3.2 - Physical Chemistry Flashcards
Define enthalpy
The thermal energy stored in a chemical system
Define enthalpy change
The heat energy transferred in a reaction at constant pressure
What are the standard conditions?
298K and 100KPa
Why do we use standard conditions?
- To make meaningful calculations
How do we convert from kPa to Pa?
Multiply by 1000
Describe the graph of an exothermic reaction?
- Goes from high to low (reactants have a higher enthalpy than products) - Products are more stable than reactants - Enthalpy change is negative
Describe the graph of an endothermic reaction?
- Goes from low to high (products have a higher enthalpy than reactants) - Reactants are more stable than reactants - Enthalpy change is positive
On an exothermic/endothermic enthalpy graph, how can you show enthalpy change?
It is the direct difference between reactants and products
Define activation energy?
The minimal amount of energy needed for a reaction to take place
On an exothermic/endothermic enthalpy graph, how can you show activation energy?
The difference between the HIGHEST point of the activation energy and the enthalpy of reactants
Give an example of an exothermic reaction
Oxidation
Give an example of an endothermic reaction
Photosynthesis
Define enthalpy change of formation
The enthalpy change when one mole of compound is formed from its elements in their standard states under standard conditions
Define enthalpy change of combustion
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
Define enthalpy change of neutralisation
The enthalpy change when one mole of water is produced when an acid and alkali react under standard conditions
What is meant by standard states
Physical states under standard conditions
What formula do we use to determine enthalpy change directly from experimental results?
q = mcΔT q = Heat loss OR gained (joules) m = Mass of water in the colorimeter (grams) c = Specific heat capacity of water (4.1.8J/grams/Kelvin) ΔT = Change in temperature (Kelvin)
What does the q stand for in q = mcΔT?
Enthalpy change (measured in joules)
What does the m stand for in q = mcΔT?
Mass of water (measured in grams)
What does the c stand for in q = mcΔT?
Specific heat capacity ( 4.18 J/grams/Kelvin)
What does the ΔT stand for in q = mcΔT?
Change in temperature (measured in Kelvin)
When using q = mcΔT how can we find ΔH?
ΔH = Q/number of moles
When using ΔH = Q/number of moles, what must we be careful about?
Whether the reaction is exothermic or endothermic. If its exothermic add a negative in front of Q, if it’s endothermic leave it positive
How do we calculate enthalpy change using q = mcΔT?
1) Write out the formulas required 2) Write down the values you have been given 3) Convert anything if necessary 4) Input numbers into the formula 5) Divide by 1000 to get from J to kJ 6) Watch out for the type of reaction (Exo/endo)
Before using the ΔH = Q/number of moles formula, what must you do?
- Establish whether the reaction is exothermic or endothermic and add its sign to Q - Convert J to kJ (divide by 1000)
Define average bond enthalpy
The mean energy needed for one mole of a given type of gaseous bond to undergo homolytic fission
Why are calculated bond enthalpies different from the actual bond enthalpies?
They are different because, bond enthalpies used in calculations are an average. The actual bond enthalpy will vary depending on the rest of the chemical.
What are the 3 steps of every reaction?
Step 1) Bonds break Step 2) Atoms rearrange Step 3) Rearranged atoms form bonds together
Is bond-breaking endothermic or exothermic?
Endothermic
Is bond-making exothermic or endothermic?
Exothermic
If you need more energy to break bonds than is released when making bonds, the reaction is ….
Endothermic
If you need more energy to make bonds than to break bonds, the reaction is ….
Exothermic
How do you calculate enthalpy change from average bond enthalpies?
1) Balance the equation 2) Calculate the sum of the average bond enthalpies for reactants 3) Calculate the sum of the average bond enthalpies for products 4) Input these figures into the formula : ΔH = Enthalpy of REACTANTS- Enthalpy of PRODUCTS
What formula do we use when working out enthalpy change from average bond enthalpies?
ΔH = The sum of the enthalpies of REACTANTS - The sum of the enthalpies of PRODUCTS
Define Hess’ law
A chemical law that states that ‘ the enthalpy change in a chemical reaction is independent of the route taken2
Define enthalpy change of formation
The enthalpy change when one mole of compound is formed from its elements in their standard states under standard conditions
In the Enthalpy change of formation, Hess’ law version, which ways are the arrows point?
Upwards
Define enthalpy change of combustion
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
In the Enthalpy change of combustion, Hess’ law version, which ways are the arrows point?
