy can be used to increase the temperature of a known mass of water

• Measure the starting mass of the fuel
• Add a known mass of water to a copper calorimeter
• Mount the copper calorimeter over the fuel and take the starting temperature of the water
• Combust the fuel for a few minutes and take the final temperature of the water
• Take the mass of the unused fuel and calculate the mass of the fuel burnt

Question 17

A bomb calorimeter:

Answer 17

A bomb calorimeter is a sophisticated piece of equipment that minimises heat loss as much as possible. It uses pure oxygen, to ensure complete combustion is achieved

Question 18

Bond enthalpy:

Answer 18

• Bond enthalpies tell you how much energy is needed to break each different bond
• When calculating bond enthalpies, you must include the overall enthalpy change of the chemical equation in your calculation

Question 19

Average bond enthalpy:

Answer 19

The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission

Question 20

Breaking and making bonds:

Answer 20

Question 21

Hess’ law:

Answer 21

Hess’ law states that the enthalpy change on a chemical reaction is independent of the route it takes

Question 22

Enthalpy cycle:

Answer 22

An enthalpy cycle is a pictorial representation showing alternative routes between reactants and products

Question 23

Hess’ cycle - enthalpy change of reaction:

Answer 23

Question 24

Hess’ cycle - enthalpy change of formation:

Answer 24

Question 25

Rate of reaction:

Answer 25

The rate of reaction is defined as the change in concentration of a reactant or a product in a given time. Usually:

• Concentrations of reactants are highest at the start of a reaction - this is when the rate is at its fastest (the initial rate)
• The rate slows down as reactant concentrations decrease (reactants get used up)
• When any reactant has a concentration of zero (it is used up) the reaction stops, and the rate of reaction is zero

Question 26

Factors affecting the rate of a chemical reaction:

Answer 26

• Temperature
• Pressure (for gaseous reactants only)
• Concentration
• Surface area
• Adding a catalyst

Question 27

The effect of temperature on reaction rate:

Answer 27

An increased temperature results in an increased rate of reaction as the reacting particles gain kinetic energy. More of the particles now have more than or sufficient activation energy. After gaining kinetic energy, the particles start to move more, and move more quickly and vigorously. There are more collisions per second, and so more successful collisions per second, meaning that there is an increased rate of reaction

Question 28

The effect of pressure on reaction rate:

Answer 28

When the pressure of a gas is increased, the molecules are pushed closer together. The same number of molecules occupies a smaller volume. As the molecules are closer together, they will collide more times per second, and so there will be more successful collisions per second, as more of the collisions are likely to occur with sufficient energy to overcome the activation energy. Therefore, the rate of reaction increases

Question 29

What is a catalyst?

Answer 29

A catalyst is a substance that increases the rate of a reaction without being used up during the process. Catalyst provide an alternative pathway of lower activation energy

Question 30

Homogeneous catalysts:

Answer 30

If a catalyst for a reaction is in the same phase (state) as the reactants, then it is a homogeneous catalyst. Enzymes within bodily fluids such as blood or saliva are examples of homogeneous catalysts

Question 31

Heterogeneous catalysts:

Answer 31

If a catalyst for a reaction is in a different phase from the reactants, it is a heterogeneous catalyst. Phases do not just include states, for example if all of the chemicals are immiscible liquids, the catalyst could still be heterogeneous, as it could be in a different liquid layer from the reactants. An example of a heterogeneous catalyst is a catalytic converter in a car - solid metal (platinum, rhodium or palladium) acting as catalyst to turn harmful gaseous exhaust fumes into less harmful substances

Question 32

The economic importance of catalysts:

Answer 32

Catalysts help to lower the energy demands of processes. By doing so, they reduce costs and also help the environment. Less fossil fuel needs to be burned to generate the required energy, and this also means lower carbon dioxide emissions
- An important industrial application of catalysts are within catalytic converters. Catalytic converters play an important role in improving our air quality by reducing toxic emissions from vehicles and preventing photochemical smog

Question 33

Haber Process: Economic importance

Answer 33

• The Haber process for the production of ammonia is of great economic importance. The ammonia is used as the basis for fertiliser manufacture, improving crop yields to feed the ever-increasing world population
• A lot of energy is required to break the triple bond in nitrogen, contributing to a high activation energy. Iron is used to catalyse this reaction, weakening nitrogen’s triple bond, and lowering the activation energy and therefore the costs - both financial and environmental

Question 34

What is the Boltzmann distribution?

