Physical Chemistry Flashcards
What is enthalpy change, ΔH?
The heat energy transferred in a reaction at constant pressure
What are the units of ΔH (enthalpy change)?
kj mol^-1
What are standard conditions?
100kPa (1 atm) pressure and a temperature of 298K (25°C)
What is standard state?
A substance’s physical state in standard conditions
What are the conditions needed to measure standard enthalpy changes?
Standard state and standard conditions (as enthalpy change is affected by temperature and pressure)
What is standard enthalpy change of reaction?
The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
What is standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in standard states
What is standard enthalpy of neutralisation?
The enthalpy change when solutions of an acid and an alkali react together to form 1 mole of water under standard conditions
What are exothermic reactions?
Reactions that give out energy to the environment
Describe exothermic reactions in terms of energy of products and reactants
The products have less energy than the reactants (as energy is released to the environment)
Will ΔH be +ve or -ve for exothermic reactions?
-ve
Will ΔH be +ve or -ve for endothermic reactions?
+ve
Give an example of an exothermic reaction?
Combustion
What are endothermic reactions?
Reactions that take in energy from their surroundings
Describe endothermic reactions in terms of energy of products and reactants
Products have more energy than reactants
Give an example of an endothermic reaction?
Thermal Decomposition
What is an enthalpy profile diagram?
A graph showing how the enthalpy changes during a reaction
What is the activation energy?
The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
What is the activation energy shown by in enthalpy profile diagrams?
The difference between the highest point on the graph and the reactants
When is a substance most stable, in terms of energy?
When it’s lost all its internal energy
Lower positions on an enthalpy profile diagram will be…
… more stable than higher positions
What is bond enthalpy?
The energy required to break a bond between 2 atoms
What is average bond enthalpy?
The energy needed to break one mole of a bond in the gas phase, averaged over the different compounds that the bond is found in
Is bond breaking endothermic or exothermic?
Endothermic, because it needs energy to break the bonds
Is bond forming endothermic or exothermic?
Exothermic, as it releases energy
Stronger bonds release….. energy when they are formed
More
Stronger bonds take….. energy to breaks
More
What equation links enthalpy change of reaction, total energy absorbed and total energy released?
Enthalpy change of reaction = Total energy absorbed - Total energy released
When measuring enthalpy changes in the lab, why do you use a polystyrene cup?
It’s an insulator, so that you don’t lose or gain much heat through the sides
How do you find the enthalpy changes in the lab for a reaction such as neutralisation?
Undergo the reaction in a polystyrene cup and place a thermometer to measure the temperature change
How do you find the enthalpy changes in the lab for a reaction such as combustion?
Use a bomb calorimeter containing a known mass of water. You burn a known mass of the reactant and record the temperature change of the water
What is the equation for enthalpy change?
q = mcΔT
In the equation q = mcΔT, what does q stand for?
Heat lost or gained (measured in J). This is the same as the enthalpy change if the pressure is constant
In the equation q = mcΔT, what does m stand for?
Mass (in g) of solution in the insulated container
In the equation q = mcΔT, what does c stand for?
Specific heat capacity of the solution/water (4.18 J g^-1 K^-1)
In the equation q = mcΔT, what does ΔT stand for?
The change in temperature of the solution
What is the specific heat capacity of a substance?
The amount of heat energy it takes to raise the temperature of 1 g of a substance by 1°C
What are the 3 steps for calculating the standard enthalpy change of combustion using data from an experiment in a lab?
- Calculate the amount of heat lost of gained during the combustion using q = mcΔT then convert units from J to kJ
- Calculate the number of moles of fuel that caused this enthalpy change, from the mass that reacted
- Calculate the standard enthalpy change of combustion by dividing the actual heat change of the reaction, q (in kJ) and the number of moles of fuel that burned
What are the 3 steps for calculating the standard enthalpy change of reaction using data from an experiment in a lab?
- Calculate the amount of heat lost of gained during the combustion using q = mcΔT then convert units from J to kJ
- Calculate the number of moles of one of the reactants that caused this enthalpy change
- Calculate the standard enthalpy change of reaction using the equation ΔH°r = (Heat lost or gained (measured in kJ) / number of moles that reacted) x number of moles reacting in balanced chemical equation
Why might a measured enthalpy change value be different than the one in the data book?
