Energy

Question 0

First law of thermodynamics

Answer 0

Energy may be exchange between a chemical system and the surroundings but the total energy remains constant

Question 1

Enthalpy change

Answer 1

Is the heat energy exchange with the surroundings at constant pressure

Question 2

In an exothermic reaction

Answer 2

• Heat is given out (to the surroundings)

- The reacting chemicals lose energy

Question 3

In an endothermic reaction

Answer 3

• Heat is taken in (from the surroundings)

- Chemicals gain energy

Question 4

Standard conditions are

Answer 4

• A pressure of 100kPa
• A stated temperature: 298K (25 degrees) is usually used
• A concentration of 1 mol dm^-3 (for aqueous solutions)

A standard state is the physical state of a substance under standard conditions

Question 5

The standard enthalpy change of reaction

Answer 5

Is the enthalpy change that accompanies a reaction in the molar quantities that are expressed in a chemical equation under standard conditions, all the reactants and products being in their standard states

Question 6

The standard enthalpy change of combustion

Answer 6

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

Question 7

Standard enthalpy change of formation

Answer 7

The enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions

Question 8

Specific heat capacity of a substance

Answer 8

The energy required to raise the temperature of 1g of a substance by 1 degree

Question 9

Hess’ Law

Answer 9

If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 10

Bond enthalpy

Answer 10

The enthalpy change required to break and separate 1 mole of bonds in the molecules of a gaseous element or compound so that the resulting gaseous species exert no forces upon each other

Question 11

Bond breaking is an

Answer 11

Endothermic process and requires energy

Question 12

Bond making is

Answer 12

An exothermic process and releases energy

Question 13

The rate of reaction is usually measured as

Answer 13

The rate if change of concentration of a stated species in a reaction

The units of rate are mol dm^-3 s^-1

Question 14

The rate of reaction may be affected by

Answer 14

• The concentration of the reactants
• The surface area of solid reactants
• A temperature change
• The presence if a catalyst

In some reactions, such as photosynthesis, the rate is effected by the presence and intensity of reaction

Question 15

The activation energy of a reaction is

Answer 15

The energy required to start a reaction by breaking bonds

• Activation energy is often supplied by a spark or by heating the reactants
• Reactions with a small activation energy often take place very readily
• A large activation energy may ‘protect’ the reactants from taking part in the reaction (at room temperature)

Question 16

Characteristics of the Boltzmann distribution

Answer 16

• Most gas molecules have energies within a comparatively narrow range
• The curve will only meet the energy axis at infinite energy. No molecules have zero energy
• Only those molecules with more energy than the activation energy of the reaction are able to react

Question 17

If the concentration of a reactant in solution or in a gas mixture is increased

Answer 17

• There are more particles present per volume
• More collisions take place each second
• More collisions exceed the activation energy every second
• Therefore the rate of reaction increase

Question 18

A catalyst speeds up the rate of a reaction by

Answer 18

Providing an alternative route for the reaction with a lower activation energy

Activation energy with catalyst Ec < Activation energy without catalyst, Ea

Question 19

What is a heterogenous catalyst?

Answer 19

A catalyst in a different phase from the reactants

Many reactions between gases are catalysed using a solid catalyst

Question 20

What is homogenous catalyst?

Answer 20

A catalyst in the same phase as the reactants.

Many reactions between gases in the atmosphere are catalysed using a gaseous catalyst

Homogeneous catalysis also takes place in many aqueous reactions in which both the reactants and the catalyst are dissolved in water

Question 21

A change in total pressure may alter the equilibrium position of a system involving gases.

The direction favoured depends upon…

Answer 21

The total number of molecules on each side of the equilibrium

Question 22

Define the term Lattice Enthalpy

Answer 22

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions, under standard conditions

Question 23

Why can lattice enthalpies not be determined directly?

Answer 23

Because it is impossible to use gaseous ions to form one mole of an ionic lattice experimentally

Question 24

What is meant by the term Electron Affinity

Answer 24

The enthalpies change involved in adding one electron to each atom in one mole of gaseous ions to form one mole of -1 ions

Question 25

What are the three main types of electrochemical cell?

Answer 25

Rechargeable, Non-rechargeable and Fuel Cells

Question 26

How does a methanol fuel cell work?

Answer 26

It is reacted with water in a reformer to produce hydrogen