Using & Controlling Reactions: Measuring Energy Changes

Question 1

Exothermic Reactions

Answer 1

• release energy to surroundings
• increases surrounding temperature
• ^H < 0 (H[reactants] > H[products])
• eg. combustion, respiration, acid-base neutralisation

Question 2

Endothermic Reactions

Answer 2

• absorbs energy of surroundings
• decreases surrounding temperature
• ^H > 0 (H[reactants] < H[products])
• eg. photosynthesis, melting, evaporation, some dissolutions

Question 3

Calorimetry

Answer 3

measurement of heat change in chemical reactions

Question 4

Enthalpy

Answer 4

Heat energy = H

Question 5

^H

Answer 5

= H[products] - H[reactants]
= heat released/absorbed in a reaction at constant pressure
= enthalpy change for a reaction

Question 6

Molar Enthalpy of Combustion

Answer 6

= quantity of heat energy released or absorbed when 1.00 mol of a substance is burnt completely in excess oxygen
= (Cp x ^T x m(water))/1000n

Question 7

Molar Enthalpy of Neutralisation

Answer 7

= quantity of heat energy released or absorbed when 1.00 mol of H+ from an acid is neutralised by a base in an aqueous solution
= (Cp x ^T x m(solution))/1000n

Question 8

Molar Enthalpy of Combustion Experiment errors

Answer 8

extremely low accuracy due to systematic errors:

• not all released H is absorbed by water (air+metal of calorimeter)
• carried out in air (20% O2), not complete combustion
• evaporation from wick of spirit burner
• reaction is not instantaneous, water returns to room temperature
• improvement: use “bomb calorimeter” (pre-weighed sample of alcohol combusted in pure oxygen inside a steel “bomb”, immersed in water… combustion is complete & rapid

Question 9

Molar Enthalpy of Solution

Answer 9

= quantity of heat energy released or absorbed when 1.00mol of the substance dissolves in sufficient water
= (Cp x ^T x M(solution))/1000n

Question 10

Thermochemical Equations

Answer 10

indicate:

• mole ratio of reactants
• state of each reactant and product
• ^H