Energetics, Equilibria and Kinetics

Question 1

Exothermic

Answer 1

Reaction that releases more energy during bond making than it takes on for bond breaking

Question 2

Endothermic

Answer 2

Reaction that requires more energy for bond breaking than it releases during bond making

Question 3

Standard conditions

Answer 3

100kPa

298K

Question 4

Specific Heat Capacity

Answer 4

Amount of heat needed to raise the temperature of 1g of a substance by 1K

Question 5

Standard Enthalpy Change of Neutralisation

Answer 5

Enthalpy change when one mole of water is formed in a reaction between acid and alkali under standard conditions

Question 6

Hess’ Law

Answer 6

Enthalpy change for a reaction is independent of the route taken

Question 7

First Law of Thermodynamics

Answer 7

Energy can be neither made nor destroyed only converted from one form into another

Question 8

Factors affecting bond dissociation enthalpy

Answer 8

Directly proportional to bond strength and inversely proportional to bond length

Question 9

State why standard enthalpy of formation for oxygen is quoted as zero

Answer 9

All elements in their standard states have a standard enthalpy of formation of zero, as there is no change involved in their formation

Question 10

Bond dissociation enthalpy

Answer 10

Enthalpy change required to break a given covalent bond in a particular molecule in the gaseous state

Question 11

Mean bond enthalpy

Answer 11

Enthalpy change required to break a given covalent bond averaged over a range of different molecules all in the gaseous state

Question 12

Describe steps to determine an accurate minimum/maximum temperature for an enthalpy change

Answer 12

• start timer
• record temperature every 30 seconds
• add reactant
• plot a graph of temperature against time
• extrapolate minimum temperature to the point reactant is added

Question 13

Explain why the value for enthalpy of formation obtained using mean bond enthalpies is different to the actual value

Answer 13

Mean bond enthalpies are not SAME as actual as they are averaged over a different molecules and bond dissociation enthalpy varies between molecules

Question 14

Explain how enthalpy change will differ if referred to the formation of water in liquid state as oppose to gaseous

Answer 14

Value will be more exothermic since energy condensation is an exothermic process/energy is released to condense water vapour

Question 15

Enthalpy of Formation

Answer 15

Enthalpy change when one mole of a compound is formed from its constituent elements with reactants in products in their standard states under standard conditions

Question 16

Enthalpy of Combustion

Answer 16

Enthalpy change when one mole of a substance completely burns in oxygen under standard conditions

Question 17

Suggest why calculated value for enthalpy change of combustion differs from the actual value

Answer 17

• Heat loss to surroundings
• Incomplete combustion
• Heat capacity of apparatus

Question 18

Equilibrium

Answer 18

• Concentration of reactants and products does not change
• Rate of forwards reaction = Rate of backward reaction
• Closed system
• Conditions do not change

Question 19

Le Châtelier’s Principle

Answer 19

For any factor that changes a system at equilibrium the position of equilibrium will shift to oppose that change

Question 20

Describe how increasing temperature affects the position of equilibrium

Answer 20

Position of equilibrium shifts to favour endothermic reaction to decrease temperature

Question 21

Describe how increasing pressure affects the position of equilibrium

Answer 21

Position of equilibrium will shift to the side with fewer moles of gas to decrease pressure

Question 22

Describe how catalysts affect position of equilibrium

Answer 22

Have no effect because they increase the rate of the forward and backward reaction by the same amount

Question 23

State why volume does not need to be known to calculate Kc for certain reactions

Answer 23

Volume cancels in Kc equation / same number of moles on either side of the equation

Question 24

Dynamic

Answer 24

Ongoing/Continuos

Question 25

Homogenous

Answer 25

All reactants are in the same phase

Question 26

Describe how to calculate Kc value for the reverse reaction if Kc for forward reaction is known

Answer 26

Reciprocal of Kc value for forward reaction

Question 27

Suggest how you can tell from equations that a substance is behaving as a catalyst

Answer 27

- substance is unchanged | - offers alternative route (forms intermediate species)

Question 28

Activation energy

Answer 28

Minimum energy (1) required to break bonds for a reaction to occur (1)

