energetics Flashcards
what is enthalpy
the heat energy that is stored in a chemical system
how is enthalpy shown
H
how is enthalpy measured
from temperature changes when a chemical reaction takes place
delta H
heat energy change at constant pressure
what are the units of heat energy change (delta H)
kJ per mole
what happens in terms of heat in an exothermic reaction
heat energy is given out to the surroundings
what happens to the temperature of the surroundings when an exothermic reaction taxes place
temperature of surroundings increases
why is delta H negative in exothermic reactions
the chemicals lose heat energy
important exothermic reactions
combustion of fuels and respiration
what happens in terms of heat in endothermic reactions
heat energy is taken in from the surroundings
what happens to the temperature of the surroundings in endothermic reactions
the temperature of the surroundings decreases
why is delta H positive in endothermic reactions
the chemicals gain heat energy
what do endothermic reactions require
the input of heat energy
important endothermic reactions
thermal decomposition of calcium carbonate
photosynthesis
what can enthalpy profile diagrams be used to illustrate
the enthalpy change for a reaction
what do enthalpy profile diagrams show
the products, reactants and enthalpy change
activation energy
the minimum energy required to start a reaction by the breaking of bonds
do the products or reactants have a higher enthalpy in exothermic reactions
reactants
do the products or reactants have a higher enthalpy in endothermic reactions
products
what is required to start the exothermic reaction
energy is required to break the bonds, even though the products have a lower energy than the reactants
what will provide the energy needed to continue to overcome the activation energy
the net energy, once the barrier has been overcome
why is energy needed in endothermic reactions
the break the bonds and start the reaction
what do most endothermic reactions need to provide the necessary energy
to be heated continuously
what is bond enthalpy
the energy required to break one mole of a given covalent bond in the molecules in the gaseous state
why are bond enthalpy values always positive
bond breaking is endothermic
What do bond enthalpies give an indication of
The relative strength of a covalent bond
How do bond enthalpies give an indication of the relative strength of a covalent bond
The stronger the bond, the more endothermic the bond enthalpy
Why do chemists use mean bond enthalpy values
The same covalent bond may appear in different compounds and the value of the bond will be slightly different in each compound
Mean bond enthalpy
The energy required to break a covalent bond, averaged for that type of bond in a range of different compounds
What happens in terms of the bonds in a chemical reaction
Bonds in the reactants are broken and new bonds are formed to make the products
What is required to break bonds
Energy
What is released when new bonds form
Energy
What is the enthalpy change of a reaction
Sum of bonds broken - some of bonds formed
Limitations of bond energy calculations
Using mean bond enthalpies to calculate the enthalpy change of a reaction often leads to a value that is less accurate than a value obtained from Hess’s Law
Why is using mean bond enthalpies to calculate enthalpy change less accurate than a value obtained from Hess’s law
Bond enthalpies used are only an average value- not specific to compound in question
We are assuming all species are in the gaseous state- lots of compounds won’t be eg. Ethanol
What are the standard conditions for standard enthalpy of formation
298k and 100kPa
Standard state
The physical state of a substance under standard conditions
Standard enthalpy of formation
The enthalpy change that occurs when one mole of a compound is formed from its constituent elements with all reactants and products in their standard states
What is the standard enthalpy of formation of an element in its standard state
0
What do all products of standard enthalpy of formation need to be
1 mole
What is the relationship between negativity of standard enthalpy of formation of a compound and the stability of it
More negative = more stable
Standard enthalpy of combustions
The enthalpy change that occurs when one mole of a compound reacts completely in oxygen with all reactants and products in their standard states
Calorimetry
The method used to determine enthalpy changes by experiment
What does calorimetry involve
Measuring the temperature change of a given amount of water as the reaction occurs- and converting this to a quantity of heat energy
Specific heat capacity of water
4.18 J required to hear 1g of water
What does heat change =
Mass of water x specific heat capacity x temperature change
Q=mc DELTA T
What is q
Heat energy released or absorbed in joules
What is m
Mass of water in grams
What is the mass of water in grams equal to
Volume in cm3
What is c
The specific heat capacity of water
What is delta t
Temperature change in K
Same as change in celcius
What must you do to q when you’ve initially calculated it using mcDELTA T
Divide it by 1000 to convert J to kJ
Heat given out or taken in by one mole=
Q(in kJ)/ no of moles
What are the units of delta H for experimental determination of enthalpy changes
kJmol-1
When is delta H negative in calorimetry
If temp increases during reaction (exothermic reaction)
When is delta H positive during calorimetry
If temperature decreases during a reaction (endothermic)
Why may the experimental value for enthalpy of combustion be different to the data book value
- heat loss to surrounding
- incomplete combustion
- some methanol may evaporate
- some water may evaporate
What can cooling curves be used for
As a method of accounting for heat loss with reactions in solutions
How to use data to find maximum temperature rise
Plot a graph of temperature against time using results and determine max temperature change accompanying reaction
Extrapolate back to where lines meet (ish) to establish max temperature rise
First law of thermodynamics
Energy cannot be created or destroyed but it can be changed from one form to another
Where does hess’s law apply the first law of thermodynamics
To chemical reactions
Hess’s law
The enthalpy change for a chemical reaction is independent of the route taken
Why is it not always possible to measure the enthalpy change of a reaction directly
- reaction may have more than one possible product
- practically impossible to measure
What is Hess’s law used for
To calculate enthalpy changes which cannot be measured directly using an enthalpy cycle
What can we do using Hess’s law if we know two of the enthalpy changes
Calculate the third
What does delta H1 equal
Delta H 2 + delta H 3
How to remember enthalpy cycles of combustion
C=R-P
Where should you always do the vectors for enthalpy of combustion
Products to element
Where should you do vectors in calculations for reactions from enthalpy of formation values
Elements to products
How to remember enthalpy cycles of formation
F=P-R