Answer 34

The Boltzmann distribution is the distribution of energies of molecules at a particular temperature, often shown as a graph

Question 35

Important features of the Boltzmann distribution:

Answer 35

• The area under the curve is equal to the total number of molecules in the sample. The area does not change with conditions
• There are no molecules in the system with 0 energy - the curve starts at the origin
• There is not maximum energy for a molecule - the curve gets close to (the bottom square) but does not touch or cross the Energy (x) axis
• Only the molecules with an energy greater than the activation energy, E_a, are able to react

Question 36

The effect of temperature on the Boltzmann distribution:

Answer 36

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, The end of the curve must finish in the same square (but above) the original temperature curve

Question 37

The effect of catalysts on reaction rate and Boltzmann distributions:

Answer 37

• Catalysts lower the activation energy of reactions. They do not change the distribution of energy within molecules. However, by lowering the activation energy, more particles will automatically be above the activation energy barrier
• On collision, more molecules in the system will overcome the new lower activation energy of the reaction. There will be more successful collisions per unit time, and the rate of reaction will increase

Question 38

When is a system in dynamic equilibrium?

Answer 38

A chemical system is in dynamic equilibrium when:

• The concentrations of the reactants and products remain constant
• The rate of the forward reaction is the same as the rate of the reverse (backward) reaction

The system must remain isolated

Question 39

Factors affecting the position of equilibrium:

Answer 39

• Concentrations of reactants or products
• Pressure in reactions involving gases
• Temperature

Question 40

The effect of pressure on equilibrium (Le Chatelier’s principle):

Answer 40

Increasing the total pressure of the system, causes the equilibrium position to shift to the side with fewer moles of gas, as this will decrease the pressure

Question 41

The effect of concentration on equilibrium (Le Chatelier’s principle):

Answer 41

Increasing the concentration of a reactant favours the forwards reaction, and causes the equilibrium position to shift to the right , in order to form more products

Question 42

The effect of temperature on equilibrium (Le Chatelier’s principle):

Answer 42

Increasing the temperature favours the endothermic reaction, and so the equilibrium positions shifts in the direction of the endothermic reaction

Question 43

The effect of a catalyst on equilibrium:

Answer 43

A catalyst does not alter the position of equilibrium or the composition of an equilibrium system:

• A catalyst speeds up the rate of the forward reaction and reverse reactions equally
• A catalyst increases the rate at which equilibrium is established but does not affect the position of the equilibrium

Question 44

Equilibrium vs yield:

Answer 44

Many important chemical processes exist as equilibrium systems. Examples include:

• the preparation of ammonia from nitrogen and hydrogen in the Haber process
• the conversion of sulphur dioxide into sulphur trioxide in the Contact process

In industry, chemists strive to achieve the highest possible yield of a desired product. However this has to be balanced against the optimum position of equilibrium, wichita allows industrial processes to be as cheap and energy-efficient as possible - to create the highest profit

Question 45

The Haber process:

Answer 45

The raw materials, nitrogen and hydrogen, must be readily available:

• nitrogen is obtained from the air by fractional distillation
• hydrogen is prepared by reacting together methane (from natural gas) and water

Ammonia is produced by the forward reaction in this equilibrium. The optimum conditions are high pressure and low temperature. This is because:

• the products have less moles than the reactants, so is favoured by high pressures
• the forward reaction is exothermic (ΔH is negative), so is favoured by using a low temperature

However, there are drawbacks to using these theoretical conditions:

• although a low temperature should produce a high equilibrium yield, the reaction would take place at a very low rate. At low temperatures, comparatively few N₂ and H₂ molecules have enough energy to overcome the required activation energy
• a high pressure increases the concentration of the gases, increasing the reaction rate. So, a high pressure should produce both a high equilibrium yield and a high rate. However, large quantities of energy are required to compress gases, adding significantly to the running costs. There are also safety implications - any failure in the systems could potentially allow chemicals to leak into the environment, endangering those working on site

Question 46

The modern ammonia plant:

Answer 46

A modern ammonia plant needs to produce a sufficient yield of ammonia at a reasonable cost and in as short a time as possible. In practice, a compromise is made between yield and rate:

• Temperature - this must be high enough to allow the reaction to proceed at a realistic rate, whilst still producing an acceptable equilibrium yield. A temperature of 400-500°C is typically used
• Pressure - a high pressure must be used, but it must not be so high that the workforce is put in danger or the environment threatened. A pressure of 200 atmospheres is typically used
• Catalyst - an iron catalyst is added to speed up the rate of reaction, allowing the equilibrium to be established faster, and lower temperatures to be used. Less energy is needed to generate heat, reducing costs

The compromise conditions used covert only 15% of nitrogen and hydrogen into ammonia. The ammonia produced is liquefied and removed. Unreacted nitrogen and hydrogen gases are then passed through the reactor again. Eventually, virtually all the nitrogen and hydrogen will have converted into ammonia

Question 47

The equilibrium constant, K_c, and the equilibrium law

Answer 47

• Chemists consider the position of an equilibrium using the equilibrium constant, K_c
• K_c = products/reactants

the lower case letters represent the number of moles of each species

Question 48

The significance of the K_c value

Answer 48

• If K_c = 1, then the equilibrium position is halfway between the reactants and products. This would mean that the ratio of the product concentration to the reactant concentration is 1:1
• If K_c>1, the reaction favours the products (equilibrium lies to the right)
• If K_c<1, the reaction favours the reactants (equilibrium lies to the left)