C7 - Energy Changes
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State the law of conservation of energy
Energy cannot be created or destroyed; it can only be transferred from one place to another
What is an exothermic reaction?
A reaction where energy is transferred to the surroundings.
Give two examples of exothermic reactions
Combustion, respiration
What happens to the temperature of the surroundings during an exothermic reaction?
Increases
What is an endothermic reaction?
A reaction where energy is transferred from the surroundings
Give two examples of endothermic reactions
Thermal decomposition, citric acid and sodium hydrogen carbonate
What happens to the temperature of the surroundings during an endothermic reaction?
Decreases
State two uses of exothermic reactions
Self-heating cans, hand warmers
State two uses of endothermic reactions
Some cooling packs for sports injuries
What is a reaction profile?
A diagram which shows whether the reactants have more or less energy than the products
State which of bond breaking and bond making is endothermic and which is exothermic
Breaking: endothermic
How do we work out the overall change of a reaction?
Work out the difference between the energy needed to break all the bonds in the reactants and the energy released to form all the bonds in the products
What is a chemical cell?
A unit which contains chemicals which produce electricity through a reaction
What are the main components of a chemical cell?
An anode, a cathode and an electrolyte
What is a battery?
Two or more cells connected in series
What happens to the electrodes in a chemical cell?
The more reactive metals depletes and the less reactive one increases in mass
Why can some cells not be recharged?
Because the reaction is not reversible
How can certain cells be recharged?
Applying an external electric current
How does the reactivity of the metal electrodes affect the size of the potential difference?
The greater the difference in reactivity
What is a fuel cell?
A cell which uses a fuel and oxygen (or air) to generate electricity
What are the products in a hydrogen fuel cell?
Water
State three advantages of a hydrogen fuel cell?
Do not need to be recharged, non pollutants are produced, can be different sizes for different uses
State three disadvantages of hydrogen fuel cells?
Hydrogen is highly flammable, hydrogen is sometimes produced though non-renewable means, hydrogen is difficult to store
Give the overall equation in a hydrogen fuel cell
2H2 (g) + O2 (g) → 2H2O (l)