Chemistry Energy Changes

What happens to energy in an exothermic reaction?

• Energy is transferred to the surroundings.

• Surroundings become warmer.

• The reaction releases more energy when forming bonds than it absorbs when breaking bonds.

What are three common examples of exothermic reactions?

• Combustion (e.g., burning fuels)

• Oxidation reactions (e.g., respiration, rusting)

• Neutralisation (acid + base reactions)

What are two common everyday uses of exothermic reactions?

• Hand warmers (oxidation of iron or crystallisation of salts)

• Self-heating cans (calcium oxide + water reaction)

What happens to energy in an endothermic reaction?

• Energy is taken in from the surroundings.

• Surroundings become cooler.

• The reaction absorbs more energy when breaking bonds than it releases when forming bonds.

What are two common examples of endothermic reactions?

• Thermal decomposition (e.g., calcium carbonate → calcium oxide + CO₂)

• Reaction of citric acid and sodium hydrogencarbonate

What is an everyday use of endothermic reactions?

Sports injury packs (dissolving ammonium nitrate in water absorbs heat, cooling the pack).

How can you identify whether a reaction is exothermic or endothermic?

• Exothermic: Temperature of surroundings increases.

• Endothermic: Temperature of surroundings decreases.

How is an exothermic reaction shown in a reaction profile diagram?

• Reactants have more energy than products.

• Energy change (ΔH) is negative.

How is an endothermic reaction shown in a reaction profile diagram?

• Products have more energy than reactants.

• Energy change (ΔH) is positive.

What is activation energy?

• The minimum energy required for a reaction to occur.

• Represented as the peak of a reaction profile.

What effect does a catalyst have on a reaction?

• Lowers the activation energy by providing an alternative pathway.

• Increases the rate of reaction without being used up.

What is a chemical cell?

• A system where a chemical reaction produces electricity.

• Uses two different metals and an electrolyte.

What factors affect the potential difference of a chemical cell?

• Type of metals used (greater difference in reactivity = higher voltage).

• Type of electrolyte (affects ion movement).

• Concentration of the electrolyte.

Why do non-rechargeable batteries eventually stop working?

• The reactants are used up and the chemical reactions stop.

• Example: Alkaline batteries.

What is a fuel cell?

• A device that produces electricity continuously if it is supplied with fuel and oxygen.

• Uses a redox reaction to generate power.

What fuel is commonly used in fuel cells?

Hydrogen gas.

What is the overall reaction in a hydrogen fuel cell?

• Hydrogen + Oxygen → Water

• 2H₂ + O₂ → 2H₂O

What happens at the anode in a hydrogen fuel cell?

• Hydrogen gas loses electrons (oxidation).

• Produces H⁺ ions and electrons.

• H₂ → 2H⁺ + 2e⁻

What happens at the cathode in a hydrogen fuel cell?

• Oxygen gains electrons (reduction).

• Reacts with H⁺ ions to form water.

• O₂ + 4H⁺ + 4e⁻ → 2H₂O

What are the advantages of hydrogen fuel cells?

• No harmful emissions (only produces water).

• Continuous supply of electricity if fuel is available.

• Lightweight and compact compared to batteries.

What are the disadvantages of hydrogen fuel cells?

• Storage issues (hydrogen is explosive and difficult to transport).

• Expensive to produce hydrogen (often requires fossil fuels).

• Few refuelling stations available.

How do rechargeable batteries differ from hydrogen fuel cells?

• Rechargeable batteries store and release energy in a reversible reaction.

• Fuel cells need a continuous fuel supply.

What is a redox reaction?

A reaction where oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously.

Why do fuel cells have a higher efficiency than combustion engines?

• Fewer energy transfers (less wasted as heat).

• Direct chemical-to-electrical conversion.

How does a hydrogen fuel cell compare to fossil fuel power plants?

• Fuel cells are cleaner (no CO₂, NOₓ, or SO₂ emissions).

• Higher efficiency in energy conversion.

• More sustainable, but hydrogen production still requires energy.

What is bond breaking?

• Endothermic process (energy is absorbed).

• Reactants need energy to break bonds.

What is bond formation?

• Exothermic process (energy is released).

• Products release energy when new bonds form.

How do you calculate the overall energy change in a reaction?

Energy change = Energy needed to break bonds - Energy released when bonds form

If a reaction has a positive overall energy change, what does it mean?

• More energy was absorbed than released.

• The reaction is endothermic.

If a reaction has a negative overall energy change, what does it mean?

• More energy was released than absorbed.

• The reaction is exothermic.