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as-level chemistry > Physical Chemistry > Flashcards

Physical Chemistry Flashcards

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1
Q

What is chemical energy?

A
  • Potential energy stored in chemical bonds
  • When these bonds break or when these bonds form in reactions the chemical energy of the substances changes.
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2
Q

What is enthalpy?

A
  • Thermal energy that is stored in a chemical system.
  • It is impossible to measure directly but can be measured by the energy absorbed or released to the surroundings during a chemical change (normally temperature change)
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3
Q

What is meant by a ‘system’?

A
  • The system is the actual chemical reaction (the atoms and bonds involved)
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4
Q

What is meant by ‘surroundings’?

A
  • Everything else
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5
Q

What is meant by the conservation of energy?

A
  • Energy is not destroyed or created only transferred into another form.
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6
Q

What is enthalpy change?

A
  • the heat exchange with the surroundings during a chemical reaction, at a constant pressure
  • the difference between the enthalpy of the products and the enthalpy of the reactants
  • H change = H products - H reactants
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7
Q

What happens in an exothermic reaction?

A
  • Enthalpy of the products is smaller than the enthalpy of the reactants
  • there is a heat loss from the chemical system to the surroundings
  • Enthalpy change is negative
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8
Q

What happens in an endothermic reaction?

A
  • Enthalpy of the products is greater than the enthalpy of the reactants
  • There is a heat gain to the chemical system from the surroundings
  • Enthalpy change is positive
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9
Q

What is activation energy?

A
  • The minimum energy required to start a reaction by breaking bonds in the reactants.
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10
Q

What are standard conditions?

A
  • 100 kPa
  • 273 K
  • All substances are in their natural states (the most stable form)
  • The are given the plimsoll line symbol (circle with line through it)
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11
Q

What is the enthalpy change of formation?

A
  • Is the energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
  • The standard enthalpy change of formation for an element in its standard state is 0kJmol -1 because according to the definition there is no change (as no compound is formed), so no energy is released or taken in.
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12
Q

What is the enthalpy change of combustion?

A
  • Energy change that takes place when 1 mole of a substance is completely combusted.
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13
Q

What is the enthalpy change of neutralisation?

A
  • Energy change of neutralisation is the energy change associated with the formation of 1 mole of water from a neutralisation reaction.
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14
Q

What are the factors effecting rate of reaction?

A
  • Temp
  • Conc
  • Pressure
  • Surface area
  • Catalyst
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15
Q

Effect of conc on RofR

A
  • Increased concentration means more molecules in the same volume.
  • The molecules will be closer together and so there is a greater chance of them colliding with sufficient energy to overcome the activation energy.
  • Collisions more frequent
  • Therefore RofR increases
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16
Q

Effect of pressure on RofR

A
  • Increased pressure means molecules are pushed closer together.
  • The same number of molecules in a smaller volume
  • More collisions are likely to occur with sufficient energy to overcome the concentration.
  • Therefore the RofR increases
17
Q

What is a catalyst?

A
  • A substance that increases the rate of a reaction without being used up during the process.
  • It lowers the activation energy of the reaction by providing an alternative route for the reaction with a lower activation energy.
18
Q

What is a heterogenous catalyst?

A
  • A catalyst which is in a different phase as the reactants.
19
Q

Effect of temperature on the RofR

A
  • Temp increases, kinetic energy increases and molecules move faster
  • more frequent successful collisions as a result
  • a higher proportion of molecules have an energy that is greater than the activation energy
20
Q

What is a homogenous catalyst?

A
  • A catalyst which is in the same phase as the reactants
21
Q

What is the equation for finding the molar enthalpy change of a substance?

