2.7.7

Energy Changes During Reactions

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Energy Changes in Chemical Reactions

A chemical reaction happens when a substance undergoes a change into another substance. This requires energy.

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Chemical reactions

  • Substances that are at the start of chemical reactions are called reactants.
  • The substances that are made at the end are called the products.
  • All the atoms in a chemical reaction are conserved. This means they are not lost in the reaction.
  • The mass of the reactants will always equal the mass of the products.
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Exothermic reactions

  • Exothermic reactions are reactions that give off heat to the surroundings.
  • This causes the surroundings to get warmer.
  • Examples of exothermic reactions include:
    • Combustion (burning).
    • Respiration.
    • Neutralisation.
    • Rusting.
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Endothermic reactions

  • Endothermic reactions are reactions that take in heat from the surroundings.
  • This causes the surroundings to get cooler.
  • Examples of endothermic reactions include:
    • Photosynthesis.
    • Cold packs.
    • Evaporation of water.
    • Melting ice.
    • Thermal decomposition (when heat breaks down a substance into smaller parts).

Jump to other topics

1Biology

1.1Cells, Tissues & Organs

1.2Reproduction & Variation

1.3Ecological Relationships & Classification

1.4Digestion & Nutrition

1.5Plants & Photosynthesis

1.6Biological Systems & Processes

2Chemistry

2.1Particles

2.2Chemical Reactions

2.3Atoms, Elements, Compounds

2.4The Periodic Table

2.5Materials & the Earth

2.6Reactivity

2.7Energetics

2.8Properties of Materials

3Physics

3.1Energy

3.2Forces & Motion

3.3Waves

3.4Electricity & Magnetism

3.5Matter

3.6Space Physics

4Thinking Scientifically

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