Physical chemistry Flashcards
What is enthalpy
the thermal energy (heat) stored in a chemical system
What does the law of conservation of energy state
states that energy cannot be created or destroyed only moved from one place to another
What do exothermic reactions do
they transfer the excess chemical energy into thermal energy which is transferred to the surroudnings, heat is gained
What do endothermic reactions do
transfer thermal energy from the surroundings to chemical energy, heat is lost
Give examples of exothermic reactions
- combustion
- respiration
Give examples of endothermic reactions
- thermal decomposition
- photosynthesis
- electrolysis
What is chemical energy
A special form of potential energy that lies within chemical bonds
What can you measure and cannot measure with enthalpy
- it is impossible to measure the enthalpy of the reactants or products
- but we can measure the energy absorbed or released to the surroundings
- can vary but chemists usually measure energy changes in reactions by monitoring thermal energy
What is a system
the actual chemical reaction, so like the atoms and bonds involved in the chemical reactions
What is a surroundings
everything else such as the aqueous solution that the substances are dissolved in
Heat loss in a chemical system
heat gain to the surroundings
Heat gain in a chemical system
heat lost to the surroundings
An enthalpy change is…
- the heat exchanges with the surroundings during a chemical reaction at constant pressure
- the differences between the enthalpy of the products and enthalpy of the reactants
ENTHALPY OF PRODUCTS-ENTHALPY OF REACTANTS
Describe an exothermic graph
- product enthalpy smaller than reaction enthalpy
- heat loss
- -ve change in H
Draw an exothermic graph
DRAWWWW IT NOW
Describe an endothermic graph
- enthalphy of products is greater than enthalpy of reactants
- heat gain to the chemical system
- +ve change in H
DRAW AN ENDOTERMIC GRAPH
DRAWWWW IT NOW
Describe how the activation energy is used in exothermic reactions
even though products have a lower energy than reactants there still has to be an input of energy to break the first bond and start the reaction
- activation energy is supplied by a spark this overcomes the activation energy and once this has overcome the net output of energy it provides the activation energy os the reaction can continue
What do simple enthalpy profile diagrams show us
it shows us what will happen to the enthalpies of the reactants and products during the course of the reaction
What happens if there was no activation energy in exothermic reactions
- the exothermic reactions and fuels would spontaneously combust and be spontaneous
What is the activation energy
the activation energy is the minimum energy required to start a reaction by breaking bonds in the reactants
What is calorimetry
calorimetry is the quantitive study of energy in a chemical reaction
What is the specific heat capacity
specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1K
you cannot measure the enthalpy directly but…
You can measure the enthalpy change
Change in enthalpy equation
q=mc(triangle)T Q= heat exchanged with surroundings measured in J M= mass of substance heated or cooled in g C= specific heat capacity T= change in temperature then it goes to n=m/mr or n= cxv then (change)H =Q/n
describe the process for the direct measurement of enthalpy of a reaction
- use insulated cup, put lid on with a hole for the thermometer to reduce heat loss to the surroundings
- add a measured mass of reactant, take temperature every 4 minutes till it is stable
- at 5 minute add the second reactant, do not measure the temperature in this minute
- monitor temp every minute for another 5 minutes
- plot a graph and repeat the experiment
Describe the direct measurement of enthalpy of combustion
- measure the starting mass of fuel in the spirit burner
- add a known mass of water to a copper calorimeter
- take starting temperature
- combust the fuel for a few minutes and take final temperature of water
- take mass of unused fuel and calculate the mass of fuel burnt
describe a bomb calorimeter
- minimises heat loss
- uses pure oxygen to ensure complete combustion is achieved
What is thermodynamics
this focuses on energy in a chemical system which is the reactants and products
Why do we use standard conditions
for chemists calculate meaningful calculations so all values must be measured under standard consitons
100Kpa - 100,000 Pa
273K- add to covert degrees to kelvin
given by circle with a horizontal line through it symbol
What are the standard states
the physical state a substance is in under the standard conditions
100Kpa, 298K
What is the average bond enthalpy
The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission and break
How to break and made a bond
- reactant bonds are broken this take in energy and is endothermic
- atoms rearrange to form products
- products are formed and release energy this is exothermic
IN AN ENDOTHERMIC REACTION MORE
ENERGY IS NEEDED TO BREAK THE REACTANT BONDS THEN IS RELEASED WHEN TEH RPODUCT BONDS ARE FORMED - TAKES IN ENERGY
IN AN EXOTHERMIC REACTION MORE
ENERGY IS RELEASED WHEN THE PRODUCT BONDS ARE FORMED THAN IS NEEDED TO BREAK REACTANT BONDS - RELEASES ENERGY
