Energy Changes Flashcards
What is an exothermic reaction?
Transfers energy to surroundings
What is an endothermic reaction?
Energy is transferred from the surroundings
What does the energy change cause?
A temperature change
Energy is always _________ in chemical reactions
Conserved
What is a reaction profile?
Shows whether a reaction is exothermic or endothermic
What is activation energy?
The minimum amount of energy particles must have to react when they collide
What are “the surroundings”?
Everything that isn’t in the reaction
What is breaking a bond?
Requires energy so is endothermic
What is forming a bond?
Exothermic because it releases energy
What are bonds?
Atoms are held together by strong chemical bonds. In a reaction, these bonds are broken and new bonds are formed.
When is the overall reaction exothermic?
When more energy is released making bonds than required to break bonds
When is the overall reaction endothermic?
If a less energy is released making the bonds than required to make the bonds
What are bond calculations?
Energy required to break bonds - energy required to make bonds
Positive number = endothermic
Negative number = exothermic
What are examples of exothermic reactions?
Oxidation
Combustion
Neutralisation
What are everyday uses of exothermic reactions?
Self heating cans
Hand warmers
What are examples of endothermic reactions?
Thermal decomposition
Citric acid and sodium hydrogen carbonate
What are everyday uses of endothermic reactions?
Sports injury packs
What are batteries?
Two or more cells connected in series
Why are some batteries rechargeable?
An external electric current is applied which reverses the reaction
Why are some batteries not rechargeable?
Some batteries, like alkaline batteries, are not rechargeable because the reaction is not reversible. Once the reactants are used up, the chemical reactions stops.
What is a metal displacement reaction?
One metal is oxidised and loses electrons. These electrons are transferred to another metal, which gains electrons so is reduced.
What is a chemical cell?
Uses the movement of electrons in a metal displacement reaction to generate a current.
What does a greater difference in reactivity of metals lead to?
The greater the potential difference produced.
What are hydrogen fuel cells?
Use a fuel and oxygen from the air to produce a potential difference.
Often generate electricity from hydrogen and oxygen
2 atoms of hydrogen are oxidised to form two molecules of water.
What are the half equations in alkaline fuel cells?
2H2 + 4OH- –> 4H2O + 4e-
O2 + 2H2O + 4e- –> 4OH-
What are advantages of hydrogen fuel cells?
Only waste is water
Do not need to be electrically recharged
What are disadvantages of hydrogen fuel cells?
Hydrogen is highly flammable and hard to store
Hydrogen is often produced from non-renewable resources
