Flashcards in Energy and redox Deck (32):
Heat of combustion definition
The enthalpy change that occurs when a specific quantity of the fuel burns completely in oxygen
Improvements for spirit burner prac
Shield the flame, insulate the container and cover the container with insulated lid (polystyrene), keep the flame closer to the can/water beaker
Reducing heat loss during calorimetry experiment
Using insulation around the container and the lid
Placing insulation around the burning fuel
Function of thermometer
To record the initial and final temperature in order to calculate the temp change
Function of stirrer
To distribute the thermal energy evenly throughout the calorimeter
Function of water
Serves as a heat bank due to high specific heat capacity
Function of electrical coil heater
TO deposit known amount of electrical energy to calibrate the calorimeter
Function of insulated lid
TO minims energy lost or gained from the surrounding
Function of electrical ignition
TO supply energy (spark) to overcome the activation energy of the combustion reaction
Function of the bomb
A special compartment that can withstand the high pressure of gases involved in combustion reaction (bomb is not insulated)
Define electrochemical/galvanic cell
Device that converts chemical energy to electrical energy
Why is electrical energy released in electrochemical cells?
-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
What is the external circuit
Path of electron movement, electric current flows through the wire and the light globe connecting the negative and positive electrodes
What is the purpose of a salt bridge?
-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
Why do we need to replace the salt bridge with every new cell?
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
What happens if oxidant reductant are in contact with each other?
No electricity because oxidant reductant directly contacted, no flow of electrons through external circuit
Contain multiple cells that work as a unit or contain more than two half cells
Three basic types of cell define
-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
When does a primary cell run out?
-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.
Why can't you recharge primary cells?
-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
Describe the design of primary cells
-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
What is the function of a metal cap in dry cells?
It protects the brittle graphite cathode. It is an electron conductor. Cant use cardboard, plastic, wood, etc
Why is the salt bridge a moist paste of electrolyte?
-Separates oxidant and reductant not allowing them to directly contact
-Must be moist then ions are ready to move, completing internal circuit
How is a secondary cell recharged?
-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
What changes occur during recharging?
--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
Why can't secondary cells be recharged indefinitely?
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
What is a measure of the performance of a battery?
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.
Factors affecting battery life
-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
How does temperature affect battery life?
-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
Nature of the electrode in a fuel cell
-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.
Advantages of fuel cells
• 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.