Possibly to do with heat loss to the surroundings, or incomplete combustion
What does Hess’s Law state?
The total enthalpy change of a reaction is always the same, no matter which route is taken
If ΔH values are for formation ΔfH, what is the energy change equal to?
Sum of products - sum of reactants
ΔfH = p - r
If ΔH values are for combustion ΔcH, what is the energy change equal to?
Sum of reactants - sum of products
ΔcH = r - p (think CRaP)
What is collision theory?
States that a reaction won’t take place between 2 particles unless they collide in the right direction and they collide with at least a certain minimum amount of kinetic energy
What is the activation energy?
The minimum amount of kinetic energy that particles need to have in order to react when they collide (in order to start breaking the bonds)
What is Boltzmann Distribution?
A theoretical model that describes the distribution of kinetic energies of molecules in a gas
What is the effect of temperature on rate of reaction?
When you increase the temperature, the molecules will , on average, have more kinetic energy and move faster.. This means a higher proportion of molecules will have at least the activation energy needed to react
Describe what Boltzmann Distribution means?
With molecules in a gas/liquid, they don’t all have the same kinetic energy. Some have more and some have less
When drawing a graph of Boltzmann Distribution, how do you transform the graph if the temperature is increasing?
Move the curve to the right (but keep the same area under the curve)
When drawing a graph of Boltzmann Distribution, how do you transform the graph if the temperature is decreasing?
Move the curve to the left (but keep the same area under the curve)
What is the effect of concentration on rate of reaction?
A higher concentration means a faster rate of reaction
Why does a higher concentration mean a faster rate of reaction?
The particles in the solution will be closer together so they’ll collide more often
What is the effect of increasing pressure on the rate of reaction for gases?
The rate of reaction will increase
Why does increasing the pressure on gases cause the rate of reaction to increase?
Because the particles are closer together, increasing the chance of collisions
If one of the reactants is a solid, what is the effect of increasing surface area on the rate of reaction?
A larger surface area makes the reaction faster
What is a catalyst?
A substance that increases rate of reaction by providing an alternative pathway with a lower activation energy
At the end of a reaction, the catalyst is…?
Chemically unchanged
Why are catalysts used in industrial processes?
Because you can save a lot of money and energy, as they don’t get used up in the reaction so you only need tiny amounts to catalyse huge amounts
What catalyst does the Haber process use?
An iron catalyst to increase the rate of forming ammonia from nitrogen and hydrogen
How does a catalyst lower the activation energy of reaction?
When a suitable catalyst is present, the reactant molecules bind to the catalyst. This makes it easier to break the bonds, so the activation energy decreases. The broken reactant molecules then form product molecules, and break away from the catalyst
What is a heterogeneous catalyst?
A catalyst that is in a different phase from the reactants (different physical state)
Name a process that uses a heterogeneous catalyst
Haber process (gases are passed over a solid iron catalyst)
What is a homogeneous catalyst?
A catalyst that is in the same physical state as the reactants
What state are homogeneous catalysts usually found as?
An aqueous catalyst for a reaction between 2 aqueous solutions
How does a homogeneous catalyst work?
One or more reactants combine with the catalyst to from an immediate species, which then reacts to form the product and reform the catalyst
How does using catalysts effect the environment?
Using catalysts means a lower temperature and pressure can be used. This means less CO2 is produced and less fossil fuels are used
Howe do catalysts reduce waste?
They allow a different reaction to be used to make the same product, but with a greater atom economy
What are catalytic converters made from?
Alloys of plutonium, palladium and rhodium
How do catalytic converters work?
They reduce pollution into the atmosphere by speeding up the reaction:
2CO + 2NO —> 2CO2 + N2
What is an issue with catalysts?
They don’t last forever so eventually they need to be disposed of
What is the problem of disposing catalysts?
Most contain nasty toxic compounds, which may leach into the soil if they’re sent directly to a landfill
In what causes would a catalyst be recycled rather than sent to landfill?
If it contains precious metals such as plutonium
What does uncatalysed mean?
No catalyst was used
What is the reaction rate defined as?
The change in the amount of reactants or products per unit time
What is reaction rate?
How fast a reaction takes place
What is the units of reaction rate?
‘whatever you’re measuring per unit time’ (e.g. g s^-1)
What is the equation for rate of reaction?
Rate of reaction = Amount of product formed or reactant used / Time
What are the 5 steps to measure the progress of a reaction when the product is a gas?
