C7: Energy Changes Flashcards
What are exothermic reactions?
Chemical reactions that transfer energy to its surroundings. There is a higher energy content in the reactants.
What are endothermic reactions?
Chemical reactions that take in energy from its surroundings. There is a higher energy content in the products.
What are examples and use of exothermic reactions?
Combustion, neutralisation, displacement.
Hand warmers and self heating cans.
What are examples and uses of endothermic reactions?
Thermal decomposition, photosynthesis.
Instant cold packs for sports injuries.
Describe how to carry out the temperature changes required practical.
- Measure 25cm3 of hydrochloric acid into a polystyrene cup and place it in a beaker to make it more stable.
- Measure and record the temperature of hydrochloric acid.
- Measure 5cm3 of sodium hydroxide and add it to the polystyrene cup.
- Quickly put a lid on the cup and gently stir the solution using a thermometer through the hole in the lid.
- When the reading on the thermometer becomes constant, record the temperature.
- Add further 5cm3 amounts of sodium hyroxide up to 40cm3.
- Repeat investigation and calculate a mean maximum temperature reached for each sodium hydroxide volumes.
What is activation energy?
The minimum amount of energy needed for particles to collide and start a reaction.
What is the difference between endothermic and exothermic reactions in terms of bond energy?
• In exothermic reactions, the energy released when new bonds are formed is greater than the energy taken in to break the bonds in the reactants.
• In endothermic reactions, the energy taken in to break the bonds is greater than the energy released to make the bonds in the products.
What are cells?
Cells contain chemicals which react to produce electricity and a voltage.
A simple cell can be made by connecting 2 different metals in contact with an electrolyte.
The greater the difference in reactivity between the 2 metals used, the higher the voltage produced by the cell.
What are batteries?
2 or more cells connected together in series to provide a greater voltage.
What are non rechargeable cells/batteries?
Chemical reactions stop when on of the reactants hase been used up. E.g. alkaline batteries.
What are rechargeable cells/batteries?
Chemical reactions are reversed when external electric current is applied. They eventually stop working and are hard to dispose of.
Is using hydrogen as fuel good for the environment?
It prevents global warming as it won’t produce carbon dioxide however it still requires electricty from non renewable fossil fuels.
What are the advantages of fuel cells?
They do not need to be recharged, no pollutants produced, can be a range of sizes for different uses.
What are the disadvantages of fuel cells?
Hydrogen is flammable, hydrogen is produced by non renewable sources and is difficult to store.
