Energetics

Question 1

What is enthalpy change?

Answer 1

The heat energy change at constant pressure

Question 2

What is the enthalpy change in an

a) endothermic
b) exothermic

Reaction?

Answer 2

a) positive

b) negative

Question 3

At what temperature does standard enthalpy change occur at?

What pressure?

Answer 3

298K

100kPa

Question 4

What is standard enthalpy of combustion?

Answer 4

The enthalpy change when 1 mole if a substance
Is completely burnt in Oxygen
Under standard conditions, all substances in standard states

Question 5

What is standard enthalpy of formation?

Answer 5

The enthalpy change when 1 mole of a compound
is formed from its elements
Under standard conditions, all substances in standard states

Question 6

What is the enthalpy of formation for an element

Answer 6

Zero by definition

Question 7

State Hess’ Law

Answer 7

The enthalpy change in a reaction is independent if the route taken, and depends only in the initial and final states

Question 8

What does
q=mc🔺t
Stand for?

Answer 8

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

Question 9

The q in 🔺H=q/n is in what unit?

Answer 9

kJ

Question 10

What is the method for Calorimetry?

Answer 10

• 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

Question 11

What are the two reasons the value obtained maybe be less than that in the data book?

Answer 11

Heat loss to surroundings

Incomplete combustion

Question 12

Describe the method for calorimetry using two solutions

Answer 12

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.

Question 13

Why do you stir the solution in calorimetry?

Answer 13

To ensure the reactants mix thoroughly fit complete reaction

To ensure temperature is even throughout solution

Question 14

Why would values be slightly inaccurate in laboratory calorimetry with two solutions?

Answer 14

Heat loss to surroundings

Incomplete reaction

Question 15

Define ‘mean bond enthalpy’

Answer 15

Enthalpy required to break 1 mole of covalent bonds averaged over a range of compounds

Question 16

Why is mean bond enthalpy not 100% accurate?

Answer 16

The values are average values from a range of compounds

Question 17

What is the equation for the enthalpy change of a reaction (bond enthalpy)?

Answer 17

🔺H= (bonds broken) - (bonds formed)

Question 18

Define activation energy

Answer 18

The minimum amount of energy required for the reaction to occur

Question 19

Why increase the frequency of collisions

Answer 19

Increasing frequency of collisions increases rate of reaction but the percentage of successful collisions will remain the stay

Question 20

Define rate of reaction

Answer 20

Change in concentration
————————————-
Time

Question 21

How can you increase frequency of collisions

Answer 21

Increasing concentration of reagents
Increasing surface area
Increasing pressure
Increasing temperature

Question 22

How does increasing concentration increase rate

Answer 22

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

Question 23

When is the rate at its maximum

Answer 23

At the start of the reaction as Reagents are used up and their concentration falls

Question 24

Two facts about the graph which shows how concentration varies over time

Answer 24

Gradient is negative at the start of reaction

Gradient falls to zero at the completion of the reaction

Question 25

How would a graph for a reaction with 2x moles of reactants and higher concentration compare to a standard reaction

Answer 25

It would be twice the height, with a steeper gradient due to the increased rate of reaction

Question 26

How would the graph of a reaction with a lower concentration but same moles compare to that of a standard reaction

Answer 26

Same height, with a less steep gradient due to the decreased rate of reaction

Question 27

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?

Answer 27

It would be half the height, with a steeper gradient due to its increased rate

Question 28

How is surface area increased for a solid?

Answer 28

Break it into smaller pieces

Question 29

How does increasing the surface area increase rate?

Answer 29

More collisions can occur therefore there are more successful collisions in a given time

Question 30

How can pressure be increased without changing the temperature

Answer 30

Using a smaller reaction vessel

Question 31

How does volume affect pressure

Answer 31

A larger volume creates a lower pressure | A smaller volume creates a higher pressure

Question 32

Other than decreasing volume, how else can pressure be increased?

Answer 32

Increasing the number of molecules in the same volume reaction vessel

Question 33

What does an increase in pressure mean?

Answer 33

That there are more particles in. A given volume, therefore there are more successful collisions in a given time

Question 34

What factor has the biggest l effect on rate Why

Answer 34

Temperature It increases number of collisions and percentage of successful collisions in a given time

Question 35

How does increasing temperature increase percentage of successful collisions

Answer 35

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

Question 36

Why do particles have different energies

Answer 36

The collisions are random and so energy is transferred randomly

Question 37

What does the Maxwell-Boltzmann curve show

Answer 37

The distribution of the energies of particles | Number of molecules(y) vs. Energy (x)

Question 38

What is the peak of the curve?

Answer 38

Most probable energy

Question 39

What does the area under the Maxwell-Boltzmann curve show

Answer 39

Total number of particles in the system

Question 40

What changes and what stays the same in a Maxwell-Boltzmann curve when the temperature is increased?

Answer 40

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

Question 41

Why can a small temperature increase result in a large increase in rate

Answer 41

Many more particles have energy>Ea

Question 42

What happens to a Maxwell-Boltzmann curve if the temperature is decreased

Answer 42

Curve shifts left and peak is higher | Much fewer particles have energy>Ea

Question 43

What kind of curve should be drawn on a rate/time graph?

Answer 43

Exponential

Question 44

Define catalyst

Answer 44

A substance that speeds up the rate of reaction | But is not used up

Question 45

How does a catalyst work?

Answer 45

By providing an alternative route which has a lower Ea than the uncatalysed route

Question 46

How does a catalyst affect enthalpy change

Answer 46

It has no effect so 🔺H is equal for both reactions

Question 47

How does a catalyst increase rate?

Answer 47

More molecules are able to react as Ea is lowered

Question 48

If a question asks about the behaviour of particles in an equilibrium what must you reference

Answer 48

Kinetics, rate, and the time taken to reach equilibrium

Question 49

If a question asks about a catalyst what must always be included in the answer

Answer 49

How the number of successful collisions in a given time is affected and what this means for the rate

Question 50

What does area under the curve represent

Answer 50

Total number of molecules IN SAMPLE

Question 51

Explain the process that causes some molecules in this sample to have very low energy

Answer 51

Collisions cause some molecules to lose energy

Question 52

If asked to draw a Maxwell Boltzman curve what measurement must you take?

Answer 52

Must level off less than 5mm from x axis