GCSE Chemistry A (Triple) OCR

This subject is broken down into 82 topics in 5 modules:

  1. Particles 4 topics
  2. Elements, Compounds and Mixtures 17 topics
  3. Reactions and Products 11 topics
  4. Chemical Reactions 26 topics
  5. Global Challenges 24 topics
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  • 82
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  • 28,012
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This page was last modified on 28 September 2024.

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Chemistry A (Triple)

Particles

States of Matter

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States of Matter

States of Matter

Particles

  • There are three common states of matter - solid, liquid, and gas.
  • The particles are arranged and move differently in these three states.
  • Solids have a fixed shape and volume. In a solid, particles vibrate around fixed positions and are close together.
  • Liquids fill the bottom of a container, and they can be poured. In a liquid, particles can move around each other and are close together.
  • Gases completely fill their containers. In a gas, particles move freely and are far apart from each other.

Changes of State

  • Melting and Freezing: The change from solid to liquid is melting, and from liquid to solid is freezing. During these changes of state, the temperature remains constant.
  • Boiling and Condensing: The transition from liquid to gas is boiling or evaporation, and from gas to liquid is condensation. During these changes, the temperature remains constant.
  • Sublimation and Deposition: Some substances can change directly from solid to gas (sublimation) and from gas to solid (deposition) without becoming a liquid.

Understanding Kinetic Theory

  • The kinetic theory helps to explain the different properties of solids, liquids, and gases.
  • All particles are in constant motion. The energy of this motion is called kinetic energy.
  • Higher temperatures translate to more kinetic energy, meaning that particles move faster.

Energy Needed for Changes of State

  • The energy needed to change a substance from one state to another is known as the Latent Heat.
  • Specific latent heat is the energy required to change 1 kg of a substance from one state to another without changing its temperature.
  • The specific latent heat of fusion refers to the change from solid to liquid (or vice versa).
  • The specific latent heat of vaporisation refers to the change from liquid to gas (or vice versa).

Internal Energy

  • The energy stored by particles within a system is called internal energy.
  • An increase in internal energy can result from an increase in heating (leading to higher kinetic energy) or by doing work on the system (leading to an increase of potential energy).

Properties of Gases

  • Random motion and collisions of molecules causes pressure.
  • Increasing the temperature of a gas will increase the speed and kinetic energy of the molecules, thus increasing pressure.
  • The relationship between pressure, volume and temperature is outlined by the equation of state PV=nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant and T is temperature.

Course material for Chemistry A (Triple), module Particles, topic States of Matter

Chemistry A (Triple)

Chemical Reactions

Endothermic Reactions

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Endothermic Reactions

Understanding Endothermic Reactions

  • An Endothermic Reaction is a chemical reaction that absorbs energy from its surroundings, typically in the form of heat.
  • Such reactions need heat to proceed and therefore the surroundings usually feel colder once the reactions are complete.
  • The easy recognition of endothermic reactions is a drop in temperature during the reaction.
  • Photosynthesis, the process by which green plants use sunlight to synthesise foods, is a prime example of an endothermic reaction.

Energy Changes in Endothermic Reactions

  • In endothermic reactions, the energy required to break the bonds in the reactants is greater than the energy released when new bonds are formed in the products.
  • Therefore, the total energy of the products is higher than the total energy of the reactants. This energy difference is often called the enthalpy change (∆H).
  • The enthalpy change for endothermic reactions is positive as energy is taken in.

Endothermic Reactions in Everyday Lives & Industries

  • Endothermic reactions are extremely important for a lot of daily processes and industrial applications.
  • For example, the process of cooking, like baking bread or boiling an egg, is endothermic.
  • Sports injury packs are another example, as they become cold when squeezed which triggers an endothermic reaction.
  • Various endothermic processes in industries include metal extraction and the production of ammonia.

Profiles of Endothermic Reaction & Mastery Tips

  • Understanding the profile of endothermic reactions is key to being able to identify them. In graphs of energy against reaction progress, the products line lies above the reactants line indicating a gain in energy.
  • Energy changes may be calculated using bond energies with practice, and understanding these calculations forms an essential part of mastering chemistry.
  • A regular practice of different problems associated with endothermic reactions will ensure a solid understanding of this concept.
  • Always remember, in an endothermic reaction, energy is taken in from the surroundings and the enthalpy change (∆H) is positive.

Course material for Chemistry A (Triple), module Chemical Reactions, topic Endothermic Reactions

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