Enthalpy changes

Question 1

Define enthalpy

Answer 1

Enthalpy is the measure of heat energy in a chemical system.

Question 2

Define enthalpy change

Answer 2

Difference in enthalpy between the products and reactants of a chemical reaction. ΔH = H(products) - H(reactants)

Question 3

Define Exothermic enthalpy changes

Answer 3

Energy (chemical) is transfered from the system to the surroundings (in the form of heat).

Question 4

Define endothermic enthalpy changes

Answer 4

Energy (heat) is transfered from the surroundings to the system (in the form of chemical energy).

Question 5

Define activation energy

Answer 5

The minimum energy required for a reaction to take place.

Question 6

What happens to the temperature of the surroundings in an endothermic process?

Answer 6

tempertaure of the surroundings decreases

Question 7

What happens to the temperature of the surroundings in an exothermic process?

Answer 7

Temperature of the surroundings increases

Question 8

In an exothermic enthalpy profile diagram are the products higher or lower than the reactants and why?

Answer 8

Lower as the products have lost chemical energy lowering their enthalpy.

Question 9

In an endothermis enthalpy profile diagram are the reactants higher or lower than the products and why?

Answer 9

Higher as the products have gained chamical energy increasing their enthalpy.

Question 10

what are the standard conditions and standard states (4)

Answer 10

1. Pressure = 100 KPa
2. Temperature = 298 K
3. Concentration = 1 moldm -3
4. State = physical state of a substance under standard conditions.

Question 11

Define the enthalpy change of reaction

Answer 11

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical reaction.
Under standard conditions, with all reactants and products in their standard states.

Question 12

Define the enthalpy change of formation

Answer 12

The enthalpy change that occurs when one mole of a substance is formed from its elements.
Under standard conditions, with all reactants and products in their standard states.

Question 13

Define the enthalpy change of combustion

Answer 13

the enthalpy change that occurs when one mole of a substance reacts completely with oxygen.
Under standard conditions, with all reactants and products in their standard states.

Question 14

Define the enthalpy change of neutralisation

Answer 14

The enthalpy change that occurs when an acid and a base react completey to form one mole of water.
under standard conditions, with all reactants and products in their standard states.

Question 15

What equation is used to calculate energy/ enthalpy change

Answer 15

q= m c ΔT

Question 16

Describe the process to find the enthalpy change of combustion

write this down

Answer 16

1. Measure a volume of water into a beaker (200ml).
2. Record the intial temperature of the water in the beaker by placing a thermometer in the beaker.
3. Add combustable fuel (methanol) to a spirit burner and measure its weight.
4. Place the spirit burner under the beaker of water.
5. Light the spirit burner and stir the water using the thermometer.
6. After 3 minutes measure the temperature of water, then extinguish the flame.
7. Measure the mass of the spirit level.
8. calculate the moles of fuel burnt by dividing the mass lost (initial - final) by the mr of the fuel.
9. calculate the energy transfered to the water, q = m c ΔT.
10. Divide the enregy gained by water, by the moles of fuel used to get a value of kJ mol -1, enthalpy of combistion value.

Question 17

What Inaccuracies are there when finding the enthalpy of combustion?

Answer 17

1. Heat may be lost to surroundings other than the water, such a the beaker or air.
2. Incomplete combustion may occur.
3. Fuel (e.g. methanol) may evaporate.
4. Conditions may not be standard.

Question 18

Describe how to determine the enthalpy change of reaction.

Answer 18

1. Place one solution into a polystyrene cup.
2. Add a thermometer to the cup and measure the initial temperature of the solution.
3. Add a excess of powder or solution into the cup.
4. Stir the solution.
5. Measure the temperature regularly recording the maximum temperature.
6. Use q = m c ΔT to calculate the energy produced.
7. Calculate the moles of the initial solution in the cup (not excess solution or powder)
8. Divide the energy transfered by the moles of solution/ substance not in excess to get a enthalpy of reaction.

Question 19

Describe how to determine the enthalpy change of neutralisation.

Answer 19

1. Place one solution into a polystyrene cup acid or base (must be the one of known mass or conc and vol).
2. Add a thermometer to the cup and measure the initial temperature of the solution.
3. Add a excess of the other solution into the cup.
4. Stir the solution.
5. Measure the temperature regularly recording the maximum temperature.
6. Use q = m c ΔT to calculate the energy produced.
7. Calculate the moles of the water produced by the reaction.
8. Divide the energy produced by the reaction, by the mols of water produced, to get the enthalpy of neutralisation.

Question 20

Define average bond enthalpy

Answer 20

The energy required to break one mole of a specific type of bond in a gaseous molecule.

Question 21

why is there an average bond enthalpy?

Answer 21

The actual bond enthalpy can vary in different chemical enviroments of different molecules. The average bond enthalpy is calculated from actual bond enthalpies.