Chapter 5,6,7 SL: Thermochemistry

Question 1

What is heat?

Answer 1

Heat is the transfer of energy between objects of varying temperatures, and also a measure of the total energy of a substance, therefore depending on the amount of substance present.

Question 2

What is temperature?

Answer 2

Temperature is a measure of the average kinetic energy of particles in a substance.

Question 3

What are the laws of Thermodynamics?

Answer 3

The Law of Conservation of energy; it cannot be created or destroyed - only transferred from one form to another.
Heat will spontaneously flow from an object of greater temp until both objects reach thermal equilibrium.

Question 4

What is absolute temperature measured in and is proportional to?

Answer 4

Absolute temp is measured in Kelvin and is directly proportional to the average K.E of particles in a sample.

Question 5

What is enthalpy and the standard enthalpy change of reaction?

Answer 5

Enthalpy is the heat energy releases/absorbed by a system at a constant pressure.
The standard enthalpy change of reaction occurs at standard conditions 100 kPa and 298K.

Question 6

What is the difference between open, closed and isolated systems?

Answer 6

In open systems, matter and energy can move freely between the system and its surroundings, which includes the reaction mixture.
In a closed system, only energy can move between the system and its surroundings.
In an isolated system, no matter or energy can move between the system and its surroundings.

Question 7

What are exothermic reactions, give some examples and what is the enthalpy change?

Answer 7

when a chemical reaction transfers heat energy to its surroundings.
Examples: Combustion + Neutralisation reactions
The enthalpy change is negative, as energy is transferred from the system to the surroundings.
Products of reaction have a lower enthalpy, thus are more energetically stable.

Question 8

What are endothermic reactions, give some examples and what is the enthalpy change?

Answer 8

When a chemical reaction absorbs heat energy from its surroundings.
Examples: Thermal Decomposition of calcium carbonate
The enthalpy change is positive, as energy is transferred from the surroundings to the system.
Products of the reaction have a higher enthalpy, thus are less energetically stable.

Question 9

What is Hess’ Law?

Answer 9

The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same.

Question 10

What is the enthalpy change of formation?
(ΔHF) or (ΔH⦵F )

Answer 10

The enthalpy change when one mole of a compound is formed from the elements in their standard states under standard conditions.

Question 11

What is the molar enthalpy change of combustion?
(ΔHƟC)

Answer 11

The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 12

What is the enthalpy change of neutralization?
(ΔHN)

Answer 12

The enthalpy change when an acid and base react together to form one mole of water.

Question 13

What happens during bond breaking and bond formation in exo/endothermic reactions?

Answer 13

Bond breaking is endothermic whereas bond formation is exothermic.
Exothermic Reaction: energy released by products forming > energy needed absorbed while breaking of reactant bonds.
opposite in endothermic reactions

Question 14

What is bond enthalpy (H) / bond dissociation energy (E) ?

Answer 14

Bond enthalpy (H) or Bond dissociation energy (E): It is defined as the energy required to break one mole of chemical bonds in the gaseous state.
The energy required to break a bond is equal to the energy released by the bond forming.

Question 15

What is the average bond enthalpy and why is it taken?

Answer 15

The enthalpy change when one mole of bonds are broken in the gaseous state averaged for the same bond in similar compounds.
As the same type of chemical bond has different bond enthalpies in different compounds and identical bonds in molecules with two (or more) types of bond also have different bond enthalpy values.

Question 16

What is collision theory and what are its two conditions?

Answer 16

Collison theory states how chemical reactions can only occur once reacting particles collide with each other and also follow:
Collision with the correct orientation
Sufficient energy to overcome the energy barrier, i.e the activation energy (EA)

Question 17

What is the activation energy (EA)

Answer 17

The minimum energy required for a chemical reaction to take place

Question 18

What is the transition state on an enthalpy change diagram?

Answer 18

The Transition State is the highest energy state on a reaction coordinate; the point at which bonds are being both broken and formed.

Question 19

What does a Maxwell-Boltzmann distribution curve show?

Answer 19

In an ideal gas, the K.E of molecules is spread over a range of values
total area under the curve = the total number of particles in a sample.
The area under the curve is also directly proportional to the number of molecules having a value of K.E in that range.
The most probable K.E occurs at the maximum point of the curve.

Question 20

How will a Maxwell-Boltzmann curve represent gases with the same temp?

Answer 20

At the same temperature, all gases have the same average K.E, thus the distribution of the curve is dependant on the molar mass.
Gas particles with lower molar masses have on average higher speeds such gases with heavier molar masses.
Therefore will have flatter curves with peaks being further rightwards.

Question 21

How will a Maxwell-Boltzmann curve represent gases with increasing temp?

Answer 21

As the temperature increases, the curve flattens out as particles gain K.E and the most probable K.E increases, however the area under the curves remains constant as the number of particles is the same, however a higher proportion surpass the EA.

Question 22

What is the rate of reaction?

Answer 22

The rate of reaction is the change in the concentration of a reactant or product per unit time (mol dm-3 s-1)

Question 23

Why is temp a factor that affects the rate of reaction?

Answer 23

Increasing the temperature increases the rate of reaction through:
Increasing the frequency of collisions as particles move faster due to increased K.E at higher temperatures
At higher temperatures, a greater proportion of particles will collide with energy greater or equal to the activation energy.
For most reactions, an increase by 10 C, doubles the rate of reaction.

