Chemistry GCSE Revision: Energy Changes

All chemical reactions involve energy and energy changes in various forms, including heat, light, and sound. For example, exploding dynamite is simply a rapid chemical reaction that releases a large amount of energy in a matter of microseconds. When heated, dynamite pushes nearby objects and the surrounding air outwards, sending shockwaves and shrapnel.

Energy changes in a chemical reaction can also be very slow, taking years or even decades, such as the slow rusting of metals. Chemical reactions like these are barely noticeable – but they still involve energy changes. This is because chemical reactions involve either the breaking or formation of bonds; and when this happens, energy is either released, absorbed, or both.

Exothermic And Endothermic Reactions

In terms of energy changes, a chemical reaction can be classified as either an exothermic or endothermic reaction:

  • Exothermic reactions release heat energy 
  • Endothermic reactions absorb heat energy

A chemical reaction is simply the breakdown of a molecule or compound into its simpler constituents. Everyday events, such as when iron and oxygen react to form rust, or when you light a fire or strike a match, are examples of chemical reactions

A chemical reaction involves two or more reactants that produce new products: when one chemical reacts with another chemical, its original bonds are broken and new bonds are formed. This releases energy. 

The elemental or active group constituents of one reactant then rearrange to combine with the constituents of the other reactant. In some cases, elemental forms of the constituents are released.

This is what happens when carbon dioxide is added to water, forming carbonic acid, or when vinegar is added to baking soda, causing it to froth and bubble as it becomes sodium acetate. It’s also what happens in our stratosphere with ozone (O3) molecules that are broken down by UV light to form O2

Ozone is a more complex molecular form of oxygen that has three atoms in a molecule and is written as O3. When ultraviolet rays hit an ozone molecule, an extra oxygen atom is dislodged, forming O2 and a free oxygen. This reaction also contributes to warming the upper atmosphere because it’s an exothermic reaction, i.e. when the ozone molecule is broken down, heat energy is released. 

What Is An Exothermic Chemical Reaction?

Exothermic chemical reactions release energy as a result of the reactants chemically combining and releasing byproducts. The most common and well-known types of exothermic chemical reactions are between acids and bases. These are otherwise known as neutralisation reactions, and their byproducts are salt and water.

Here’s an example of a simple neutralisation reaction:

Acid-base, neutralization reaction of hydrochloric acid and sodium hydroxide, infographic

As you can see in the example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the resulting products are water (H2O) and table salt, a.k.a. sodium chloride (NaCl). If you want a more precise balanced reaction, you can use the titration method. You can also use the balanced equation to mathematically determine the reaction and its byproducts. All you need to know for this is the precise molar mass of each reactant needed to have a complete neutralisation reaction. You can also calculate the energy released and confirm it experimentally.

For strong acids and alkalis, the enthalpy or energy of neutralisation involved is always negative because energy is released. It’s also important to remember that the term ‘strong’, when referring to either acid or alkali, doesn’t mean that it has a very low or very high pH. It actually refers to the tendency of these substances to be in a fully ionized state when dissolved in water. Therefore, the basic assumption is that these substances are fully ionized when they react with each other.

So, expanding on the example we gave above, the standard balanced equation for the chemical reaction between hydrochloric acid and sodium hydroxide solution is:


The active participants in the reaction are the hydroxide and the hydrogen ions. So, the chemical reaction can also be written as:


Other common examples of exothermic chemical reactions are the following:

  • Combustion reactions
  • Rusting of iron
  • Thermite reaction
  • Water and calcium chloride
  • Sodium sulfite and bleach
  • Potassium permanganate and glycerol
  • Sodium and chlorine to make sodium chloride (table salt)
  • Water and any strong acid
  • Water and any anhydrous salt
  • The biological respiration process

Speaking of the biological respiration process, anaerobic respiration in biological organisms is the most common and complex form of an exothermic chemical reaction. It’s basically the conversion of glucose into energy by using ATP and oxygen, and the main chemical waste byproduct is carbon dioxide.

What Is An Endothermic Chemical Reaction?

Endothermic chemical reactions are reactions that need energy to occur. Instead of releasing energy, they absorb energy to either split or join atomic bonds. The most common and best example of this is photosynthesis, which uses the energy of light to produce glucose out of the basic ingredients of water and carbon dioxide. This is an endothermic reaction because of the necessity for sunlight, i.e. light energy, to be absorbed by the plant:

Here are some other common examples of endothermic chemical reactions:

  • Dissolving ammonium chloride in water
  • Cracking alkanes
  • Thermal decomposition reactions
  • Mixing water and ammonium nitrate
  • Mixing water with potassium chloride
  • Ethanoic acid with sodium carbonate
Endothermic and Exothermic Reactions infographic diagram showing relation between reactant energy and product

Chemical Cells

Chemical cells are otherwise known as electrochemical cells. An electrochemical cell is a setup or a device that includes electrodes (cathodes and anodes), a chemical electrolyte (either solid or liquid), and the flow of electricity. Chemical cells either generate electricity from chemical reactions or use electricity to power chemical reactions.

Chemical cells that generate electrical currents are called voltaic cells or galvanic cells. They’re named in honour of two great scientists and inventors, Alessandro Volta and Luigi Galvani. Chemical cells that use electricity to generate chemical reactions, such as the splitting of water into hydrogen and oxygen, are called electrolytic cells, and these are used by many industrial processes, like electroplating.

The voltage output or the necessary voltage depends on several factors, including:

  • The type of chemicals in the electrolyte solution
  • The type of metals used as electrodes
  • The temperature of the immediate surrounding

Chemical reactions also depend on the reactivity of the electrodes and the dissolved solutes. For example, the direction of the current in a battery directly depends on the reactivity of the electrodes. Metals have different levels of reactivity, as shown in the example ranking below, from the least reactive to the most reactive:

  • Silver
  • Copper
  • Zinc
  • Magnesium

When immersed in an electrolyte such as an acid solution, electrodes will either give up or absorb electrons. This will largely depend on the reactivity of the electrodes. We can demonstrate the basic principles behind voltaic or galvanic cells using the simple balanced equation shown below, which shows the reaction of metallic zinc with cupric ion:


In this reaction, you can see that the less reactive copper is replaced by the more reactive zinc in the solution as an ion. Meanwhile, the less reactive metal precipitates from the solution.

GCSE Revision Guides:

Exothermic and Endothermic Reactions Sample Questions

Chemical Cells Sample Questions


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