Energetics

Question 1

What is an endothermic and exothermic reaction?

Answer 1

A reaction that give out heat is exothermic.

A reaction that absorbs heat is endothermic.

Question 2

Describe a simple calorimetry experiment for combustion

Answer 2

Put 50g of water into a copper can

• Record the temperature of the water
• Weigh the spirit burner and lid
• Place the spirit burner underneath the copper can and light its wick.
• Stir constantly until the water reaches about 50ºC
• Put the flame out using the burner lid
• Record the final temperature of the water
• Weigh the spirit burner and lid again
• calculate the enthalpy change using mcΔt

Question 3

Describe a simple calorimetry experiment for dissolving

Answer 3

CaO + H2O —-> Ca(OH)2
1. 50 cm3 of water is measured and transferred into a polystyrene cup.
2. The initial temperature of the water is measured and recorded.
3. Calcium oxide is added and the maximum temperature is measured and recorded.
4. The temperature rise is then calculated.
Equation: mcΔt

Question 4

Describe a simple calorimetry experiment for dissolving

Answer 4

CaO + H2O —-> Ca(OH)2
1. 50 cm3 of water is measured and transferred into a polystyrene cup.
2. The initial temperature of the water is measured and recorded.
3. Calcium oxide is added and the maximum temperature is measured and recorded.
4. The temperature rise is then calculated.
Equation: mcΔt

Question 5

Describe a simple calorimetry experiment for neutralisation

Answer 5

NaOH + HCL —-> NaCl + H2O
1. 50 cm3 of dilute hydrochloric acid is measured and transferred into a polystyrene cup.
2. The initial temperature of the dilute hydrochloric acid is measured and recorded.
3. Sodium hydroxide is added and the maximum temperature is measured and recorded.
4. The temperature rise is then calculated.
Equation: mcΔt

Question 6

calculate molar enthalpy change from heat energy change

Answer 6

Calculate amount of energy transferred: Joules x mass x temperature
Find the number of moles: Mass / Mr
calculate molar enthalpy: amount of energy / moles

Question 7

Describe endothermic and exothermic reactions in terms of bonds

Answer 7

Exothermic: breaking up bonds in a compound costs more energy than you get out when the new ones are made. That means that when the reaction is finished, more energy has been absorbed than is released.
Endothermic: you get much more energy released when the new bonds are formed than was used to break the original ones. This surplus energy is given out as heat during the reaction.

Question 8

use average bond energies to calculate the enthalpy change during a simple chemical reaction

Answer 8

Enthalpy change, ΔH = Energy needed to break all the bonds - Energy released to make all the new bonds