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.
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)
3
Q
What is meant by a ‘system’?
A
- The system is the actual chemical reaction (the atoms and bonds involved)
4
Q
What is meant by ‘surroundings’?
A
- Everything else
5
Q
What is meant by the conservation of energy?
A
- Energy is not destroyed or created only transferred into another form.
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
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
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
9
Q
What is activation energy?
A
- The minimum energy required to start a reaction by breaking bonds in the reactants.
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)
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.
12
Q
What is the enthalpy change of combustion?
A
- Energy change that takes place when 1 mole of a substance is completely combusted.
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.
14
Q
What are the factors effecting rate of reaction?
A
- Temp
- Conc
- Pressure
- Surface area
- Catalyst
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
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
Effect of a catalyst on the shape of the Boltzmann distribution curve
- Same shape of curve
- Lower activation energy
- Greater proportion of molecules can react
26
What is Hess' law?
- Hess' law states that the enthalpy change in a chemical reaction is independent of the route it takes.
27
What does 'dynamic equilibrium' mean?
- 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
When is a chemical system in dynamic equilibrium?
- 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
What factors effect equilibrium?
- concentration of the reactants or products
- pressure in reactions involving gases
- temperature
30
What effect does increasing the concentration of a reactant have on the equilibrium?
- 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
What effect will increasing the concentration of products have on the equilibrium?
- 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
What effect will increasing pressure have on the equilibrium?
- 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
What effect will adding a catalyst have on the equilibrium?
- No effect on the equilibrium.
- The catalyst will increase the rate of reaction of both sides of the reaction by the same amount
34
What is the equation for Kc? (the equilibrium constant)
- 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
What does the value of Kc say about the equilibrium position?
- 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
What is Kc?
- It is the equilibrium constant -> a numerical value to gage the position of the equilibrium at a specific temperature.
37
What is Hess' law?
- States that the enthalpy change in a chemical reaction is independent of the route it takes
38
What is an enthalpy cycle?
- A pictorial representation showing alternative routes between reactants and products
AS-Level Chemistry
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