Module 3.2 Flashcards
What is enthalpy
A measure of heat energy in a chemical system
It can be thought of the energy stored in bonds
What is the law of conservation of energy
The law that:
energy cannot be created or destroyed only transferred
How is enthalpy change measured
By measuring the energy transferred from the system to the surroundings
And the energy transferred from the surroundings to the system.
What is an exothermic reaction
A reaction with a negative change in energy between products and reactants
- The temperature of the surroundings increase
What is an endothermic reaction
A reaction where enthalpy change is positive between products and reactants
- the temperature of the surroundings decreases
What is activation energy
The minimum energy required for a reaction to begin
How do you draw an enthalpy diagram
write the reactants and products on the diagram
Draw the activation energy from the reactants to the top of the curve
Draw the enthalpy change from the reactants to the products
What are the standard conditions for change in enthalpy
100kpa pressure
298k temperature
1mol/dm3 concentration
What is the standard state of a substance
the physical state of the substance under standard conditions
What is the standard enthalpy change of reaction (ΔrH)
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions and in standard states
What is the standard enthalpy change of formation (ΔfH)
The enthalpy change that takes place when 1 mol of a substance IS FORMED FROM ITS PRODUCTS under standard conditions, with all products and reactants in their standard states,
e.g
C(s) + 2H2(g) → CH4(g)
What is the standard enthalpy change of combustion (ΔcH)
The change in enthalpy that takes place when 1 mole of a substance reacts with oxygen under standard conditions with all reactants and products in their standard states
how is energy calculated in a reaction under standard conditions (calorimetry)
q = mcΔT
q = energy (J)
m = mass (g)
c = specific heat capacity (J/g/k)
ΔT = change in temperature (k)
What is the standard enthalpy change of neutralisation (ΔneutH)
The enthalpy change that accompanies the reaction of an acid and base to form 1 mole of H2O under standard conditions whith all reactants and products in their standard states
How is the enthalpy change of combustion in a reaction calculated (equation)
Find the q=mcΔT of the solution
Then divide the energy (which must be converted to KJ) by the moles of a specific substance
q(KJ) / mol = ΔcH (KJ/mol)
What is Hess’s Law
The law that enthalpy changes in a chemical reaction are independent of the route they take
∆H1 + ∆H2 → ∆H
How is a regular Hess cycle drawn
The direct route is between the products and reactants
The indirect route is from the products to the alternative route (alternative products / reactants) to the products
In the indirect route an arrow points down to the alternative products from the reactants
And an arrow points up from the alternative products to the products
What is the ∆Hf of elements
0
If the element is on both sides of the reaction its enthalpy change is 0
How do you draw the Hess cycle for enthalpy change of formation
Write the constituent elements below the reaction.
Draw arrows pointing up from the constituent elements to the reactants and the products
How do you draw the Hess cycle for enthalpy change of combustion
Write the equation
Underneath, write the products of combustion (usually H2O and CO2)
Draw arrows pointing to the reactants of combustion, from the products and the reactants
What are average bond enthalpies
The mean energy needed for 1 mole of a given gaseous bond to undergo homolytic fission (breaking covalent bonds)
What type of reaction is bond breaking
Endothermic
- energy is absorbed to break bonds
What type of reaction is bond forming
Exothermic
- energy is released when bonds are formed
Why are reactions exothermic
If more energy is released when forming bonds than energy is absorbed to break bonds, there is a negative energy change
This means the enthalpy change is negative
Why are reactions endothermic
If more energy is absorbed when breaking bonds, than is released forming new bonds there is a positive enthalpy change.
The reaction is endothermic
How do you calculate ∆H using average bond enthalpies
∆H of reactants - ∆H products
(Break - make)
When calculating using average bond enthalpies do moles matter
Yes - multiply the average bond enthalpy by the number of moles
In Hess cycle calculations, do you need to do anything with moles
Yes
Multiply the correct enthalpy changes by the correct moles
What is Calorimetry
A quantitative study of energy in a chemical reaction
A mathematical equation is then used to calculate enthalpy change ( q = mc∆T)
Describe the experiment allows the direct measurement of enthalpy of combustion
(Specifically copper calorimetry)
Combusting a fuel can be used to increase the temperature of a known mass of water
-Measure the fuels starting mass
-Add a known mass of water to the copper calorimeter
-Mount the copper calorimeter above the fuel and measure the starting temperature of fuel
-Combust the fuel using a spirit burnerfor a few minutes (e.g 5) and record the waters final temperature
-Take the mass of the unused fuel and calculate the fuel used
Calculate the energy change of water
Find the moles of fuel burnt
Energy ÷ moles = enthalpy
How can enthalpy of reaction be measured (experiment - the simpler one)
-Place an insulated polystyrene cup into a beaker, with a hole for a thermometer
-Add a known measurement of a known concentration liquid and record its temperature (every minute) until its stable
-Add the second reactant and do not record the temperature
- Record the temperature every minute for 5 minutes
Plot a graph to calculate the temperature change in the reaction
Then calculate the change in enthalpy
How do you calculate density
Density = mass / volume
Why might the calculated enthalpy change of an experiment be different to the actual value
- Heat energy could be lost to the surroundings
- The reaction may not go to completion ( e.g incomplete combustion)
- Non - standard conditions
Why might the calculated enthalpy change of an experiment be different to the actual value WHEN USING AVERAGE BOND ENTHALPY
It is an average bond enthalpy - the values are not exact
Bond energy / enthalpy assumes all molecules are gasseous
What is the collision theory
For a reaction ot occur, reactants must possess energy that is equal to or greater than the activation energy
Are reactions with a high activation energy faster than thoes with a low activation energy
No -a s it will take onger to reach the required energy
How does tmeperature effect rate of reaction
AS temperature increases, the kientic energy of the particles increases. More particles have Ek greater than or equal to the activation energy, so there are more frequent successful collisions
Rate of reaction is greater
How does increasing concentration effect the rate of reaction
increasing concentration means there are more molecules in the same volume, particles are more likley to collide as there are more of them
There are more frequent successful collisions, so the rate of reaction is greater
How does increasing pressure effect the rate of reaction
The molecules are pushed closer together, the same number of molecules occupy a smaller volume.
