Energy Change (C5) Flashcards
Describe an Endothermic reaction
The reactants take in energy from the surroundings during the reaction
Describe an Exothermic reaction
The reactants give out energy to the surroundings during the reaction
What happens during any reaction?
Bonds are made or broken
What happpens when bonds are broken?
Energy is taken in (endothermic)
What happpens when bonds are made?
Energy is released (exothermic)
Examples of exothermic reations
Combustion
Neutralisation
Oxidation
Examples of endothermic reactions
Reaction between citric acid and sodium hydrogen carbonate (i.e.: lemon juice and baking soda)
Thermal Decomposition
Describe the reaction profile of an endothermic reaction
The energy increases to the activation energy and then falls a bit but not as low as the original reactants.
Describe the reaction profile of an exothermic reaction
The energy increases to the activation energy and then falls lower than the original reactants.
What is the heighest point on the reaction profile?
Activation Energy
What is the activation energy?
The minimum amount of energy the particles need when colliding to react.
How can we measure the amount of energy released by a chemical reaction?
Take the temperature of the reactants, mix them in a polyester cup and measuring the temperature at the end of the reaction
What is the biggest problem with measuring energy released from a chemical reaction using a polyester cup and how can it be reduced?
A lot of energy is lost to the surroundings
It can be reduced by placing a lid on the cup and insulating it with cotton wool
What variables must be controlled when investigating energy transferred in a reaction?
Mass
Concentration
How often the tempreature is taken
In a bond energy equation, if the answer is positive it means that…
More energy was taken in than released (endothermic)
In a bond energy equation, if the answer is negative it means that…
More energy was released than taken in (endothermic)
What are electrochemical cells?
A basic system by which chemical reactions create electricity
How do electrochemical cells work?
Two electroldes are in an electrolyte with ions which are reacting and go to either pole
The two electrolytes are connected by a wire and thus the delocalised electrons flow through
What changes the charge or voltage of a cell?
Type of electrode
Type of electrolyte
The greater the difference in reactivity of the electrodes….
…the greater the voltage of the cell
What is a battery?
Two combined cells
What is the voltage of any given battery?
The sum of the voltage of its individual cells
What does it mean if a battery is non-rechargable?
The chemical reactions that occur at the electrodes are IRREVERSABLE and so the reactants are eventually used up
What type of batteries are alkaline batteries?
Non-rechargable
How do rechargable batteries work?
The reaction is reversible so when hooked up to an external electric current it is reversed and the reactants can be re-used
Describe Fuel Cell
An electrical cell that is supplied with fuel and oxygen (or air) and uses energy from the reaction between them to produce electrical energy efficiently
Describe how a hydrogen fuel cell works
Hydrogen is oxidised to release energy
The only bi-product is water
What acid is used as an electrolyte in a hydrogen fuel cell?
Phosphoric Acid
What is at the cathode in a hydrogen fuel cell?
Oxygen
What is at the anode in a hydrogen fuel cell?
Hydrogen
What is the reaction at the cathode in a hydrogen fuel cell? Give the equation?
2O2++4H++4e–>2H2O
What is the reaction at the anode in a hydrogen fuel cell? Give the equation?
H2->2H++2e-
What is the overall equation for the reaction in a hydrogen fuel cell?
2H2+O2->2H2O
Advantages to Hydrogen Fuel Cells
- Non pollutant
- Less expensive than batteries
- Store more energy than batteries
Disadvantages to Hydrogen Fuel Cells
- Needs a lot of space (gas)
- It is explosive
- Often made by other hydrocarbons