Downwards
What is the formula to find enthalpy change of formation from an enthalpy change of formation Hess’ law cycle?
Enthalpy change of formation = The SUM of the enthalpy change of formation of the products - The SUM of the enthalpy change of formation of the reactants
What is the formula to find enthalpy change of formation from an enthalpy change of combustion Hess’ law cycle?
Enthalpy change of formation = The SUM of the enthalpy change of combustion of the reactants- The SUM of the enthalpy change of combustion of the products
Explain how concentration affects the rate of reaction
If you increase the concentration of reactants in a solution, the particles will become closer together, on average. As particles are closer, they’ll collide more frequently If there are more collisions, the rate of reaction will increase. Therefore, an increase in concentration lads to an increase in the rate of the reaction
Explain how pressure (gases only) affects the rate of reaction
When the pressure of a gas is increased, the gaseous molecules are pushed closer together. As particles are closer together, they’ll collide more frequently and more successfully. If there are more successful collisions, the rate of reaction will increase. Therefore, an increase in pressure leads to an increase in the rate of reaction.
Explain how surface area affects the rate of reaction
Using several smaller pieces of a substance instead of one large piece increases the surface area. This means that more collisons can happen , so the rte of reaction increases.
Explain how temperature affects the rate of reaction
Using a higher temperature solution means the particles of reactants in the solution have more energy. This means more particles move faster and can collide ore successfully and more often. Therefore, an increase in temperature will lead to a higher rate of reaction
What does the collision theory state?
For a reaction to occur : - Particles must collide - Particles that collide, must collide with ENOUGH ENERGY and at the CORRECT ORIENTATION
How can we increase the rate of reaction
Increasing pressure Increasing concentration Using a catalyst Increasing temperature Increasing surface area
How can we measure the reaction rate?
- Measure the time taken to produce a gas - Measure change in mass over time - Measure time it takes for a colour change - Measure time it takes for a solution to go cloudy.
How do we find a gradient?
Change in y/Change in x
How do we find a gradient from a curved graph?
1) Draw a tangent at the point 2) Elongate it and do change in y/change in x
What is meant by a catalyst?
A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy. The catalyst is NOT used up in the process
Explain the use of catalysts
Catalysts are very useful because they lower the activation energy of a reaction by providing another pathway for bonds to be broken and remade. If the activation energy is lower, it means that more particles will have enough energy to react. Therefore, catalysts increase the rate of reaction
What is meant by a heterogeneous catalyst?
When the catalyst used in a reaction in a different phase from the products
What is meant by a homogeneous catalyst?
When the catalyst used in a reaction in a same phase from the products
Why are catalusts important?
Catalysts are important because they allow for lower temperatures to be used in a reaction, so reduced the demand for energy. This reduction in the demand for fossil fuels will mean that the supply of fossil fuels will also fall, meaning that less fossil fuels will be burnt as a result. Therefore, less CO2 is produced, so the reaction becomes sustainable
What are the benefits of using a catalyst?
Reduced demand for fossil fuels so less CO2 emissions Coal is a fossil fuel, so less usage will allow it to replenish over time. - Greater atom economy
Describe the techniques and procedure used to investigate reaction rates (change in mass)
1) When the product is a GAS, its formation can be measured by using a MASS BALANCE 2) The amount of product formed is the mass DISAPPREARING from the container 3) When the reaction starts, you should start a STOP CLOCK or timer. Then take measurements at regular time intervals 4) Make a table with a column “time” and a column for “mass” and fill it in as the reaction goes on 5) When the gas’ mass stops decreasing, the reaction has finished.
Describe the techniques and procedure used to investigate reaction rates (volume of a gas given off)
1)You can use a gas syringe to measure the volume of product formed 2) When the reaction starts, you should start a STOP CLOCK or timer. Then take measurements at regular time intervals 3) Make a table with a column “time” and a column for “mass” and fill it in as the reaction goes on 4) When the gas’ mass stops decreasing, the reaction has finished.
What is a Boltzmann distribution curve?
A curve showing the distribution of energies of molecules at a particular temperature
Outline the key features of a Boltzmann distribution curve
- No molecules have zero energy - Most molecules possess an average amount of energy - Some molecules have MORE energy than the activation energy, and so can react
What will increasing temperature do to the shape of the boltzmann distribution curve?
Shifts it to the right Bring the peak down to a higher energy level but with less molecules at that level
What will happen if we increase temperature on a Boltzmann Distribution curve?