- Set up a mass balance and place on top a conical flask with the reactants in it
- Measure the start mass
- When the reaction starts, start the stopwatch
- Take regular mass readings at regular intervals
- Make a table of column time and column mass
The rate of reaction… as time progresses
Slows down
Why does the rate of reaction slow down as time progresses?
The reactants get used up, and so collisions between molecules become less likely
How do you work out the rate of reaction from a mass-time graph?
It’s the gradient (draw a tangent at a given point and find gradient)
What is dynamic equilibrium?
In a reversible reaction, dynamic equilibrium is reached when the concentration of reactants and products are constant, and the forward and backwards reactions are going at the same rate
What is the only condition needed for a dynamic equilibrium?
For the reaction to be in a closed system, so nothing can get in or out
What is Le Chatelier’s Principle?
If there’s a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change
In a reversible reaction, what does it mean if the position of equilibrium moves left?
The backwards reaction is faster, so you’ll get more reactants
What is the effect of a catalyst on the position of equilibrium?
No effect, as they speed up the forward and reverse reaction at the same rate
For Le Chatelier’s Principle, what happens if you increase the concentration of reactant?
The equilibrium tries to get rid of the extra reactant, so it shifts to the right so that more product is made
For Le Chatelier’s Principle, what happens if you increase the concentration of product?
The equilibrium tries to get rid of the extra product, so it shifts to the left, making the reverse reaction faster, so that more reactant is made
For Le Chatelier’s Principle, what happens if you increase the pressure?
The equilibrium shifts to the side with the fewest gas molecules to reduce the pressure
For Le Chatelier’s Principle, what happens if you decrease the pressure?
The equilibrium shifts to the side with the most gas molecules to increase the pressure
What state does the reactants and products have to be in for a change in pressure to make any difference to the position of the equilibrium?
Changing the pressure only affects equilibria involving gases
If there are the same number of moles of gas on each side of the reaction, what happens to the equilibrium when you change the pressure?
It won’t affect the position of the equilibrium
For Le Chatelier’s Principle, what happens if you increase the temperature?
Increasing the temperature means adding heat energy, so the equilibrium shifts in the endothermic (positive ΔH) direction to absorb this energy
For Le Chatelier’s Principle, what happens if you decrease the temperature?
Decreasing the temperature means removing heat energy, so the equilibrium shifts in the exothermic (negative ΔH) direction to try and replace this energy
If the forward reaction is endothermic, what will the reverse reaction be?
Exothermic
What is yield?
The amount of product you get from a reaction
What are the industrial conditions for the production of ethanol?
Pressure: 60-70 atm
Temperature: 300 °C
Phosphoric acid catalyst
Why did they chose the industrial conditions for the production of ethanol to be what they are?
To balance cost and yield (due to Le Chatelier’s Principle)
How do you investigate the position of equilibrium due to a change in temperature?
The brown gas NO2 exists in equilibrium with the colourless gas N2O4
- Place 2 sealed tubes of containing the equilibrium mixture in a water bath (one in a warm and one in a cool)
- The tube in the warm water will change to a dark brown colour
- The tube in the cold water will go a paler colour
Why aren’t ideal conditions for maximum yield used in reality?
Because of the cost of generating the conditions or their safety
What is a homogeneous reaction?
A reaction where all the reactants and products are in the same physical state
What is the equation to work out the equilibrium constant (Kc) for the general reaction aA + bB ⇌ dD + eE (where lower case = number of moles)?
Kc = ( [D]^d x [E]^e ) / ( [A]^a)x [B]^b )
the square brackets mean the concentration in mol dm^-3
What is the equilibrium constant?
A ratio worked out from the concentration of the products and reactants once a reversible reaction has reached equilibrium
When calculating equilibrium constant, what goes on the top line and what goes on the bottom line of the fraction?
Products on top, reactants on bottom
How do you work out the units of the equilibrium constant?
Put the units in the equation on their own and simplify the fraction
What does it mean if the equilibrium constant is a large number?
At equilibrium there will be lots of product and not much reactant
What does it mean if the equilibrium constant is a small number?
At equilibrium there will be lots of reactant and not much product
How do you estimate the position of equilibrium?
Kc will increase when the quantities of the products increase, so an increase in Kc means the equilibrium has shifted to the right