A
  • Q = m * c * change in T
  • When Q is the heat exchanged with the surroundings expressed in joules or kilojoules.
  • When m is the mass of the substance in grams.
  • When C is the specific heat capacity of the substance. C means the energy needed to raise the temperature of 1 gram of a substance by one kelvin and is measured in J / g / K
  • To find the molar enthalpy change you must then divide the result by the moles of the product that has reacted.
22
Q

Describe the Haber process

A
  • N2 (g) + 3H2 (g) <=> 2NH3 (g)
  • Iron catalyst
  • Heterogenous
  • 450 degrees, 200atm
23
Q

What does a Boltzmann distribution diagram show?

A
  • Energy on x axis and number of molecules on y axis
  • Shows that there are some molecules that are faster moving and have high energy
  • Shows that there are some molecules that are slower moving and have a low energy
  • Shows that the majority of molecules have an average energy.
  • The area underneath the graph shows the total number of molecules in the sample.
  • There are no molecules in the system with zero energy. Curve doesn’t start at origin but slightly above
  • There is no maximum energy for a molecule so curve never touches zero.
24
Q

How would changing the temperature in a system change the shape of the curve in a Boltzmann distribution diagram?

A
  • The area underneath the graph would have to stay equal
  • The activation energy also does not change.
  • Instead the peak moves to a higher energy with a lower height with a higher temperature and as a result there is a greater proportion of molecules which exceed the Activation Energy.
  • At a lower temperature, the peak would have be at a lower temperature and would be at a higher height.
25
Q

Effect of a catalyst on the shape of the Boltzmann distribution curve

A
  • Same shape of curve
  • Lower activation energy
  • Greater proportion of molecules can react
26
Q

What is Hess’ law?

A
  • Hess’ law states that the enthalpy change in a chemical reaction is independent of the route it takes.
27
Q

What does ‘dynamic equilibrium’ mean?

A
  • There is no observable change.
  • Dynamic -> it is in constant motion.
  • Equilibrium -> As fast as the reactants are converted into products, the products are being converted back into reactants.
28
Q

When is a chemical system in dynamic equilibrium?

A
  • The concentrations of the reactants and the products remain constant.
  • the rate of the forward reaction is the same as the rate of the reverse reaction
  • The system must be isolated
29
Q

What factors effect equilibrium?

A
  • concentration of the reactants or products
  • pressure in reactions involving gases
  • temperature
30
Q

What effect does increasing the concentration of a reactant have on the equilibrium?

A
  • Increasing the concentration of a reactant causes the equilibrium to shift in the direction that decreases this reactant’s concentration:
  • The system opposes the change by decreasing the concentration of the reactant by removing it
  • The position of equilibrium moves to the side with the lowest concentration.
31
Q

What effect will increasing the concentration of products have on the equilibrium?

A
  • Causes the equilibrium to shift in the direction that decreases this product’s 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.
32
Q

What effect will increasing pressure have on the equilibrium?

A
  • Will only change the position of equilibrium if there are gases present
  • The side with more moles of has is the side at a higher pressure.
  • It hence causes the equilibrium to shift to the side with fewer has molecules as this will decrease the pressure
33
Q

What effect will adding a catalyst have on the equilibrium?

A
  • No effect on the equilibrium.
  • The catalyst will increase the rate of reaction of both sides of the reaction by the same amount
34
Q

What is the equation for Kc? (the equilibrium constant)

A
  • Kc = Concentration of each product to the power of how many moles there are of it multiplied together / Concentration of each reactant to the power of how many moles there are of it multiplied together
35
Q

What does the value of Kc say about the equilibrium position?

A
  • When Kc is >1 there are more products than reactants and the equilibrium lies to the right.
  • When Kc is <1 there are more reactants than products and the equilibrium lies to the left.
  • When Kc is = 1 the equilibrium lies halfway between reactants and products.
36
Q

What is Kc?

A
  • It is the equilibrium constant -> a numerical value to gage the position of the equilibrium at a specific temperature.
37
Q

What is Hess’ law?

A
  • States that the enthalpy change in a chemical reaction is independent of the route it takes
38
Q

What is an enthalpy cycle?

A
  • A pictorial representation showing alternative routes between reactants and products

as-level chemistry (4 decks)

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