5.2 - Energetics

Question 1

What is enthalpy change?

Answer 1

The enthalpy change of a reaction is the heat energy it releases or absorbs per mole.

Question 2

What is the symbol for enthalpy change?

Answer 2

ΔH

Question 3

What does a positive sign tell us about the enthalpy change?

Answer 3

It was an endothermic reaction because heat energy was absorbed.

Question 4

What does a negative sign tell us about the enthalpy change?

Answer 4

It was an exothermic reaction because heat energy was released.

Question 5

What does the heat energy of a reaction depend on?

Answer 5

1. Breaking bonds, which absorbs energy.
2. Making bonds, which releases energy.

Question 6

In an endothermic reaction will the products end up with more or less energy than the reactants?

Answer 6

more

Question 7

In an exothermic reaction will the products end up with more or less energy than the reactants?

Answer 7

less

Question 8

Equation for enthalpy change?

Answer 8

ΔH = Bonds Broken − Bonds Made

Question 9

What is a reaction profile diagram?

Answer 9

A reaction profile diagram shows the path the reaction takes from reactants to products.

Question 10

What does activation energy look life on an energy profile diagram?

Answer 10

a bump

Question 11

What is calorimetry?

Answer 11

Calorimetry is an experimental method used to measure an enthalpy change (ΔH).

Question 12

Calorimetry expriment for an exothermic reaction?

Answer 12

1. The reaction releases heat energy.
2. That heat energy is absorbed by the nearby water.
3. The temperature of the water increases.

Question 13

What 2 formulae do we use to convert our measurements to ΔH?

Answer 13

Q = mxcxΔt (heat energy = mass x heat specific capacity x change in temperature)

ΔH = -Q / moles (enthalpy change = negative heat energy / moles)

Question 14

Method for combustion calorimetry?

Answer 14

1. Weigh out 50 g of water and place in a can above the burner.
2. Measure the initial mass of the burner and the initial
   temperature of the water.
3. Ignite the burner, stir water for 30 seconds, then extinguish
   the burner.
4. Measure the final mass of the burner and the final (maximum)
   temperature of the water.

Question 15

What are the sources of error in combustion calorimetry?

Answer 15

1. Heat loss – most of the heat energy dissipates into the air instead of the water.
2. Incomplete combustion – this releases less heat energy than expected

Question 16

Method for solution calorimetry?
(example Mg + CuSO4)

Answer 16

1. Pipette 25 cm3 of CuSO4(aq) into a polystyrene cup.1 cm3 solution has a mass of 1 g
2. Measure the initial temperature of the solution.
3. Add 1 g of Mg(s) and stir continuously.
4. Measure the maximum temperature of the solution.

Question 17

What is the value for heat specific capacity?

Answer 17

4.18 J/g/ degrees C.

Question 18

What happens to the water temperature when we do calorimetry experiments for endothermic reactions?

Answer 18

The reaction absorbs heat energy from the water, so the water cools down.