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Energy Changes
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Energy Changes
Endothermic reactions
- A reaction where energy is transferred in from the surroundings
- Used for reactions that need to reduce temperature such as instant cold packs and Thermal decompositon reactions
- Bond Breaking
Exothermic reactions
- A reaction where energy from reaction is transferred to the surrondings
- Used for: Combustion, oxidation, neutralisation reactions and hand warmers
- Bond Making
Reaction Profiles
- Can show relative difference in energy reactants and products on reaction profiles
- Exothermic
- Energy released when new bonds are formed is greater thean energy required when bonds are broken
- Endothermic
- Activation energy needed to start reaction
- Energy released when new bonds are formed is less than energy needed to when bonds are broken
Bond Energy calculations
- Energy must be supplied to create or break bonds
- Can calculate overall energy change through bond energies
Chemical Cells and batteries
- Metals tend to lose electrons to form postive ions
- When two metals are dipped into a salt solution and joined by a wire the more reactive metal will donate electrons to the less reactive metal
- The Greater the difference in reactivity between metals the more voltage produced
- Non renewable batteries stop working when one of the reactants is used up while Rechargeable batteries use reverse reactions to regenerate the product to its original form
Fuel cells
- Most of world relies on fossil fuels
- Hydrogen can be alternate (can be burnt in combustion engines or used to power verichles
- Hydrogen gas can be oxidised to provide a source of electrons to the hydrogen fuel cell - waste product is water
- Hydrogen is highly flammable, sometimes produced by unrenewable means and is difficult to store