Energetics

Question 0

What must occur for two substances to react?

Answer 0

Bonds within the reactants must break and new bonds are formed to make the products.

Question 1

How is energy contained in a substance?

Answer 1

In the kinetic forces of the movements of atoms and molecules, even if the substance is crystalline and the particles can only vibrate.

There is also potential energy associated with the chemical bonds.

Question 2

What must happen in order to break the bonds of the reactants?

Answer 2

Heat must be absorbed so the atoms vibrate faster and faster and the bonds can be broken.

Question 3

What must happen to the products so they can become stable?

Answer 3

Heat must be lost so that the atoms move/vibrate less and can stay together.

Question 4

What can be said about an exothermic reaction?

Answer 4

If heat is given out then the energy lost through formation of the products is greater than the energy taken in to break the bonds in the reactants.

Question 5

What can be said about a endothermic reaction?

Answer 5

That more energy is taken in to break the bonds in the reactants then is given out by the formation of the products.

Question 6

What is Enthalpy?

Answer 6

The total energy stored in a substance.

Question 7

Can enthalpy be measured?

Answer 7

It is impossible to measure enthalpy. However enthalpy change can be measured.

Question 8

What is the symbol given to enthalpy change?

Answer 8

ΔH

Question 9

What can be said if ΔH is a negative value?

Answer 9

The reaction can be said to be exothermic.

Question 10

What can be said if ΔH is a positive value?

Answer 10

The reaction can be said to be endothermic.

Question 11

How is enthalpy change always stated?

Answer 11

Under standard conditions (298K)

ΔHᶿ₂₉₈

Question 12

What is standard enthalpy of formation and its notation?

Answer 12

ΔHfᶿ

This is the energy absorbed if one mole of a substance is formed from its elements in their standard states.

Question 13

What is the ΔHfᶿ of an element?

Answer 13

Zero.

Question 14

What is the standard enthalpy change of combustion and its notation?

Answer 14

ΔHcᶿ

This is the heat absorbed when 1 mole of a substance is completely burnt in oxygen at constant pressure.

Question 15

Describe a reaction for measuring enthalpy changes for a solution.

Answer 15

The reaction can be carried out in plastic cups which have a negligible heat capacity as their mass is small compared with the mass of the solutions and do not allow heat loss to the surroundings. The temperature of the two solutions before reaction are taken and and averaged. After mixing the maximum (or minimum) temperature is taken. Knowing the volumes and molarity of the solutions the molar enthalpy change can be calculated.

Question 16

What is the law of conservation of energy?

Answer 16

The law states that energy cannot be crated or Destroyed but can only be changed from one form to another.

Question 17

Apply the law of conservation of energy to chemical principles.

Answer 17

If one set of chemical substances if converted to another set by any route the total energy change must be the same.

Question 18

When did G.H Hess pen his law and what is it?

Answer 18

In 1840

Hess’ law:- the total energy change accompanying a chemical change is independent of the route by which the chemical change takes place.

Question 19

What is the value of Hess’ law?

Answer 19

It allows you to calculate enthalpy changes that cannot be determined directly by experement.

Question 20

What is the standard bond dissociation enthalpy for a molecule?

Answer 20

This is the energy that must be absorbed to separate two atoms in a bond.

Question 21

What problem occurs when trying to apply bond dissociation enthalpies to molecules with more than one bond e.g methane?

Answer 21

The energy required to break the first C-H bond in not the same as that required to remove a hydrogen atom from a CH₃⠁radical.

Question 22

How do we solve the problem associated with bond dissociation energies for complex molecules (more than one bond).

Answer 22

By dividing the standard enthalpy change (ΔH) by the number of bonds present to give the average standard bond enthalpy.

Question 23

What is average standard bond enthalpy also known as?

Answer 23

The bond energy term.

Question 24

What is the sum of the bond energy terms equal to for any molecule?

Answer 24

The enthalpy change for atomising that compound in the gaseous state.

Question 25

What can bond energy terms be used to give us?

Answer 25

To give an approximate estimate of the standard enthalpy change which occurs in the reaction. Where the difference between the sum of the bond energy terms in the reactants and the sum of the bond energy terms in the products is the ΔHᶿ