Unit 2.3 Enthalpy Flashcards
What is enthalpy?
The heat content that is stored in a chemical system
What is chemical energy?
A special form of potential energy that lies within chemical bonds
How can you measure enthalpy?
By measuring the heat exchange with the surroundings
- Heat loss in a chemical system = heat gain by surroundings
- Heat gain in a chemical system = heat loss by surroundings
What is the law of conservation of energy?
Energy is never lost or created, it is simply transferred from 1 place to another
What is the chemical system?
The reactants and products
How can you calculate enthalpy change?
Enthalpy of products - enthalpy of reactants
What is an exothermic reaction?
- A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants
- Resulting in heat loss to the surroundings
- ΔH is negative
What is an endothermic reaction?
- A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants
- Resulting in heat being taken in from the surroundings
- ΔH is positive
Definition of activation energy
The minimum energy required to start a reaction by the breaking of bonds
Why is activation energy important for exothermic reactions?
Without it, they would take place spontaneously
Definition of ‘the standard enthalpy change of reaction’
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
What are standard conditions?
- A pressure of 100 kPa (1 atmosphere)
- A stated temperature, usually 298K (25°C)
- A conc of 1 mol dm ^-3 (for reactions with aqueous solutions)
Definition of standard state
The physical state of a substance under standard conditions
Definition of ‘the standard enthalpy change of combustion’
The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states
Definition of ‘the standard enthalpy change of formation’
The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
What is the enthalpy change of formation of an element?
0 kJ mol^-1
How can you calculate heat exchange, Q?
m - the mass of the surroundings involved in the heat exchange
c - the specific heat capacity of the surroundings
ΔT - the temperature change of the surroundings
Q = mcΔT
Definition of standard heat capacity
The energy required to raise the temperature of 1g of a substance by 1°C
When using an equation to represent the enthalpy change of combustion, what must you be careful not to do?
DO NOT put a balancing number in front of the substance being burnt
(If you do, more than 1 mol would have been combusted)
How can you determine the enthalpy change of combustion by carrying out an experiment?
- Burn a known mass of a substance in air
- Heat a known mass of water
- Measure the temp change in the water
- Use Q = mcΔT to calculate the heat gained by the water
- The heat loss from chemicals is negative Q
- You can then use moles = mass/molar mass to work out the moles of the substance
- Divide Q by the moles to find how much heat is lost by 1 mol of the substance
Why might the experimental value of the standard enthalpy change of combustion be different to the data book value?
- There may have been incomplete combustion
- There may have been heat loss to the surroundings
Definition of average bond enthalpy
The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species
Is bond breaking an exothermic or an endothermic process?
Endothermic - energy is needed to break bonds in the reactants
Is bond making an exothermic or an endothermic process?
Exothermic - energy is released as new bonds are formed in the products
How can you calculate enthalpy change using bond enthalpies?
ΔH = Σ(bond enthalpies of bonds broken) - Σ(bond enthalpies of bonds made)
In an exothermic reaction, are the bonds that are formed stronger or weaker than the bonds that are broken?
Stronger
Why is it not always possible to measure the enthalpy change of a reaction directly?
- High activation energy
- Slow reaction rate
- More than 1 reaction taking place
What does Hess’ law state?
If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route
If you want to work out the enthalpy change for a particular route:
Reactants —-(A)—-> product
And you are given a different route which involves an intermediate:
Reactants —-(B)—-> intermediate —-(C)—-> product
How can you work out A?
A = B + C (because of Hess’ law)
What is an enthalpy cycle?
A diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess’ law.
How can you use Hess’ law to work out enthalpy change, using enthalpy change of formation values?
Set up an enthalpy cycle with the reactants, products and the elements
- Reactants —(A)—> products
- Elements —(B)—> reactants
- Elements —(C)–> products
So by following the arrows, you can find that C = A + B
Work out the values for B and C to find A
Don’t forget to include the number of moles, and that the standard enthalpy change of formation for an element is 0!
How can you calculate the rate of reaction?
Change in concentration/time
Units: mol dm^-3 s^-1
What does the collision theory state?
A chemical reaction can only take place when the reacting molecules collide
When 2 molecules collide, a reaction might take place if:
- The molecules have sufficient energy to overcome the activation energy of the reaction
- The molecules collide in the correct orientation
What effect does concentration have on the reaction rate?
If the concentrations of the reactants are increased, the rate of reaction also increases:
- Increased conc gives more molecules in the same volume
- The molecules will be closer together and there is a greater chance of the molecules colliding
- Collisions will be more frequent
What effect does pressure have on the reaction rate?
If the pressure of a gas is increased, the rate of reaction also increases:
- The molecules are pushed closer together
- So the same number of molecules occupy a smaller volume
- There is a greater chance of the molecules colliding
- Collisions will be more frequent
What is a catalyst?
A substance that increases the rate of a chemical reaction without being used up in the process
How does a catalyst work?
- It lowers the activation energy of the reaction by providing an alternative route with lower energy for the reaction to follow
- The catalyst may react to form an intermediate, but it is later regenerated so that it does not undergo any permanent change
Why are catalysts useful for many industrial processes?
