5) Chemical energetics

Question 1

Enthalpy

Answer 1

• the total chemical energy inside a substance
• can be endothermic (+) or exothermic (-)

Question 2

Differences between endothermic and exothermic reactions

Answer 2

table in notes

Question 3

Transition state

Answer 3

stage during reaction at which chemical bonds are partially broken and formed (very unstable and is higher in energy than the reactants and products

Question 4

Activation energy (Ea)

Answer 4

minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction (energy needed to reach the transition state)

Question 5

Standard conditions

Answer 5

Pressure- 101kPa
Temp- 298K (25 degrees Celsius)
Each substance involved in the reaction is in its normal physical state

Question 6

Standard enthalpy change of reaction

Answer 6

• The enthalpy change when the reactants in the stoichiometric equation react to give the products in their standard states
• Exothermic and endothermic

Question 7

Standard enthalpy change of formation (commonly asked)

Answer 7

• The enthalpy change when 1 mole of a compound is formed from its elements in their standard states
• Exothermic and endothermic

Question 8

Standard enthalpy change of combustion

Answer 8

• The enthalpy change when 1 mole of a substance is burnt in excess oxygen in their standard states
• Exothermic

Question 9

Standard enthalpy change of neutralisation

Answer 9

• The enthalpy change when 1 mole of water is formed by reacting an acid and alkali in their standard states
• Exothermic

Question 10

Bond energies

Answer 10

• enthalpy changes take place, because bonds are being broken and formed
• energy in the form of heat is needed to overcome attractive forces between atoms, therefore bond breaking is endothermic
• energy released from the reaction to surroundings in the form of heat when new bonds are formed, therefore bond formation is exothermic

Question 11

Enthalpy change of reaction formula

Answer 11

enthalpy change for bonds broken + enthalpy change for bonds formed

Question 12

Enthalpy change of formation formula

Answer 12

energy of products - energy of reactants

Question 13

Exothermic rxn

Answer 13

• if more energy is released when new bonds are formed than energy is required to break bonds

Question 14

Endothermic rxn

Answer 14

• if more energy is required to break bonds than energy is released when new bonds are formed

Question 15

Measuring enthalpy changes

Answer 15

• a calorimetry is a technique used to measure changes in enthalpy of chemical reactions
• a calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or a metal can
• the energy transferred as heat can be calculated by q= m x c x triangle T

Question 16

Specific heat capacity

Answer 16

the energy required to increase the temperature of 1g of a substance by 1 degree Celcius

Question 17

Formula to calculate any changes in enthalpy per mole of a reactant or product

Answer 17

enthalpy change= m x c x triangle T
- when there is a rise in temp, the value for enthalpy change becomes - suggesting the reaction is exothermic
- then there is a fall in temp, the value for enthalpy change becomes + suggesting the reaction is endothermic

Question 18

Hess’s law

Answer 18

the total enthalpy change in a chemical reaction is dependent on the route by which the chemical reaction takes place as long as the initial and final conditions are the same

Question 19

Example of hess’s cycle for enthalpy change of formation

Answer 19

in notes

Question 20

Example of hess’s cycle for enthalpy change of combustion

Answer 20

in notes

Question 21

Hess’s cycle- mean bond enthalpy

Answer 21

in notes

Question 22

Calorimetry (combustion energy transferred)

Answer 22

1) Calculate energy first (substitute all the values into equation)
2) Calculate n of fuel to work out enthalpy (mass/Mr)
3) Enthalpy= first answer/n

• if there is a rise in temp it will be negative (exothermic)
  *if there is a fall in temp it will be positive (endothermic)

Question 23

Calorimetry (of solutions)

Answer 23

• polystyrene cup is used to prevent heat loss
• add acid first- measure temp
• add alkali/solid- stir and measure temp change
• similar formula to work out enthalpy change