energetics(I)

Question 1

define enthalpy of a substance

Answer 1

it is a measure of energy content of a substance, which reflects its stability

Question 2

define enthalpy change of reaction

Answer 2

it is the energy change in a chemical reaction when the molar quantities of reactants specified in the chemical equation react to form products

Question 3

what are the conditions for standard conditions

Answer 3

298k(25 degree celcius), 1 bar(100000Pa) and 1 mol dm^-3

Question 4

state equation to find out enthalpy change of reaction

Answer 4

sum of enthalpies of products - sum of enthalpies of reactants

units for enthalpy change: kJ mol^-1.

Question 5

define endothermic reaction

Answer 5

it is a chemical reaction in which energy is absorbed from the surroundings

Question 6

state the properties of endothermic reaction

Answer 6

1. enthalpy change is positive
2. temperature of surroundings decreases in the process
3. products are energetically less stable than reatants.

Question 7

define exothermic reaction

Answer 7

it is a chemical reaction in which energy is released to the surroundings

Question 8

state the properties of an exothermic reaction

Answer 8

1. enthalpy change is negative
2. temperature of surroundings** increases** in the process
3. products are energetically more stable than reactants.

Question 9

define standard enthalpy change of reaction

Answer 9

it is the energy change when molar quantities of reactants as specified by the chemical equation react to form products under standard conditions( 1 bar and 298K)

Question 10

define standard enthalpy change of formation

Answer 10

it is the energy change when one mole of the substance is formed from its constituent elements under standard conditions .

note that the standard enthalpy change of formation of an element is always zero.

Question 11

state the equation for the standard enthalpy change of formation

Answer 11

sum of standard enthalpy change of products - sum of standard enthalpy change of reactants

Question 12

define standard enthalpy change of combustion

Answer 12

it is the **energy evolved **(heat released, exothermic) when 1 mole of the substance is completely burnt in oxygen under standard conditions.

hydrocarbons/those with oxygen burn completely in oxygen to form carbon dioxide(g) and H2O(l)

take note that water is in liquid state under standard conditions.

Question 13

state the equation for the standard enthalpy change of the combustion

Answer 13

sum of standard enthalpy change of combustion of reactants - sum of standard enthalpy change of combustion of products.

Question 14

define the standard enthalpy change of neutralisation

Answer 14

it is the energy evolved when one mole of water(in dilute aqueous solution) is formed from the neutralisation between an acid and base under standard conditions.

neutralisation is always exothermic

Question 15

why is the standard enthalpy change of neutralisation involving a weak acid/base slightly less negative/less exothermic as compared to one with a strong acid/base?

Answer 15

1. Weak acids/bases are only slightly dissociated in aqueous solution. Ionisation, which involves bond breaking, is an endothermic process.
2. Some of the energy evolved from the neutralisation process is used to further dissociate the weak acid/weak base completely.
3. Thus, the enthalpy change of neutralisation involving a weak acid/base is less negative than that berween strong acid and strong base.

Question 16

state the equation to deduce the amount of heat released/absorbed

Answer 16

mass of solution/total volume of solution x specific heat capacity of solution x temperature change

heat capacity of solution x temperature change

Question 17

define specific heat capacity,c

Answer 17

it is the quantity of heat required to raise the temperature of 1g/1cm^3 of the substance by 1K

units: J g^-1 K-1/ J cm^-3 K^-1

Question 18

define heat capacity, C

Answer 18

a substance is the quantity of heat required to raise the temperature of Xg (X cm^-3) of the substance by 1K

units: J K^-1

Question 19

how is specific heat capacity related to heat capacity

Answer 19

C = m x c

Question 20

state how enthalppy change can be determined using heat released/absorbed calculated.

Answer 20

standard enthalpy change = +/- (heat released/absorbed)/n

n refers to the amount(mole) of limiting reagent or specified product depending on the type of enthalpy change being determined.

add the -/+ sign to indicate exothermic/endothermic reaction respectively.

Question 21

state the assumptions made for specific heat capacity/heat capacity questions

Answer 21

1. heat released by reaction = heat absorbed by surrounding solution/heat absorbed by reaction = heat lost by surrounding solution.
2. Negligible heat lost to/gained from the surroundings due to insulation, assuming 100% efficiency.

Question 22

why do we need to extrapolate temperature-time graph.

Answer 22

this is so that we can find the true maximum/minimum final temperature by compensating for the time taken for the thermometer to respond to temperature changes.

Question 23

define bond dissociation energy

Answer 23

it is the energy absorbed(endothermic) to break 1 mole of a particular covalent bond in a specific molecule in the gaseous state to form gaseous atoms

use bond energy values from data booklet

Question 24

define bond energy

Answer 24

it is the average energy absorbed to break 1 mole of covalent bonds in the gaseous state to form gaseous atoms under standard conditions

energy absorbed in bond breaking in reactants: endothermic
energy released in bond formation in products: exothermic

Question 25

state the equation using bond energies to find enthalpy change of reaction

Answer 25

sum of bond energies of reactants minus bond energies of products.