Question 29

Suggest why at a constant temperature most molecules eventually react

Answer 29

Some particles always have energies higher than the activation energy (1) and molecules gain energy due to collisions (1)

Question 30

Catalyst

Answer 30

A substance that increases the rate of a chemical reaction (1) without being permanently changed or used up (1)

Question 31

Describe how a catalyst works

Answer 31

Catalyst lowers the activation energy (1) by providing an alternative route for the reaction (1) so that more particles have energies higher than the activation energy (1)

Question 32

Suggest a way collision frequency between particles can be increased without changing temperature

Answer 32

Increase concentration/ pressure | NOT catalyst because they ONLY cause more energetic collisions not a higher frequency

Question 33

Suggest why a small increase in temperature leads to a large increase in reaction rate

Answer 33

Many more particles have energies higher than the activation energy

Question 34

Explain why reactions occur faster at high temperatures

Answer 34

More molecules have energies higher than the activation energy (1) causing more energetic collisions (1) so there is a higher frequency of successful collisions (1)

Question 35

Give two requirements for a reaction to occur between gaseous molecules

Answer 35

Collision Sufficient energy (higher than activation energy) Correct orientation collision (last option)

Question 36

Disadvantages of high pressure/temperatures

Answer 36

Expensive to generate | High cost of plant (to resist high temperature /pressure, i.e. strong pressure vessel and equipment)

Question 37

Suggest why operating temperature increases without the use of a catalyst

Answer 37

Activation energy is higher than with catalyst (1) so temperature increased so more molecules have energies higher than activation energy (1)

Question 38

Suggest how yield can be increased without altering temperature or pressure

Answer 38

Recycle reactants

Question 39

Explain why rate of reactions decrease over time

Answer 39

Reactants are used up so lower frequency of collisions per unit time

Question 40

Transition state

Answer 40

some bonds are in the process of being made while others are being broken

Question 41

Suggest why Maxwell-Boltzmann distribution is asymptotic

Answer 41

No particles are zero energy and there is no maximum energy

Question 42

Suggest why catalysts are used in the form of a gauze

Answer 42

large surface area | cheaper (since gauze is thin wire mesh)

Question 43

Rate of reaction

Answer 43

Change in concentration over time

Question 44

Reasons percentage yield is lower than expected

Answer 44

- reversible reaction - product lost in isolation process - product lost in other reactions

Question 45

Explain why reactions occur slow at lower pressures

Answer 45

- molecules are spread further apart | - lower frequency of successful collisions between gas particles

Question 46

How to write Kc expression for heterogenous reaction

Answer 46

Do not include solids in Kc expression since its concentration remains constant

Question 47

How does changing conditions affect the value of Kc

Answer 47

- changing concentration or addition of catalyst has no effect - temperature changes value of Kc

Question 48

Explain why increasing temperature affects the value of Kc but increasing concentration of reactants or using a catalyst does not

Answer 48

- increasing temperature favours endothermic reaction (in order to decrease temperature) so concentration of products and reactants changes, hence value of Kc changes - increasing concentration of reactants means concentration of products must increase to oppose change so ratio of products to reactants is unchanged, hence value of Kc remains constant - catalyst increases rate of forward and backward reaction by same amount so concentration of products and reactants is unchanged, hence Kc remains constant

Question 49

Enthalpy Change

Answer 49

- change in heat energy | - measured at constant pressure

Question 50

Suggest why you cannot measure enthalpy change of salt to form hydrated salt directly

Answer 50

- difficult to measure temperature of solid - difficult to prevent solid from dissolving - impossible to add precise amount of water

Question 51

Explain what happens to the most probable energy and number of molecules that have the most probable energy when temperature increases or decreases

Answer 51

temperature increases - moves to right - fewer molecules have most probably energy temperature decreases - move to left - more molecules have most probable energy

Question 52

Explain how to work out initial rate of reaction from measuring volume of product over regular intervals of time

Answer 52

- plot graph of volume against time | - draw tangent and work out gradient at beginning

Question 53

Suggest why you cannot measure the enthalpy change of the following reaction directly C(s) + 1/2O2(g) -> CO(g)

Answer 53

difficult to prevent complete combustion | temperature of gases can be measured directly so ignore