Energy Changes

12 important questions on Energy Changes

What is an endothermic reaction?

one which takes energy in from the surroundings. Shown by a fall in temperature. e.g. citric acid and sodium hydrogen carbonate

How does a disposable hand warmer work?

They use the exothermic oxidation of iron in air with a salt solution catalyst to release energy and warm up surroundings

How does a a reusable hand warmer work?

A small metal disc in the plastic pack is used to start the exothermic where the crystallisation of sodium ethane transfers heat energy to the surroundings. It is reusable because you can put the pack in boiling water to re dissolve the crystals.
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What is activation energy?

Minimum amount of energy needed to start a reaction

Bond energy calculations

1) decide whether bonds are being broken (endothermic- absorbs energy) or made (exothermic-releases energy)
2) look at the table and determine how much energy is required

What are the main parts of a battery?

The basic power unit inside a battery is called a cell and it consists of three main bits. There are two electrodes that have a chemical called electrolyte in between them. For safety they are usually packed inside a metal or plastic outer case. There are two electrical terminals marked with a plus and minus on the outside which are connected to the inside electrodes.

How do batteries work?

Electrical cells are made from zinc and copper. Electrons flow from the more reactive metal (zinc) to the less reactive metal (copper). So zinc acts as the negative terminal of the cell. REDOX

Why do batteries need different materials?

The greater the difference in reactivity between the two metals used the higher the voltage produced.

What is the difference between primary and secondary cells?

primary cells cannot be recharged and are prone to leakage whereas secondary cells can be recharged by reversing the reaction that occurs at each electrode when the cell is discharging.

What are the advantages of hydrogen fuel cells?

Do not need to be electrically recharged
no pollutants
range of sizes for different uses

What are the disadvantages of hydrogen fuel cells?

Highly flammable
hydrogen sometimes produced by non-renewable sources
difficult to stop hydrogen

What is a battery?

a self contained chemical power pack that can produce a limited amount of electrical energy.

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