Question 26

define the standard enthalpy change of atomisation of an **element**

Answer 26

it is the **energy absorbed** when **1 mole of gaseous atoms** is formed form the **element** under **standard conditions**.

Question 27

define standard enthalpy change of atomisation of a **compound**

Answer 27

it is the **energy absorbed** when **one mole of compound** is converted to **gaseous atoms** under **standard conditions**. example: CH4(g) = C(g) + 4H(g) Li2O(s) = 2Li(g) + O(g)

Question 28

define first ionisation energy

Answer 28

it is the energy absorbed when 1 mole of gaseous atoms loses 1 mole of electrons to form 1 mole of singly charged gaseous cations.

Question 29

define second ionisation energy

Answer 29

it is the energy absorbed when 1 mole of gaseous singly positively charged ions loses one mole of electrons to form 1 mole of doubly positively charged gaseous cations.

Question 30

define first electron affinity

Answer 30

it is the energy change when 1 mole of gaseous atoms acquires one mole of electrons to form 1 mole of singly charged gaseous anions.

Question 31

define second electron affinity

Answer 31

it is the energy absorbed when 1 mole of singly negatively charged gaseous ions acquire one mole of electrons to give one mole of doubly charged gaseous anions.

Question 32

why is 1st EA of non-metals usually negative?

Answer 32

this is because the **energy released** when the **nucleus attracts an electron** is **more** than the **energy taken** in to **overcome inter-electronic repulsion**.

Question 33

why is 2nd EA always positive

Answer 33

since an electron is added to a **negatively-charged ion**, energy has to be **supplied/absorbed** to **overcome the repulsive forces** between the **2 negatively charged species**.

Question 34

define lattice energy of an ionic solid

Answer 34

it is the **energy evolved** when **1 mole of the solid ionic compound** is formed from its **constituent gaseous ions** under **standard conditions** .

Question 36

state the equation to deduce strength of lattice energy.

Answer 36

(Q+ x Q-)/(R- + R+) the higher the ionic charge, the more negative the lattice energy is. the smaller the ionic radius, the more negative the lattice energy is.

Question 37

why is there a discrepancy between experimental and theoretical lattice energies for ionic compounds?

Answer 37

this is mainly due to partial covalent character.

Question 38

define standard enthalpy change oh hydration of an ion

Answer 38

it is the **energy evolved** when **1 mole of the gaseous ions** is **hydrated** under **standard conditions**. E.g. 1. Na+(g) = Na+(aq) 2. Cl-(g) = Cl-(aq)

Question 39

state the equation to deduce magnitude of standard enthalpy change of hydration

Answer 39

it is proportional to (ionice charge)/(ionic radius) it is directly proportional to the charge of the ion and inversely proportional to size of ion.

Question 40

define the standard enthalpy change of solution

Answer 40

it is the **energy change** (can be +/-) of a substance when **1 mole of the substance** is **completely dissolved in a solvent** to form an** infinitely dilute solution** under **standard conditions**. E.g. NaCl(s) = Na+(aq) + Cl-(aq) MgSO4(s) = Mg2+(aq) + SO4 2-(aq)

Question 41

describe the 2 stages of dissolving an ionic solid in water

Answer 41

1. the **separation of the ions in solid lattice into gaseous ions**. this is an endothermic process, the **reverse of lattice energy(-L.E.)** 2. **Hydration of gaseous ions** by water molecules, which is an exothermic process where **heat is released due to ion-dipole interactions** formed between ions and water.

Question 42

state the equation to find out standard enthalpy change of solution

Answer 42

-L.E. + sum of standard enthalpy change of solution the more negative the standard enthalpy change of solution is, the more soluble the ionic compound is.

Question 43

deduce solubility of ionic compounds when standard enthalpy change of solution is smaller than 0

Answer 43

inoinc compound is likely to be soluble. the **energy evolved** in **hydration** is **sufficient** to **compensate for the energy required to break down the crystal lattice**.

Question 44

deduce solubility of ionic compounds when standard enthalpy change of solution is greater than 0.

Answer 44

ionic compound is likely to be insoluble as the energy evolved in hydration is insufficient to compensate for energy required to break down the crystal lattice.

Question 45

define energetic stability

Answer 45

it arises when the enthalpy content of a bustance is relatively lower than that of the product of a chemcial reaction. this occurs when the enthalpy change of reaction is positive.

Question 46

define kinetic stability of a substance

Answer 46

it arises when the rate of chemical reaction is extremely slow, or almost negligible. this occurs when the activation energy of the chemical reaction is extremely high.