Module 3: Section 2

Question 1

Define Enthalpy change, and give its symbol and units of measurement.

Answer 1

Heat energy transferred in a reaction at a constant pressure.
Symbol : ΔH
Units : kJmol-1

Question 2

What is a endothermic and exothermic reaction?

Answer 2

Exothermic: energy is released (bond-making)

Endothermic : energy is absorbed (bond-breaking)

Question 3

In a enthalpy profile diagram, where does the arrow for
1. Ea
2. ΔH
get drawn?

Answer 3

Ea = From reactants line to the top of the arch
ΔH = Between the reactants and products line.

Question 4

Define standard enthalpy change of reaction.

Answer 4

Enthalpy change associated with a given reaction, under standard conditions.

Question 5

Define enthalpy of formation.

Answer 5

Enthalpy change when 1 mol of a compound is formed from its constituent elements, under standard conditions.

Question 6

Define enthalpy of combustion.

Answer 6

Enthalpy change when 1 mol of a compound is completely combusted in oxygen, under standard conditions.

Question 7

Define enthalpy of neutralisation.

Answer 7

Enthalpy change when 1 mol of water is formed in a neutralisation reaction, under standard conditions.

Question 8

What are standard conditions?

Answer 8

298K

100kPa

Question 9

Define average bond enthalpy.

Answer 9

Energy needed to break 1 mol of bonds in the gas phase, averaged over many different compounds.

Question 10

When calculating enthalpy changes with q=mcΔt what do they stand for?
Give the units of measurements for each.

Answer 10

q= heat lost or gained (J)
m= mass of surroundings (g)
c= specific heat capacity (4.18Jg-1K-1)
Delta t = change in temp of surroundings (K)

Question 11

What does Hess’s Law State?

Answer 11

Enthalpy change of a chemical reaction is independent of the route taken.

Question 12

Why can calculating the enthalpy of formation of hydrocarbons be difficult?

Answer 12

A mixture of hydrocarbons will likely be made, not the specific hydrocarbon required.

Question 13

How do you calculate the enthalpy change of reaction?

Answer 13

ΔH formation of products
-
ΔH formation of reactants

Question 14

How do you calculate:

1. Bond enthalpy
2. ΔC
3. ΔF

Answer 14

1. BERP
2. CORP
3. FOPR

Question 15

When will a reaction take place between 2 particles

Answer 15

1. They collide in the right direction (angle)

2. They collide with a minimum amount of kinetic energy

Question 16

What is activation energy (Ea)?

Answer 16

Minimum amount of kinetic energy particles need to break bonds and start the reaction.

Question 17

Give 5 factors that affect the speed of chemical reactions.

Answer 17

1. Temperature
2. Concentration
3. Surface Area
4. Pressure
5. Catalyst

Question 18

With a Boltzmann diagram, explain how temperature and pressure increase rate of reaction.

Answer 18

Temperature: Curve becomes lower and moves to the right. Larger proportion of molecules have an energy greater or equal to the Ea, so more particles can successfully react.
Catalyst: Ea line moves to the left, as the catalyst provides an alternative pathway of lower Ea. More particles have an energy equal to or greater than the Ea, so can react.

Question 19

What labels are on the axis of a Boltzmann Curve?

Answer 19

x axis = Kinetic energy

y axis = No. of molecules

Question 20

Define catalyst.

Answer 20

Substance that increases rate of reaction, by providing an alternative pathway of lower Ea, without itself being used up.

Question 21

Do catalysts take part in reaction?

Answer 21

Yes, but they are remade at the end.

Question 22

Give an example of a reaction where a catalyst is used.

Answer 22

Haber Process: N2 + 3H2 —-> 2NH3

Catalyst = Fe

Question 23

Define:

1. Heterogenous catalyst

2. Homogenous catalyst

Answer 23

1. Catalyst is in a different state to the reactants and products. (Haber Process)
2. Catalyst is in the same state to the reactant and products

Question 24

Give some benefits of using a catalyst.

Answer 24

Lower production costs (reduce temp…)
More product in the same time
Fossil fuel reservoirs are preserved
Less CO2 released

Question 25

What is the equation for a catalytic converter?

Answer 25

2CO + 2NO ----> 2CO2 + N2

Question 26

What is the equation for rate of reaction?

Answer 26

Change in conc / time OR y / x

Question 27

Give 3 properties of a dynamic equilibrium

Answer 27

1. forward reaction and reverse reaction occur at the same rate 2. quantities of reactants and products stops changing 3. only occurs in a closed system

Question 28

What does Le Chateliers Principle state?

Answer 28

When a system is in dynamic equilibrium and is subject to an external change in condition, the position of equilibrium will shift to minimise the change.

Question 29

Using Le Chateliers Principle, what will happen if you increase the conc of the reactants in a equilibrium reaction?

Answer 29

equilibrium shifts to the right hand side, to decrease the concentration of the reactants,

Question 30

Using Le Chateliers Principle, what will happen if you increase the pressure of a equilibrium reaction.

Answer 30

Equilibrium will shift to the side with fewer mols, to decrease the pressure and minimise the change.

Question 31

Using Le Chateliers Principle, what will happen if you increase the temperature of a equilibrium reaction, with: DeltaH = -92 kJmol-1

Answer 31

Equilibrium will try to decrease the temperature, by absorbing the extra energy, by moving in the endothermic direction. (LHS)

Question 32

What affect does a catalyst have on the position of equilibrium?

Answer 32

Catalysts have no affect on the position of equilibrium, they only reduce the time taken for equilibrium to be established.

Question 33

Give the reaction for the formation of ethanol, and state the condition needed.

Answer 33

C2H4(g) + H2O(g) ----> C2H5OH(g) Ethene + steam --------> Ethanol Conditions: 60-70 atms 300 degrees celsius Phosphoric (V) acid catalyst

Question 34

What is the equilibrium constant for the following reaction: aA + bB -------> cC + dD

Answer 34

Kc = [D]^d [C]^c ---------------- [A]^a [B]^b

Question 35

On an enthalpy profile diagram, what are the axis labels?

Answer 35

X axis = Reaction pathway | Y axis = Enthalpy kJmol^-1