Unit2) Reaction Feasibility

Question 1

What is reaction feasibility - what does it tell us

Answer 1

Can tell us the potential for a reaction to occur - how feasible it is
- tells us NOTHING about the rate of a reaction

Question 2

What is thermochemistry

Answer 2

• when a chemical reaction takes place there is often an exchange of heat energy between the reaction and its surroundings.
• the study of changes in energy which occur during chemical reactions is called thermochemistry

Question 3

What does reaction feasibility explore

Answer 3

• explores the relationship between two properties energy and entropy
• energy : is capacity to do work (useful energy)
• entropy : a measure of disorder of a system
  The relationship between these two properties is the driving force behind chemical reactions and determines their feasibility

Question 4

Absolute values of enthalpy ________ be calculated.

- but changes in enthalpy ___ be measured

Answer 4

Cannot

Can

Question 5

What is enthalpy change and how is it calculated

Answer 5

• scientists can actually measure the stored chemical energy within a substance but they can measure the change in energy between a system and its surroundings during a chemical reaction
• enthalpy change

ΔH = Hp - Hr
Where Hp and Hr are enthalpies of products and reactants

Question 6

When is enthalpy change exothermic and when is it endothermic

Answer 6

When ΔH is negative = exothermic ( heat lost to surroundings )
When ΔH is positive = endothermic ( heat taken in from surroundings )

Question 7

• The full equation for enthalpy change

Answer 7

-

- look in data booklet

Question 8

How can enthalpy changes be measured directly or indirectly

Answer 8

• direct methods include : calorimetery in which temperature changes are measured as a reaction happens
• indirect methods include Hess’s law calculations and bond enthalpy calculations

Question 9

What is standard state

Answer 9

• the standard state of an element is the most stable state of a substance under standard conditions
• standard conditions are a pressure of one atmosphere and a specific temperature , usually 298K (25 degrees )

Question 10

• What is the standard enthalpy of formation of a substance

- what’s it’s symbol

Answer 10

• The standard enthalpy of formation of a compound is the enthalpy change (energy change) when one mole of compound is formed from its elements in their standard states
• symbol ΔH f.

Question 11

What is the standard enthalpy of formation of an element in its most stable form

Answer 11

Zero

Question 12

If sum of reactants is higher ΔH is ______

If sum of products is higher ΔH is ______

Answer 12

• negative - exothermic

- positive - endothermic

Question 13

• enthalpies of formation can be found in the _____ _________ or will be stated in the question
• calculations practice

Answer 13

• data booklet

- Look in notes

Question 14

• What is bond enthalpy
• breaking bonds in reactants is _______
• making new bonds in products is ______

Answer 14

• endothermic
• exothermic

Question 15

What is

• molar bond enthalpy
• mean bond enthalpy

Answer 15

-

Question 16

What needs to be considered to predict if a reaction is likely to occur ( feasibility)

Answer 16

• energy (which can be produced or used up during a chemical reaction)
• entropy ( level of disorder ) which is changed in all chemical processes

Question 17

What is entropy

Answer 17

• entropy is basically the unavailable energy of a system / reaction
• it is better understood as a measure of the degree of disorder within a system
  (Order is characterised by repetition ; disorder by a lack of patterns , an absence of organisation , by randomness and chaos )

Question 18

• The larger the value of the entropy , the _____ the degree of disorder
• symbol of entropy
• where can you find standard entropy values of selected substances

Answer 18

• greater
• symbol S (S with little degree sign) is the entropy of 1 mol of the substance at a pressure of 1 atm and usually a temp of 298K
• standard entropy values for some selected substances are given on p17 of the data booklet

Question 19

Units of entropy

Answer 19

J K-1 mol-1

Joules per kelvin per mol

Question 20

• substances in solid state have ____ entropy values and substances in gaseous state have _____ entropy values
• explain this
• graph to show how it varies

Answer 20

• low , high
• the particles in a solid occupy approximately fixed positions. They can vibrate , but they cannot move from one place to another , solids are therefore highly ordered. Gases however have very high entropy values , gases contain particles that have complete freedom of movement and as a result are highly disordered. The entropies of Liquids lie between these two extremes
• graph is in notes

Question 21

Bigger molecules have ______ entropy

Answer 21

• Higher

- solids entropy is lowest , then liquids then gases

Question 22

What is the second law of thermodynamics

Answer 22

For a reaction to be spontaneous/ feasible there must be an increase in overall entropy (entropy must be positive ) for it to be feasible

The second law of thermodynamics states that the total entropy of a reaction system and
its surroundings always increases for a spontaneous process.

Question 23

What does the third law of thermodynamics state

- explain the graph in notes (the graph which shows how entropy varies with temperature )

Answer 23

• at 0K the particles in a solid are no longer vibrating and are perfectly ordered. Such a solid is described as a perfect crystal.
• so the entropy of a perfect crystal at 0K is zero. This is known as the third law of thermodynamics
• as the temperature increases from 0K , the entropy of a solid substance increases gradually until it’s melting point is reached
  At this point there is a rapid increase in entropy as the substance changes state from solid to liquid , there is an even larger increase in entropy at the boiling port as the substance changes state from liquid ti gas

Question 24

How to predict entropy

Answer 24

• look in brightred page 41
• look in slides
  LOOK IN NOTES

Question 25

Increase in entropy, S will have ______ value

Decrease in entropy, S will have ______ value

Answer 25

Positive

Negative

Question 26

How can change in entropy be calculated

Answer 26

S = s(products) - s(reactants )

Look at proper full equation in data booklet

Question 27

Heat energy released by the reaction system into the surroundings_____ the entropy
of the surroundings.
Heat energy absorbed by the reaction system from the surroundings_____ the
entropy of the surroundings.

Answer 27

increase

decreases