C7- Energy Changes Flashcards
Exothermic reactions
Release energy into the surroundings. Often heats up the surroundings.
Endothermic reactions
Take in energy from their surroundings
Example of an exothermic reaction
The burning of fuels such as the comustion of methane gas.
Energy in the products of exothermic reactions
Less energy than the reactants
Example of an Endothermic reaction
Thermal decomposition reactions e.g the decomposition of Calcium carbonate
Energy in the products of Endothermic reactions
The products have more energy than the reactants
Uses of Exothermic reactions
Chemical hand warmers use exothermic reactions to warm you up
Self heating cans also use exothermic reactions
What are reusable hand warmers based on?
The formation of Crystals from solutions of a salt
Uses of endothermic reactions
Chemical cold packs
How do chemical cold packs work?
They usually contain ammonium nitrate and water
When ammonium nitrate dissolves it absorbs energy from its surroundings making them colder.
What are reaction profiles?
Diagrams showing the relative amounts of energy contained in the reactants and the products.
What is on the X axis of a reaction profile?
Progress of reaction
What is on the Y axis of a reaction profile
Energy
What is the unit that energy change is measure in ?
kJ/mol or Kilojoules per mole
Exothermic reactions ( energy change during reaction)
More energy in reactants
Less energy in products
Difference in energy is the energy transferred to the surroundings
Endothermic reactions (energy change during reaction)
Less energy in reactants
More energy in the products
The difference is the energy taken in from the surroundings.
Activation energy
The minimum amount of energy needed before colliding particles of reactants have sufficient energy to cause a reaction
What is bond energy?
The energy needed to break the bond between two atoms.
The greater the difference in reactivity between the two metals used…. ( electrical cells and batteries)
The higher the voltage produced
Chemical cell set up
Join two metals together by a wire and dip them into an electrolyte and electrons will flow through the wire from one metal to another
How can some batteries be recharged?
The battery is connected to a power supply that reverses the reactions that occur at each electrode when the cell is discharging. This regenerates the original reactants.
Benefits of hydrogen as a fuel
It burns well and produces no pollutants
What are fuel cells?
These cells are fed with hydrogen and oxygen and produce water. Most of the energy released in the reaction is transferred to electrical energy.
Advantages of hydrogen fuel cells
Do not need to be electrically recharged
No pollutants are produced
Can be a range of sizes for different uses
Disadvantages of hydrogen fuel cells
Hydrogen is highly flammable
Hydrogen is sometimes produced for the cell by non-renewable sources
Hydrogen is difficult to store