- A greater proportion of molecules will have a higher energy level than the activation energy and so will be able to reaction, increasing the rate of reaction . - Increases the spread of the distribution (broader and flatter)
When we increase the temperature on a Boltzmann distribution curve, what stays the same?
The AREA under the curve
What will happen if we add on a catalyst to the Boltzmann Distribution curve?
Catalysts will lower the activation energy of the reaction. This is because they provide an alternative route for bonds to be broken and remade. If the activation energy is lower, it means that, more particles will have enough energy to react. Therefore, catalysts increase in the rate of reaction.
Define dynamic equilibrium
A state where the RATE of the forward reaction is equal to the RATE of the backwards reaction.
At dynamic equilibrium, what stays the same?
The concentrations of the reactants and the products
Does dynamic equilibrium happen in a closed on open system?
CLOSED
What factors affect where equilibrium lies?
Concentration Pressure Temperature
Define Le Chatelier’s principle
A chemical law that states “IF THE CONDITIONS OF A REACTION CHANGE, THE POSITION OF EQUILIBRIUM FOR A REACTION WILL RESPOND BY ALSO CHANGING, TO OPPOSE THE INITIAL CHANGE”
What factors can cause the position of equilibrium to change?
Concentration Pressure Temperature
If we increase temperature, what happens to equilibrium?
It shifts it in the ENDOTHERMIC direction. This is because equilibrium works to counteract an increase in temperature bu absorbing the extra heat.
If we decrease temperature, what happens to equilibrium?
It shifts it in the EXOTHERMIC direction. This is because equilibrium works to counteract a decrease in temperature by releasing more heat.
When the forward reaction is endothermic, what effect will increasing temperature have on the position of equilibrium?
Increasing the temperature when the forward reaction is ENDOTHERMIC, results in more forwards reaction. Therefore, the position of equilibrium shifts to the RIGHT to oppose this change.
When the forward reaction is endothermic, what effect will decreasing temperature have on the position of equilibrium?
Decreasing the temperature when the forwards reaction is ENDOTHERMIC, results in more backwards reaction. Therefore, the position of equilibrium shifts to the LEFT to oppose this change.
When the forward reaction is exothermic, what effect will increasing temperature have on the position of equilibrium?
Increasing the temperature when the forwards reaction is EXOTHERMIC, results in more backwards reaction. Therefore, the position of equilibrium shifts to the LEFT to oppose this change.
When the forward reaction is exothermic, what effect will decreasing temperature have on the position of equilibrium?
Decreasing the temperature when the forwards reaction is EXOTHERMIC, results in more forward reaction. Therefore, the position of equilibrium shifts to the RIGHT to oppose this change.
If we increase pressure, what affect will this have on the position of equilibrium?
Increasing the pressure, shifts the position of equilibrium to the side with FEWER gaseous moles. This works to reduce the pressure of the system
If we decrease pressure, what affect will this have on the position of equilibrium?
Decreasing the pressure, shifts the position of equilibrium to the side with MORE gaseous moles. This is because equilibrium works to raise the pressure in the system.
If we increase the concentration of a reactant, what happens to the position of equilibrium and why?
Increasing the concentration of a reactant results in the equilibrium making more product, to oppose this change. So equilibrium will shift to the RIGHT
If we decrease the concentration of a reactant, what happens to the position of equilibrium and why?
Decreasing the concentration of a reactant results in the equilibrium shifting to make the number of reactants = the number of products. So equilibrium will shift to the LEFT
If we increase the concentration of a product, what happens to the position of equilibrium and why?
Increasing the concentration of a product results in the equilibrium making more reactant to get rid of the extra product. So equilibrium shifts to the LEFT
If we decrease the concentration of a product, what happens to the position of equilibrium and why?
Decreasing the concentration of a product results in the equilibrium making more product, to make the number of reactants = the number of product. Therefore, equilibrium shifts to the RIGHT.
Compromise
INSERT
What is Kc?
The equilibrium constant
To use Kc, the reaction must be….
Homogeneous (reactants and products in the same physical state
What is the formula for Kc?
Kc = ([C]^c[D]^d) / [A]^a[B]^b) In square brackets = the concentration values Lower case letters = mole number KEY NOTE : Mole number is the power.
When using Kc, how do you find out the units?
Use a cancellation method
A value of Kc=1 means?
The position of equilibrium is HALFWAY between the reactants and the products
A value of Kc>1 means?
The reaction is product favoured, so equilibrium shifts to the right.
A value of Kc<1 means?
The reaction is reactant favoured, so equilibrium shifts to the left .