Question 24

Why is concentration/pressure a factor that affects the rate of reaction?

Answer 24

An increase in conc/pressure increases the rate of reaction due to increased frequency of collisions between reactant particles as there are a greater amount of particles per unit volume.
An increase in collisions results in an increased chance of successful collisions.

Question 25

Why is surface area a factor that affects the rate of reaction?

Answer 25

Increasing the surface area to volume ratio allows for more reactant particles to be exposed for collision in the reaction mixture, thus increasing the rate of reaction.

Question 26

Why does the rate of reaction vary with time?

Answer 26

At the beginning of a reaction, the gradient is the steeped (the rate is the fastest) due to the concentration of reactants being at its highest - increased frequency of collisions. This is also why why the average rate of reaction differs from the instantaneous rate at a given time.

Question 27

Why does a catalyst affect the rate of reaction?

Answer 27

Catalysts increase the rate of reaction through providing an alternate energy pathway with lower activation energy than the uncatalysed pathway, thus a greater proportion of particles can collide with sufficient energy for the reaction to occur. A catalyst remains chemically unchanged at the end of a reaction and can be sometimes reused.

Question 28

How does a catalyst affect the Maxwell-Boltzmann distribution curve?

Answer 28

It decreases the activation energy required therefore a greater area (proportion) of particles will have sufficient energy to collide successfully.

Question 29

What is dynamic equilibrium?

Answer 29

The forwards and backwards reactions occur at the same rate. The concentration of both reactants and products remain constant. While the concentrations may remain constant, the are not equal as in most reactions, the position of equilibrium favors either the forward or backward reaction.

Question 30

What are some conditions for equilibrium?

Answer 30

Equilibrium can only occur in a closed system where the products and reactants cannot escape. At equilibrium, there is no change in macroscopic properties such as colour, pH, pressure or concentration.

Question 31

What is physical equilibrium?

Answer 31

Equilibrium also occurs in closed systems where physical changes occur. The rate of the forward process is equivalent to that of the backward.

Question 32

Why is Kc temp dependent?

Answer 32

The equilibrium constant is the value given for a particular reaction at a constant temp, therefore is temperature dependent and would differ at different temperatures for the same reaction.

Question 33

How does the magnitude of Kc indicate the progress of the reaction?

Answer 33

If the Kc value is high, this indicates that the reaction has neared completion and a high proportion of reactants have been converted to products. (P.O.E lies to the right) If the Kc value is low, the reaction has barely proceeded in the forwards direction and only a small proportion of the reactants have been converted to products.(P.O.E lies to the left)

Question 34

What can we tell about the concentration of the reactants when Kc = 1?

Answer 34

There is an equal concentration of products and reactants at equilibrium. any values greater/lower indicate that products/reactants predominate at equilibrium

Question 35

What happens to Kc if the reaction is reversed or the coefficients are doubled?

Answer 35

If the reaction is reversed, the value of Kc will be the reciprocal of the value for the forwards reaction. If the coefficients are doubled, the value of Kc will be squared. If they are halved, the square root of Kc is taken.

Question 36

What is Qc?

Answer 36

Reaction quotient that is a measure of the relative amounts of products and reactants for a reaction that is not at equilibrium. It can be used to predict the direction the reaction will take to reach equilibrium by comparing to Kc.

Question 37

What does Le Chatelier's principle suggest?

Answer 37

When a closed system at equilibrium is subjected to change, the system will respond to minimise the effect of the change. The change results in a shift in the position of equilibrium, resulting in the forward/backward reaction being favoured in order to minimise the effect of the change.

Question 38

What will happen to Kc and Q if the conditions of a closed system at equilibrium are changed?

Answer 38

The values of Kc and Q will also change, meaning that the reaction is no longer at equilibrium until the shift in position of equilibrium results in the two values becoming equal at a new equilibrium. Changes in temperature cause a change in the value of K C whereas pressure and concentration do not.

Question 39

How can a change in pressure be caused to a system in equilibrium and what will happen?

Answer 39

Any change in pressure will only have an effect if the reactants/products are in a gaseous state. The pressure of a system at equilibrium can be changed by: Adding/removing a gaseous reactant/product Changing the volume of the reactor vessel, due to Boyle’ Law; volume and pressure are inversely related. An increase in pressure will result in the position of equilibrium shifting to favor the reaction which produces the least molecules of gaseous particle and vice versa.

Question 40

What will happen to the position of equilibrium if temperature is changed?

Answer 40

If the temperature is increased: The position of equilibrium will shift to favor the reaction that is endothermic. Whereas if the temperature is decreased: The position of equilibrium will shift to favor the reaction that is exothermic.

Question 41

What will happen to Kc if the temp is changed?

Answer 41

For an exothermic reaction, increasing the temperature will shift the equilibrium position to the left - lowering the value of KC. For an endothermic reaction, an increase in temperature will result in a shift of the position of equilibrium to the right - increasing the value of KC.

Question 42

What is the effect of a catalyst on Kc and the position of equilibrium?

Answer 42

Adding a catalyst will not result in a shift in equilibrium and KC as it increases the rate of the forward/backward reactions by the same amount