Energy and redox

Question 1

Heat of combustion definition

Answer 1

The enthalpy change that occurs when a specific quantity of the fuel burns completely in oxygen

Question 2

Improvements for spirit burner prac

Answer 2

Shield the flame, insulate the container and cover the container with insulated lid (polystyrene), keep the flame closer to the can/water beaker

Question 3

Reducing heat loss during calorimetry experiment

Answer 3

Using insulation around the container and the lid

Placing insulation around the burning fuel

Question 4

Function of thermometer

Answer 4

To record the initial and final temperature in order to calculate the temp change

Question 5

Function of stirrer

Answer 5

To distribute the thermal energy evenly throughout the calorimeter

Question 6

Function of water

Answer 6

Serves as a heat bank due to high specific heat capacity

Question 7

Function of electrical coil heater

Answer 7

TO deposit known amount of electrical energy to calibrate the calorimeter

Question 8

Function of insulated lid

Answer 8

TO minims energy lost or gained from the surrounding

Question 9

Function of electrical ignition

Answer 9

TO supply energy (spark) to overcome the activation energy of the combustion reaction

Question 10

Function of the bomb

Answer 10

A special compartment that can withstand the high pressure of gases involved in combustion reaction (bomb is not insulated)

Question 11

Define electrochemical/galvanic cell

Answer 11

Device that converts chemical energy to electrical energy

Question 12

Why is electrical energy released in electrochemical cells?

Answer 12

• Half reactions occur in two separate components of the cell
• Oxidant and reductant do not come into direct contact with each other
• Electrons can only be transferred by travelled through the external circuit
• THis flow of electrons creates and electric current. Chemical energy of the reactants is transformed into electrical energy

Question 13

What is the external circuit

Answer 13

Path of electron movement, electric current flows through the wire and the light globe connecting the negative and positive electrodes

Question 14

What is the purpose of a salt bridge?

Answer 14

• Internal circuit is the path of ion movement
• Completes the electric circuit, contains ions that are free to move so they can balance charges formed in the half cells
• Maintains electrical neautrality

Question 15

Why do we need to replace the salt bridge with every new cell?

Answer 15

Salt bridge can be contaminated by the ion movement from the half cell electrolyte. If a contaminated salt bridge is used, precipitation may occur in the half cells

Question 16

What happens if oxidant reductant are in contact with each other?

Answer 16

No electricity because oxidant reductant directly contacted, no flow of electrons through external circuit

Question 17

Define battery

Answer 17

Contain multiple cells that work as a unit or contain more than two half cells

Question 18

Three basic types of cell define

Answer 18

• Primary cells: which are disposable and are designed not to be recharged
• Secondary cells which are rechargeable and are designed to be reused many times
• Fuel cells: which do not run down or need recharging as long as a fuel is supplied to them

Question 19

When does a primary cell run out?

Answer 19

• Redox reactions are reversible. When the redox reaction occurs in the cell and reaches equilibrium (forward and backward reactions occur simultaneously at the same rate) the cell will be flat. (no net flow of electrons).
• The cell products formed as it discharges slowly migrates away from the electrodes and are consumed by side reactions occurring in the cell, preventing ht cell from being recharged.

Question 20

Why can’t you recharge primary cells?

Answer 20

• TO maintain a net forward reaction the soluble reaction products must migrate away from the electrodes
• During use the built up of products around the electrodes slows and can even stop the forward reaction. Warming the cell with the heat of your hand can increase the rate of recovery

Question 21

Describe the design of primary cells

Answer 21

• two half cell reactions occur in separate places within one container
• An electrolyte composed of a moist paste plays the same role as the salt bridge, in addition or acting as a separator between the two half cells so it minimises leakages and is safer

Question 22

What is the function of a metal cap in dry cells?

Answer 22

It protects the brittle graphite cathode. It is an electron conductor. Cant use cardboard, plastic, wood, etc

Question 23

Why is the salt bridge a moist paste of electrolyte?

Answer 23

• Separates oxidant and reductant not allowing them to directly contact
• Must be moist then ions are ready to move, completing internal circuit

Question 24

How is a secondary cell recharged?

Answer 24

• TO recharge a cell the cell reaction must occur in reverse, the products of the reaction must be converted back into original reactant.
• Rechaging is done by connecting the cell to a charger, a source of electrical energy, which has a PD slightly greater than the PD of the cell
• THe positive and negative terminals of the charger are connected to their respective positive and negative electrodes of the flat cell

Question 25

What changes occur during recharging?

Answer 25

–Electrical energy from the battery charger is converted to chemical energy by reversing the discharged reaction of the cell
-The polarity of the electrons does not change for the discharge or recharge reactions but the process at each electrode is reversed
refer to short notes table

Question 26

Why can’t secondary cells be recharged indefinitely?

Answer 26

As each time a cell is discharged and recharged, a small amount of reactants is converted into an inactive form. the amount of active reactant, diminishes until finally the cell will no longer be recharged

Question 27

What is a measure of the performance of a battery?

Answer 27

The number of charge discharge cycles before a battery becomes unusable
-The limited life is due to unwanted physical and chemical changes occurring within the cell.

Question 28

Factors affecting battery life

Answer 28

• Side reactions that reduce amount of active material in the battery
• Not all the discharge products remain in contact with the electrodes
• Some of the discharge products become coarse and resist current flow
• Impurities may be present in the electrodes or half cell chemicals
• Contact of the electrolyte with the electrodes can decrease

Question 29

How does temperature affect battery life?

Answer 29

• As temp increases, rate of deterioration increases. During cell discharges, energy is released and some is converted to heat energy, raising the temperature, increasing rate of side reactions and reducing battery life
• As temp decreases rate or reaction falls, batteries deliver less electric charge at a specific discharge rate, under cold conditions leading to a decrease in battery capacity

Question 30

Nature of the electrode in a fuel cell

Answer 30

• Be conductive
• Have catalytic properties that speed up the rate of electrode reactions
• Be porous so they keep the gaseous fuel and oxidising agent apart, but allow each to be in contact with the electrode.

Question 31

Advantages of fuel cells

Answer 31

• Fuel cells convert chemical energy directly to electrical energy. This is more e cient than the series of energy conversions that takes place in power stations that burn fossil fuels: chemical energy → heat energy → mechanical energy → electrical energy.
• Hydrogen fuel cells produce water and heat as by-products. No greenhouse gases, such as carbon dioxide, are released.
• Fuel cells will generate electricity as long as the fuel is supplied. Conventional batteries need to be recharged or replaced.
• Fuel cells can use a variety of fuels.
• Electricity can be generated on-site and users are not reliant on connection to an electricity grid. Waste heat can be used to heat water for a hot water system or provide heating for a home during winter.

Question 32

Disadvantages of fuel cells

Answer 32

• Fuel cells require a constant fuel supply.
  • Fuel cells are expensive. They are still a developing technology, and are not being made in large numbers so there are no
  economies of scale as there are in other industries.
  • Some types of fuel cell use expensive electrolytes and catalysts.
  • The use of fuel cells in transport will require an extensive network of hydrogen lling stations before it can become widespread.
  • Fuel cells generate a direct current (DC); electrical appliances used in the home and in industry rely on an alternating current (AC). An inverter is required to change DC to AC at the appropriate voltage.
  • At present, the hydrogen used in many fuel cells is mainly sourced from fossil fuels. This process involves energy losses and generates greenhouse gases.
  • There are signi cant issues associated with the storage and safety of hydrogen fuel.