How do you use average bond enthalpy data to predict enthalpy change
enthalpy change = sum of bond enthalpies of reactants - sum of bond enthalpies of products
why is it hard to calculate the enthalpy change for incomplete combustion
difficult or impossible to measure directly because there is often complete and incomplete combustion happing simultaneously
- but you can use average bond enthalpy data to calculate the enthalpy change
- values are averages for the bond in a variety of different environments and in different molecules this is not as accurate as direct measurements and using precise equipment such as the bomb calorimeter
define the enthalpy change of reaction
this is the energy change associated with a given reaction
define the enthalpy change of formation
this is 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
define the enthalpy change of combustion
this is the energy change that takes place when 1 mole of substance is completely oxidised under standard conditions with the elements under standard states
define the enthalpy change of neutralisation
this is the energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions
what does Hess’s law state
it states that the enthalpy change in a chemical reaction is independent of the route that it takes
What are enthalpy cycles used to measure
they are used to measure the enthalpy changes that are impossible to measure directly - they indirectly calculate the enthalpy change
what does the enthalpy cycle do
an enthalpy cycle is a pictorial representation showing alternative routes between reactants and products
ENTHALPY CALCULATIONS
PG 144-145
How do you work out the rate of reaction
change in concentration/time - measure the gradient, draw a straight line at the point that you want to measure
describe how the rate of reaction progresses during a reaction
at the start the concentrations of the reactants are the highest so the rate is at its fastest this is the initial rate
the rate slows down as the reactant concentrations decrease
when any reactant has a concentration of zero the reaction stops and the rate is 0
What factors effect the rate of a chemical reaction
- temperature (increased temperature = increased rate)
- pressure (increase pressure = increased rate)
- concentration ( increased concentration = increased rate)
- surface area ( increased surface area = increased rate)
- catalyst ( increases the rate of reaction)
How does concentration affect reaction rate
increased concentration, this means that there are more molecules in the same unit volume,, the molecules will be closer together so there is a greater chance of them colliding with sufficient energy to overcome the activation energy
collisions of molecules will be more frequent
therefore more will occur in a certain amount of time
therefore increased rate of reaction
How does pressure affect reaction ate
when pressure of a gas increased the molecules are pushed closer together
same number of molecules occupy a smaller volume
more frequent collisions therefore more collisions expected to overcome the activation energy
therefore rate of reaction increases when pressure increases
Draw enthalpy change of formation hess diagram
DRAW IT
Draw and enthalpy change of reaction from enthalpy changes of combustion diagram (Hess)
DRAW IT
whats the standard enthalpy change of formation of n2
- 0KJmol-1, as N2 element is in standard state under standard conditions
why is the experimental value different from the actual result
- heat loss
- may not happen under standard conditions
- may not have complete combustion
- fuel evaporated
when are average bond enthalpies used
- when calculating an enthalpy change but only when each species in a reaction is a gas
why calculations of enthalpy change of reaction using bond enthalpy values are not standard values
- they have to be in a gaseous state in order to be able to use average bond enthalpies
- gaseous state is not a standard state for most reactants and is not in standard condition so the enthalpy change of reaction is not standard enthalpy change
what does Hess law allow us to calculate
- indirect calculation of the enthalpy change of reaction
how can a enthalpy change of reaction be determined experimentally
- measure formation or combustion
- construct an Hess cycle
what two factors are required for a collision between two particles to be successful in causing a reaction
- 2 or more reactants collide with enough energy to overcome the activation energy
- there are reactive areas
how can you calculate rate of reaction from results between change in concentrtion over time
gradient of a line
how does a solid catalyst work in a reaction where the reactants are solutions
- the solutions adsorb onto the surface of the catalyst
in boltzman distribution why does the curve never touch the X axis
- molecules do never have the maximum amount of energy
what happens when a reaction reaches equlibrium
- no net movement of concentrations and they remain cosntant but are constantly interchanging
- foward rate of reaction = reverse rate of reaction
what determines the extent of a reaction
- equilibrium
what must you add to yellow potassium chromate to cause a colour change
- addition of acid
- yellow to organge
- chromate becomes dichromate