More frequent successful collisions, so the rate of reaction is greater
How do you measure the rate of reaction
Monitor the physical changes of the reaction, and the time,
E.g concentration changes, change in mass
You can pot this on a graphy, where the gradient is rate of reaction
What is the boltzman distribution
The distribution of molecular energies in a gas at a concent temperature
How does increasing temperature change the boltzman distribution
The peak is lower, and the curve shifts to the right
How does decreasing temperature change the boltzman distribution
The peak is higher and the curve shifts to the left
How does increasing pressure change the boltzman distribution
The graph has the same steepness but is higher than the origina
How does decreasing pressure change the boltzman distribution
The graph has the same steepness but is lower
How do you lable / draw the boltzman distribution
The peak is labled as the most probable energy
To the right of the peak lable the mean energy
To the end of the curve lable the activation energy
Draw a curve that quickly peaks, fall and then gradually lowers, do not touch the axis
y axis = number of particles with kinetic energy
x axis = kinetic energy
How do catalysts effect rate of reaction
Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway.
More particels have grreater than or equal to the activation energy, so there are more frequent successful collisons
The rate of reaction is faster
How does using a catalyst change the boltzman distribution
The activation energy is moved to the left, the curve does not change at all
What is a catalyst
A substance that increases the rate of reaction, by providing an alternative reaction pathway, without being useed up in the process
What is a homogenous catalyst
A catalyst in the same phase (state) as the reactants
What is a hetrogenous catalyst
A catalyst in a different phase from the reactants
What are the advantages and disadvantages of using a homogenous catalyst
A:
More uniform reaction conditions
Can be highly specific to certain reactions
D:
HArder to seperate the product from the mixture
Can be less stable and may degrade over time
Advantages and disadvantages of using a hetrogenous catalyst
A:
Easy to seperate from the products
Often more stable and durable
D:
Requires specific conditions for effective surface contact
MAy suffer from catalyst poisoning (where impurities block active sites)
How do hetrogenous catalysts work
Adsorption - A reactant aproches the catalyst and ssettles onto an active site
- Another reactant approches the catalyst
- Rearangement of electrons takes place, to form the product
Deadsorption - The products are released from the surface
What is La chatliers principle
If a dynamic equillibrium is disurbed by changing conditions, the reaction moves to counteract the change
What is a closed system
A system / container that reactants / products cant leave
What is dynamic equillibrium
Equillibrium in a reversible reaction where the products and reactants are being produced at the same rate
How does changing temperature effect the equillibrium contant
Increasing tempertature = The equillibrium position moves in the endothermic direction (positive direction(
Decreasing temperature = The equillibrium position moves in the exothermic direction
What does the equillibrium position show
The equilibrium position highlights that ratio of the yeield of products / reactants
If the equillibrium position moves to the backwards reaction, the yield of th ebackwards reaction increases
How does changing pressure effect the equillibrium contant
Increasing pressure = The equillibrium position moves to the side of fewest moles
Decreasing pressure = The equillibrium position moves to the side with most moles
Do catalysts effect equillibrium position
no
How do you calculate equillibrium constant
products / reactants
If there are any moles in the reaction, they are put as a power
in the reaction
N2)4 = 2NO2
write the equillibrium equation
kc = [NO2]^2 / [N2O4]
How do you find the units of the equillibrium constant
Put the moles and th eunits (mol dm^-3) and calculate the moles
What does it mean if the Kc is greater than 1
The reaction favours the products
(there is a greater yield of products)
What does it mean if the Kc is lower than 1
The reaction favours the products
How do you find the Kc, when you are give the moles of the reactants (not in equillibrium) and the moles of the products (in equillibrium)
E.g
1.6mol No 1.4mol O2
At equillibrium 1.2 mol 02 is formed
2NO + O2 = 2NO2
Finfd the ratio of moles (in the reaction)
Write out the intitial actual moles
Divide the equillibrium moles, by the ratio
Subtract the values to obtain the moles in equillibrium, for each molecule
2 + 1 = 2
1.2 + 0.6 = 1.2
1.6-1.2 = 0.6 (equillibrium moles for NO
1.4-0.6 = 0.8 (equillibrium moles for O2)
