Chapter 9-10 - Enthalpy&Reaction rates and equalibrium

Question 1

Enthalpy Change (ΔH) =
{when using hess’ law for ΔHf} ONLY

Answer 1

∑(Bond enthalpies of products) - ∑(Bond enthalpies of reactants)

Question 2

Heat energy (q)

Answer 2

q = mcΔT
[q in J] [m in g] [c for water=4.18] [change in temp so C or K]

Question 3

Define standard enthalpy change of formation (ΔfHo)

Answer 3

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

Question 4

Define standard enthalpy change of combustion (ΔcHo)

Answer 4

The enthalpy change when 1 mole of a substance is burned completely in oxygen, under standard conditions.

Question 5

Define standard enthalpy change of neutralisation (ΔneutHo)

Answer 5

The enthalpy change when an acid reacts with an alkali to form 1 mole of water, under standard conditions.

Question 6

Standard conditions:

Answer 6

pressure of 100kPa & temp. of 298K

Question 7

Define Average Bond Enthalpy:

Answer 7

The energy required to break 1 mole of a specified type of bond in a gaseous molecule, averaged over many different compounds.

Question 8

Exothermic reactions

Answer 8

• Making bonds
• giving out energy
• ΔH is negative

Question 9

Endothermic reactions

Answer 9

• Breaking bonds
• taking in energy
• ΔH is positive

Question 10

Define Activation Energy

Answer 10

The minimum energy required for a reaction to occur

Question 11

Hess’ Law

Answer 11

Total ΔH of a reaction is always the same regardless of the route taken

Question 12

How does temperature affect the rate of reaction?

Answer 12

Increasing temp. [in terms of collision frequency]
- particles move faster…
- ∴ collide more frequently…
- ∴ more successful collisions / second…
- ∴ rate of reaction increases.

Increasing temp. [in terms of activation energy]
- particles have more energy…
- ∴ more particles exceed the activation energy…
- ∴ a greater proportion of the collisions are successful…
- ∴ rate of reaction increases.

Question 13

Boltzmann distribution and temperature

Answer 13

At higher temperatures:
- same amount of molecules ∴ same area under curve
- average energy of molecules increases
- small proportion molecules still have low energy but more have a higher energy
- peak of graph is lower and shifted to the right [most probable energy of particles]
-higher end point as graph was shifted to right
- higher proportion of particles have enough energy to react

Question 14

How does Pressure/concentration affect the rate of reaction?

Answer 14

• increasing pressure/ conc. makes the particles closer together…
• ∴ particles collide more frequently…
• ∴ more successful collisions / second…
• ∴ rate of reaction increases.

Question 15

Define Catalyst

Answer 15

A substance that changes the rate of a chemical reaction without undergoing permanent changes itself.

Question 16

Homogeneous Catalyst

Answer 16

• Same physical state as reactants
• reacts with reactants to form an intermediate which then breaks down to form products and regenerates catalyst.

Question 17

Heterogeneous Catalyst

Answer 17

• different physical state from reactants [usually solid]
• reactant molecules adsorbed {weakly bonded} onto surface of catalyst, reaction takes place, products leave surface via desorption.

Question 18

Boltzmann distribution and catalysts:

Answer 18

Activation energy lowered ∴ greater proportion of particles exceed activation energy

Question 19

Define Dynamic Equilibrium

Answer 19

• exists in closed system
• when forward and reverse reactions both occur at same rate
• no overall change in concentrations of the reactants or products

Question 20

Le Chatelier’s Principle (not a required definition)

Answer 20

When a system in dynamic equilibrium is subjected to change, the equilibrium position will shift to minimise the change.

Question 21

Increasing the temp. effect on position of equilibrium

Answer 21

• shifts to the endothermic reaction (+ΔH)
• in order to oppose change and decrease temp.

Question 22

Increasing the pressure effect on position of equilibrium

Answer 22

• shifts to the side with least gas molecules
• in order to oppose change and decrease pressure

Question 23

Increasing the conc. of a reactant effect on position of equilibrium

Answer 23

• shifts to the product side and vice versa
• in order to oppose change and decrease conc. of reactant

Question 24

Effect of catalyst on equilibrium position

Answer 24

• do not change the equilibrium position
• but the reaction occurs more quickly (equal increase on both sides)

Question 25

enthalpy change (ΔH)=
{when using average bond enthalpies}

Answer 25

ΔHr= Bonds broken - Bonds made

Question 26

ΔHc is always…

Answer 26

negative (exothermic)

Question 27

ΔHf and ΔHr can be…

Answer 27

exothermic or endothermic

Question 28

Define enthalpy change of reaction

Answer 28

The enthalpy change when the number of moles of reactants as specified in the balanced equation react together under standard conditions

Question 29

Explain the bonding in a C=C double bond. Use the orbital overlap model

Answer 29

• C=C bonds are formed by the sideways overlap of p orbitals from 2 carbon atoms
• This results in a C=C bond formed above and below the plane of the molecule.