3.2 Physical Chemistry

Question 1

What is an endothermic reaction?

Answer 1

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

Question 2

What is activation energy?

Answer 2

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

Question 3

What are the standard conditions of enthalpy changes?

Answer 3

101 kPa

298K

Question 4

What are standard states?

Answer 4

Physical states under standard conditions

Question 5

What is enthalpy change of reaction?

Answer 5

Enthalpy change associated with a given reaction

Question 6

What is enthalpy change of formation?

Answer 6

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

Question 7

What is enthalpy change of combustion?

Answer 7

The energy change that takes place when 1 mole of a substance is completely combusted

Question 8

What is enthalpy change of neutralisation?

Answer 8

The energy change associated with the formation of 1 mole of water from a neutralisation reaction

Question 9

What is represented by the formula Q=mcDeltaT?

Answer 9

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

Question 10

Explain the term average bond enthalpy.

Answer 10

The mean energy needed to break 1 mole of bonds in gaseous molecules

Question 11

What is the three-step process for chemical reactions?

Answer 11

1. reactants bonds are broken - this process takes in energy so it is endothermic
2. atoms rearrange to form products
3. product bonds are formed - this process releases energy so it is exothermic

Question 12

How can the enthalpy change for a reaction be calculated?

Answer 12

Sum of bond enthalpies of reactants - Sum of bond enthalpies of products

Question 13

What is an exothermic reaction?

Answer 13

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

Question 14

What is the enthalpy cycle for combustion?

Answer 14

reactants ———————–>products
| /
| /
|_____[combustion products]_____/

—-> = |__ - /__

|__ = —-> + /__

Question 15

What is the enthalpy cycle for formation?

Answer 15

reactants ———————–> products
| /
| /
|___________ elements ____________/

Question 16

What is the formula for the rate of reaction in terms of concentration?

Answer 16

Change in concentration
Rate = —————————
Time

rate is fastest at the start

rate slows down as reactant concentrations decrease

Question 17

What are the factors affecting the rate of a chemical reaction?

Answer 17

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

Question 18

What is a catalyst?

Answer 18

A substance that increases the rate of reaction without being used up in the overall reaction

Question 19

How dos a catalyst lower activation energy?

Answer 19

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

Question 20

What is a homogenous catalyst?

Answer 20

When a catalyst for a reaction is in the same phase as the reactants

Question 21

What is a heterogenous catalyst?

Answer 21

When the catalyst for a reaction is in a different phase from the reactants

Question 22

What are the benefits of catalysts?

Answer 22

Reduce costs

Less fossil fuels burnt to generate required energy so lower carbon dioxide emissions

Increased sustainability

Question 23

What are some risks associated with catalysts?

Answer 23

Could contain toxic substances

Question 24

What is the Boltzmann distribution?

Answer 24

The distribution of energies of molecules at a particular temperature

Question 25

What are important features of the Boltzmann distribution?

Answer 25

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

Question 26

What is the effect of temperature on reaction rate in terms of Boltzmann distribution?

Answer 26

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

Question 27

What is the effect of a catalyst on reaction rate in terms of Boltzmann distribution?

Answer 27

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

Question 28

When is a chemical state in dynamic equilibrium?

Answer 28

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

Question 29

How can the position of a dynamic equilibrium be altered?

Answer 29

By changing:

concentrations of reactants or products

pressure in reactions involving gases

temperature

Question 30

What is Le Chatelier’s Principle?

Answer 30

When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise that change

Question 31

What is the effect of concentration on equilibrium?

Answer 31

Increasing concentration of a reactant causes equilibrium to shift to the right hand side, forming more products

Vice versa

Question 32

What is the effect of pressure on equilibrium?

Answer 32

Increasing total pressure of the system shifts equilibrium to the side with fewer moles

Vice versa

Question 33

What is the effect of temperature on equilibrium?

Answer 33

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

Question 34

What is the effect of a catalyst on equilibrium?

Answer 34

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

Question 35

Why do chemists need to balance equilibrium and yield?

Answer 35

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

Question 36

What is the equilibrium constant?

Answer 36

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)