3a - Energetics

Question 1

What is an exothermic reaction?

Answer 1

Reaction in which energy is given out to surrounding, decreasing energy level (temperature increases)

Question 2

What is an endothermic reaction?

Answer 2

Reaction in which energy is taken in from surrounding, increasing energy level (temperature decreases)

Question 3

How can you easily differentiate between exothermic and endothermic reactions?

Answer 3

So, in an exothermic reaction the heat exits from the chemicals so temperature rises

Question 4

What are some examples of exothermic reactions?

Answer 4

• Burning
• Neutralisation
• Reaction between Water and Calcium Oxide

Question 5

What are some examples of endothermic reactions?

Answer 5

• Electrolysis
• Thermal decomposition of Copper (II) Carbonate
• Reaction between Acetic Acid and Sodium Carbonate

Question 6

What is a calorimetry experiment?

Answer 6

Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.

Question 7

How can you calculate the heat energy change of a reaction?

Answer 7

Data from calorimetry experiment can be used to calculate the heat energy change of a reaction.

Question 8

What is the equation to calculate energy transferred?

Answer 8

Q = mc△T
Q = energy transferred
m = Mass of Water or volume of Water (*If Water is not used, this is replaced by mass or volume of other solutions)
c = specific heat capacity (4.18J/g/C)
△T = The temperature rise ( Final Temperature - Initial Temperature)

Question 9

Complete this example with the equation:
Energy from burning 0.5 g of Propane was transferred to 100 cm3 of Water to raise its temperature by 20°C. Calculate the heat energy change (in KJ)

Answer 9

Mass of Water = 100 cm3
Heat Capacity of Water = 4.2 j / g
Temperature Rise = 20°C

Energy Transferred = 100 x 4.2 x 20 = 8400 J

*1000 J = 1 KJ
So 8400 J = 8.4 KJ

Question 10

Explain the calorimetry experiment for combustion (Combustion is the name given to reactions where a fuel combines with oxygen):

Answer 10

• Using a measuring cylinder, measure 100 cm3 of Water into a Copper can (copper provides efficient heat insulation)
• Measure and record the initial temperature of Water
• Filling the spirit burner with test substance, measure and record its mass
• Place burner under Copper can and light the wick
• Constantly stir the Water and continue heating until temperature rises by about 20 - 30°C, then blow out the flame
• Measure and record the highest temperature of Water
• Measure and record the final mass of burner and remaining Alcohol

Question 11

Explain the calorimetry experiment for displacement, dissolving and neutralisation reactions:

Answer 11

• 50 cm3 of copper(II) sulfate is measured and transferred into a polystyrene cup.
• The initial temperature of the copper sulfate solution is measured and recorded.
• Magnesium is added and the maximum temperature is measured and recorded.
• The temperature rise is then calculated.

Question 12

How can you calculate the molar enthalpy change of a reaction?

Answer 12

Data from a Calorimetry Experiment can be used to calculate the molar enthalpy change of a reaction

Question 13

What is the equation for the molar enthalpy change of a reaction?

Answer 13

Molar enthalpy change = Mass of water heated x 4.18 j/g x temperature rise / Moles

Mass of water heated x 4.18 j/g x temperature rise = Amount of energy transferred (Q)

Question 14

When temperature increases is the enthalpy change negative or positive (exothermic or endothermic)

Answer 14

Exothermic because temperature increases in exothermic reactions (negative)

Question 15

When temperature decreases is the enthalpy change negative or positive (exothermic or endothermic)

Answer 15

Endothermic because temperature decreases in endothermic reactions (positive)

Question 16

What is bond energy?

Answer 16

Energy required to break a bond