They reduce costs:
- They speed up the process by lowering the activation energy
- So less energy is needed for the molecules to react
- Less energy reduces costs
Less energy also means that less fossil fuel is burnt:
- less CO2 will be released into the atmosphere
They can make a process more effective:
- By improving the percentage yield of an industrial preparation
Definition of heterogeneous catalysis
Catalysis of a reaction in which the catalyst has a different physical state from the reactants
Definition of homogeneous catalysis
Catalysis of a reaction in which the catalyst and reactants are in the same physical state
What is biocatalysis?
Any process in which the catalyst is an enzyme
What are enzymes?
- Large protein molecules that act as biological catalysts
- They are able to catalyse the reactions of large quantities of biological molecules in very short periods of time
What conditions do enzymes operate under?
- Low temps
- Atmospheric pressure
- At an optimum pH value
Why are enzymes useful in industry?
- Lower temps and pressure can be used than with conventional inorganic catalysts, saving energy and costs
- They often allow a reaction to take place which forms pure products, with no side reactions. This reduces the need for complex separation techniques, so reduces costs
- Conventional catalysts are often poisonous and difficult to dispose of, but enzymes are biodegradable
What happens in the Haber process?
Ammonia is made by reacting together nitrogen and hydrogen:
- The triple bond in nitrogen has to be broken
- This requires a large input of energy, contributing to a high activation energy
- Iron is used to catalyse this reaction, weakening the triple bond in nitrogen, and lowering the activation energy
What is the Boltzmann distribution?
The distribution of energies of molecules at a particular temperature, often shown as a graph
(Some molecules move fast and have high energy, but some move slowly and have low energy. Most have an average energy)
Name 3 features of the Boltzmann distribution.
- Area under the curve = total number of molecules in the sample
- There are no molecules in the system with 0 energy
- There is no maximum energy for a molecule
- Only the molecules with an energy greater than the activation energy are able to react
How does the Boltzmann distribution change when the temperature is increased?
It flattens and shifts to the right:
- The peak moves to a higher energy with a lower height
- A greater proportion of molecules exceeds the activation energy, so the rate of reaction increases
- The number of molecules in the system doesn’t change, so the area under the curve remains the same
How does the Boltzmann distribution change when a catalyst is added?
- The curve doesn’t change at all
- But the activation energy is lower, so a greater proportion of molecules exceeds the lower activation energy
- Reaction rate increases
What is dynamic equilibrium?
The equilibrium that exists in a closed system when:
- The rate of the forward reaction is equal to the rate of the reverse reaction
- The concentrations of the reactants + products remain the same
What is the position of equilibrium?
The extent of a reaction at equilibrium
What factors affect the position of equilibrium?
- Concentrations of the reactants and products
- Temperature
- Pressure (in reactions involving gases)
What does le Chatelier’s principle state?
When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change
What effect does increasing the concentration of a reactant have on the position of equilibrium?
It causes the position of equilibrium to move in the direction that decreases this increased reactant concentration:
- The system opposes the change by decreasing the conc of the reactant by removing it
- So the position of equilibrium moves to the right-hand side, forming more products
What effect does increasing the concentration of a product have on the position of equilibrium?
It causes the position of equilibrium to move in the direction that decreases this increased product concentration:
- The system opposes the change by decreasing the concentration of the product by removing it
- The position of equilibrium will move to the left-hand side, forming more reactants
How can you tell which side of a reaction has the highest pressure?
The side with the greater moles of gas has the highest pressure
What effect does increasing the total pressure of the system have on the position of equilibrium?
It causes the position of equilibrium to move to the side with fewer gas molecules, because this will decrease the pressure
What effect does decreasing the total pressure of the system have on the position of equilibrium?
It causes the position of equilibrium to move to the side with the greater number of gas molecules, because this will kncrease the pressure
What effect does increasing the temperature of the system have on the position of equilibrium?
It causes the position of equilibrium to move in the direction that decreases the temperature (the endothermic direction)
What effect does decreasing the temperature of the system have on the position of equilibrium?
It causes the position of equilibrium to move in the direction that increases the temperature (the exothermic direction)
What effect does a catalyst have on the position of equilibrium?
It doesn’t affect the position of equilibrium:
- It speeds up the rate of the forward and reverse reactions equally
- It increases the rate at which equilibrium is established
What conditions favour production of ammonia?
Ammonia is produced by the forward reaction:
- The forward reaction produces fewer gas molecules, favoured by using high pressure
- The forward reaction is exothermic, favoured by using low temp
What are the drawbacks to using the ‘favourable’ conditions for ammonia production?
- Although a low temp should produce a high equilibrium yield, the reaction would take place at a very low rate
- A lot of energy is required to maintain a high pressure, so it would be very expensive
- A high pressure also has a lot of safety implications
What conditions are required for the Haber process?
- Temperature of 400-500°C
- Pressure of 200 atmospheres
- An iron catalyst
What factors affect the rate of a chemical reaction?
- Temperature
- Pressure (when the reactants are gases)
- Concentration
- Surface area
- Adding a catalyst
