Energetics

Question 1

What is the conservation of energy?

Answer 1

Energy cannot be created or destroyed only stored or transferred

Chemical bonds are the forces of attraction the bind atoms together. They hold a potential energy.

Question 2

Define enthalpy

Answer 2

The thermal energy that is stored in a chemical system.

Question 3

What are the standard conditions for enthalpy reactions?

Answer 3

Pressure: 100KPa

Temperature: 298K

Question 4

What is the system and what are the surroundings?

Answer 4

System- chemical reactions (atoms and bonds)

Surroundings- everything else

Question 5

Define enthalpy change

Answer 5

The difference between the enthalpy of the products and the reactants

Change in enthalpy= Hproducts - Hreactants

Question 6

What are exothermic reactions?

Answer 6

When the enthalpy of the products is lower than the enthalpy of the reactants and therefore heat energy has been lost to the surroundings

Question 7

What are endothermic reactions?

Answer 7

When the enthalpy of the products is greater than the enthalpy of the reactants and therefore energy has been absorbed by the reactants to form bonds.

Question 8

What is activation energy?

Answer 8

The minimum energy require to break bonds in the reactants for a reaction to take place

Question 9

What is Hess’s law?

Answer 9

The enthalpy change of a reaction is independent of the route taken. It relies only on be initial and final states of the reaction.

Question 10

Define the enthalpy change for a reaction.

Answer 10

The enthalpy change when the amounts of reactants show in the equation react under standard conditions to give their products in their standard states.

Question 11

Define enthalpy change of formation.

Answer 11

The enthalpy hangs when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 12

Define the enthalpy change of combustion

Answer 12

The enthalpy change when 1 mole of a substance is burnt completely in Excess oxygen under standard conditions

Question 13

Define enthalpy of change of neutralisation.

Answer 13

The enthalpy change when solutions of acid and alkali react together under standard conditions to produce 1 mole of water.

Question 14

Define the enthalpy change of solution.

Answer 14

The enthalpy change when 1 mole of solute dissolved completely in sufficient solvent to form a solution in which the ions are far enough apart to not interact with each other.

Question 15

Define the mean bond enthalpy

Answer 15

The mean /average energy required for 1 mole of a given type of gaseous bond to undergo homolytic fission

Question 16

How do you calculate enthalpy?

Answer 16

q=mc(change in temperature)

Question 17

What is calorimetry?

Answer 17

The study of chemical energy in a chemical reaction

Question 18

Explain the coffee cup calorimeter process.

Answer 18

1. Add a measured mass of the first reactant (if solid then dissolve in the appropriate solvent)
2. Add the second reactant
3. Monitor the temperature every couple of minutes (not the minute the second reactant is added)
4. Play a graph.

Question 19

Explain the spirit burner calorimeter process.

Answer 19

1. Weigh the spirit burner containing the test liquid.
2. Add a know volume of water to the copper can
3. Use the thermometer to record the waters initial temperature
4. Light the spirit burner
5. Continuously stir the water
6. After a few mins extinguish the burner and immediately re-weigh the burner.
7. Calculate mass of fuel burnt
8. Measure final temperature of the water

Question 20

What are the possible errors and assumptions made when doing calorimetry (spirit burner)

Answer 20

Some energy produced from burning the fuel is no transferred to the water (instead the air or copper tin)

Incomplete combustion of fuel may occur

Conditions are not standard

Some alcohol or water may evaporate

Question 21

Explain in terms of bond energy an endothermic reaction

Answer 21

More energy is needed to break bonds in the reactants that is released when bonds are formed in the products so the products have a higher enthalpy and H is positive

Question 22

Explain in terms of bond energy an exothermic reaction

Answer 22

More energy is given out when the bonds in the products are formed than is absorbed when the bonds in the reactants are broken
So reactants have a higher enthalpy

Question 23

What is the equation for enthalpy change?

Answer 23

Enthalpy change = enthalpy of the products - enthalpy of the reactants

Question 24

How do you work out the enthalpy change of combustion from calorimetry?

Answer 24

1. Find Q (energy in joules)
2. Find moles of fuel burnt etc (mass of Mr)
3. Enthalpy change is Q / n (in joules)
4. Convert to KJ
5. state whether it is exo or endothermic (+/-)

Question 25

Explain why calculate mean bond enthalpies are different to those calculated form experimental formation?

Answer 25

The mean bond enthalpy are the average enthalpy from a range of compounds and therefore will not be exact

Question 26

How do you calculate the enthalpy of a reaction from the mean bond enthalpies?

Answer 26

Enthalpy = SUM(bonds broken in reactants)-SUM(bonds formed in the products)