Unit2) Reaction Feasibility Flashcards
What is reaction feasibility - what does it tell us
Can tell us the potential for a reaction to occur - how feasible it is
- tells us NOTHING about the rate of a reaction
What is thermochemistry
- 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
What does reaction feasibility explore
- 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
Absolute values of enthalpy ________ be calculated.
- but changes in enthalpy ___ be measured
Cannot
Can
What is enthalpy change and how is it calculated
- 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
When is enthalpy change exothermic and when is it endothermic
When ΔH is negative = exothermic ( heat lost to surroundings )
When ΔH is positive = endothermic ( heat taken in from surroundings )
- The full equation for enthalpy change
-
- look in data booklet
How can enthalpy changes be measured directly or indirectly
- 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
What is standard state
- 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 )
- What is the standard enthalpy of formation of a substance
- what’s it’s symbol
- 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.
What is the standard enthalpy of formation of an element in its most stable form
Zero
If sum of reactants is higher ΔH is ______
If sum of products is higher ΔH is ______
- negative - exothermic
- positive - endothermic
- enthalpies of formation can be found in the _____ _________ or will be stated in the question
- calculations practice
- data booklet
- Look in notes
- What is bond enthalpy
- breaking bonds in reactants is _______
- making new bonds in products is ______
- endothermic
- exothermic
What is
- molar bond enthalpy
- mean bond enthalpy
-
What needs to be considered to predict if a reaction is likely to occur ( feasibility)
- energy (which can be produced or used up during a chemical reaction)
- entropy ( level of disorder ) which is changed in all chemical processes
What is entropy
- 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 )
- 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
- 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
Units of entropy
J K-1 mol-1
Joules per kelvin per mol
- substances in solid state have ____ entropy values and substances in gaseous state have _____ entropy values
- explain this
- graph to show how it varies
- 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
Bigger molecules have ______ entropy
- Higher
- solids entropy is lowest , then liquids then gases
What is the second law of thermodynamics
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.
What does the third law of thermodynamics state
- explain the graph in notes (the graph which shows how entropy varies with temperature )
- 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
How to predict entropy
- look in brightred page 41
- look in slides
LOOK IN NOTES
Increase in entropy, S will have ______ value
Decrease in entropy, S will have ______ value
Positive
Negative
How can change in entropy be calculated
S = s(products) - s(reactants )
Look at proper full equation in data booklet
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.
increase
decreases
