Energy Changes Flashcards
What does exothermic mean?
Energy is transferred to the surroundings
What does endothermic mean?
Energy taken in from the surroundings
What is the “surroundings” in a reaction?
Everything else that isn’t the reaction
What is activation energy?
The minimum amount of energy particles must have to react when they collide
What are atoms held together by?
Strong chemical bonds
What happens in a reaction? (atoms)
Bonds are broken, and new ones are made between different atoms
Is breaking a bond endothermic or exothermic?
Endothermic - it requires energy
Is making a bond endothermic or exothermic?
Exothermic - making a bond
When is a reaction exothermic?
If a lot of energy is released when making bonds, and little is required to break them so overall energy is released, so exothermic.
When is a reaction endothermic?
If little energy is released when making bonds and a lot is required to break them, energy is taken in so the reaction is endothermic.
How do you work out the overall energy change of a reaction?
- Work out how much energy is required to break all bonds in the reactants
- Work out how much energy is released when making all the bonds in the products.
Overall energy transferred = energy required to break bonds - energy required to make bonds
• A positive numbers = endothermic reaction.
• A negative numbers = exothermic reaction.
What is the temperature change of the surroundings in an exothermic reaction?
Increase
What is the temperature change of the surroundings in an endothermic reaction?
Decreases
What is an example of an exothermic reaction?
Oxidation, combustion, neutralisation…
What is an example of an endothermic reaction?
Thermal decomposition, citric acid + sodium hydrogen carbonate…
What is an everyday use of exothermic reactions?
Self-heating cans, hand warmers
What is an everyday use of endothermic reactions?
Sports injury packs
What happens to the metal in metal displacement reactions?
One metal is oxidised so loses electrons.
These are transferred to another metal, so this metal is is reduced.
How do chemical cells generate a current?
Using metal displacement reactions - electron’s movement generates a current
Chemical cells: what happens to the atoms from one of the electrodes?
Electrons are lost, ionised and they move into the solution
Chemical cells: how does the LED light?
The electrons from one of the electrodes travel through the circuit to the other electrode, turning on the LED.
Chemical cells: what happens at the other electrode?
A metal ion from the solution will pick up the electrons up and become a metal atom.
Chemical cells: how can you increase the p.d. induced?
Increase the reactivity between the two metals in the cell
Battery: what does rechargeable mean?
An external electric current is applied -> reverses the reaction
Why are some batteries not rechargeable?
The reaction is not reversible.
One reactants are used up, the chemical reaction stops and no more p.d. is released.
What do fuel cells do?
Generate a potential difference using fuel and oxygen from the air
How do hydrogen fuel cells generate electricity?
Using hydrogen and oxygen. Hydrogen is oxidised to produce water.
What are advantages of hydrogen fuel cells?
- The only waste is water
- Do not need to be electrically recharged
What are disadvantages of hydrogen fuel cells?
- Hydrogen is highly flammable and difficult to store
- Hydrogen is often produced from non-renewable resources
