Energetics/ thermochemistry

Question 1

Energy

Answer 1

• ability to do work

- E = force x distance

Question 2

exothermic

Answer 2

when bonds in products = stronger than bonds in reactants, E released when bonds formed = temp rise

Question 3

average bond enthalpy

Answer 3

amount E required to break one mole of bonds in gaseous state averaged across range of compounds containing that bond

Question 4

endothermic

Answer 4

bonds in reactants = stronger than bonds in products,

E absorbed to break bonds betw atoms = temp decrease

Question 5

specific heat capacity

Answer 5

amount heat E required to raise temp of 1g of substance by 1c/ 1K
(measured in J/kg°C)

Question 6

equation for enthalpy change

Answer 6

• q = m x c x ∆T

- enthalpy change = mass x specific heat capacity x change in temp

Question 7

standard enthalpy change of reaction

Answer 7

difference betw products and enthalpy of reactants under standard conditions

Question 8

higher enthalpy =

Answer 8

< stable substance

Question 9

examples of exothermic reactions

Answer 9

• neutralisation
• combustion
• many oxidation reactions
• vaporisation
• condensation

Question 10

relationship betw ∆T, ∆H and classification of reactions as endothermic /exothermic

Answer 10

• when H of products > than reactants, E absorbed from surroundings = endothermic w/ temp decreasing
• when change = negative, E lost to surroundings = exothermic

Question 11

thermochemical equation

Answer 11

when equation = written w/ associated change in enthalpy

Question 12

activation E

Answer 12

• bonds betw reactants = broken b4 converted into products
• collide w/ sufficient E
• some KE = converted into vibrational E, which overcomes bonds when reaching certain magnitude

Question 13

examples of endothermic reactions:

Answer 13

• ionisation
• melting
• evaporation
• sublimation

Question 14

standard enthalpy change of combustion

Answer 14

enthalpy change of a combustion reactions under standard conditions

Question 15

standard enthalpy change of formation

Answer 15

enthalpy change when one mole of a compound = formed form elements under standard conditions

Question 16

Hess’s Law

Answer 16

• E diff. between 2 states = independent of route betw them

- heat evolved/ absorbed in chemical process = same no matter how many steps it takes

Question 17

lattice enthalpy

Answer 17

E required to completely separate 1 mole of solid ionic compound into its gaseous ions

Question 18

electron affinity

Answer 18

∆E when mole of electrons = accepted by 1 mole of atoms in gaseous state to form 1 mole of -ve ions

Question 19

how relative sizes + charges of ions affect lattice enthalpies of diff. ionic compounds

Answer 19

• ionic lattice w/ higher enthalpy > stability bc harder to break apart
• > charge = > enthalpy
• smaller radius = > enthalpy

Question 20

standard enthalpy of atomization

Answer 20

standard enthalpy change when 1 mol gaseous atoms = formed from element in its standard state under standard conditions

Question 21

lattice enthalpy

Answer 21

either endothermic process of turning crystalline solid into gaseous ions/ exothermic process of turning gaseous ions into crystalline solid

Question 22

enthalpy change of solution

Answer 22

enthalpy change when 1 mol ionic substance dissolves in water to give solution of infinite dilution

Question 23

hydration E

Answer 23

enthalpy change when 1 mol gaseous ions dissolves in sufficient water to give infinitely dilute solution

Question 24

entropy

Answer 24

distribution of available E among particles in system.. increase in entropy = increase in disorder

Question 25

how can increase in entropy happen?

Answer 25

- mixing diff types of particles - change in state where distance betw particles increases - increased movement of particle e.g. heating liquid/ gas - increasing no. particles

Question 26

standard entropy of a substance

Answer 26

entropy change per mole from heating substance from 0K to standard temperature of 298K

Question 27

define spontaneous reaction

Answer 27

if it causes system to move from less stable to more stable state depends on enthalpy change and entropy change