Energetics Flashcards
What is enthalpy change?
The heat energy change at constant pressure
What is the enthalpy change in an
a) endothermic
b) exothermic
Reaction?
a) positive
b) negative
At what temperature does standard enthalpy change occur at?
What pressure?
298K
100kPa
What is standard enthalpy of combustion?
The enthalpy change when 1 mole if a substance
Is completely burnt in Oxygen
Under standard conditions, all substances in standard states
What is standard enthalpy of formation?
The enthalpy change when 1 mole of a compound
is formed from its elements
Under standard conditions, all substances in standard states
What is the enthalpy of formation for an element
Zero by definition
State Hess’ Law
The enthalpy change in a reaction is independent if the route taken, and depends only in the initial and final states
What does
q=mc🔺t
Stand for?
q= heat energy (J)
m= mass of water (g; density of water is 1g/cm3)
c=specific heat capacity of water (4.18JK-1g-1)
🔺t= change in temperature
The q in 🔺H=q/n is in what unit?
kJ
What is the method for Calorimetry?
- Mass of spirit burner and fuel with cap on is recorded
- a known vol. of water is placed in a beaker and temp is recorded
- light fuel to heat water - ensure it is close to beaker and protected by heat-proof guards to minimise heat loss. Stir water.
- when temp has increased aptly, extinguish flame by placing lid on flame
- keep monitoring temp and record highest reached
- re-weigh spirit burner with lid on and calculate the mass of fuel burnt
What are the two reasons the value obtained maybe be less than that in the data book?
Heat loss to surroundings
Incomplete combustion
Describe the method for calorimetry using two solutions
Pour aqueous reactant into polystyrene cup and record temp every minute for 3 mins to ensure solution is at room temperature
On 4th minute, add the other reactant and stir. Do not record temp on 4th min
Record temperature every minute after that up to 10 mins.
Why do you stir the solution in calorimetry?
To ensure the reactants mix thoroughly fit complete reaction
To ensure temperature is even throughout solution
Why would values be slightly inaccurate in laboratory calorimetry with two solutions?
Heat loss to surroundings
Incomplete reaction
Define ‘mean bond enthalpy’
Enthalpy required to break 1 mole of covalent bonds averaged over a range of compounds
Why is mean bond enthalpy not 100% accurate?
The values are average values from a range of compounds
What is the equation for the enthalpy change of a reaction (bond enthalpy)?
🔺H= (bonds broken) - (bonds formed)
Define activation energy
The minimum amount of energy required for the reaction to occur
Why increase the frequency of collisions
Increasing frequency of collisions increases rate of reaction but the percentage of successful collisions will remain the stay
Define rate of reaction
Change in concentration
————————————-
Time
How can you increase frequency of collisions
Increasing concentration of reagents
Increasing surface area
Increasing pressure
Increasing temperature
How does increasing concentration increase rate
It increasing number of particles in a given volume and so increases frequency of collisions, therefore there are more successful collisions in a given time
When is the rate at its maximum
At the start of the reaction as Reagents are used up and their concentration falls
Two facts about the graph which shows how concentration varies over time
Gradient is negative at the start of reaction
Gradient falls to zero at the completion of the reaction
How would a graph for a reaction with 2x moles of reactants and higher concentration compare to a standard reaction
It would be twice the height, with a steeper gradient due to the increased rate of reaction
How would the graph of a reaction with a lower concentration but same moles compare to that of a standard reaction
Same height, with a less steep gradient due to the decreased rate of reaction
How would a graph of a reaction with half the moles, carried out at a higher temperature with a catalyst compare to a standard reaction?
It would be half the height, with a steeper gradient due to its increased rate
How is surface area increased for a solid?
Break it into smaller pieces
How does increasing the surface area increase rate?
More collisions can occur therefore there are more successful collisions in a given time
How can pressure be increased without changing the temperature
Using a smaller reaction vessel
How does volume affect pressure
A larger volume creates a lower pressure
A smaller volume creates a higher pressure
Other than decreasing volume, how else can pressure be increased?
Increasing the number of molecules in the same volume reaction vessel
What does an increase in pressure mean?
That there are more particles in. A given volume, therefore there are more successful collisions in a given time
What factor has the biggest l effect on rate
Why
Temperature
It increases number of collisions and percentage of successful collisions in a given time
How does increasing temperature increase percentage of successful collisions
Particles have more energy at a high temperature so move faster and so move collisions occur and there are more successful collisions in a given time
Why do particles have different energies
The collisions are random and so energy is transferred randomly
What does the Maxwell-Boltzmann curve show
The distribution of the energies of particles
Number of molecules(y) vs. Energy (x)
What is the peak of the curve?
Most probable energy
What does the area under the Maxwell-Boltzmann curve show
Total number of particles in the system
What changes and what stays the same in a Maxwell-Boltzmann curve when the temperature is increased?
Total area underneath stays the same - total number of particles stays the same
At a higher temperature the curve shifts to the right and the peak is lower
MANY more particles have energy>Ea
Why can a small temperature increase result in a large increase in rate
Many more particles have energy>Ea
What happens to a Maxwell-Boltzmann curve if the temperature is decreased
Curve shifts left and peak is higher
Much fewer particles have energy>Ea
What kind of curve should be drawn on a rate/time graph?
Exponential
Define catalyst
A substance that speeds up the rate of reaction
But is not used up
How does a catalyst work?
By providing an alternative route which has a lower Ea than the uncatalysed route
How does a catalyst affect enthalpy change
It has no effect so 🔺H is equal for both reactions
How does a catalyst increase rate?
More molecules are able to react as Ea is lowered
If a question asks about the behaviour of particles in an equilibrium what must you reference
Kinetics, rate, and the time taken to reach equilibrium
If a question asks about a catalyst what must always be included in the answer
How the number of successful collisions in a given time is affected and what this means for the rate
What does area under the curve represent
Total number of molecules IN SAMPLE
Explain the process that causes some molecules in this sample to have very low energy
Collisions cause some molecules to lose energy
If asked to draw a Maxwell Boltzman curve what measurement must you take?
Must level off less than 5mm from x axis
