Topic 5 (Energy Changes) Flashcards
What is an exothermic reaction?
A reaction that transfers energy to the surroundings, usually as heat, causing the temperature to increase.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings, usually as heat, causing the temperature to decrease.
Give two examples of exothermic reactions.
Combustion
Neutralisation (acid + alkali)
Give two examples of endothermic reactions.
Thermal decomposition
The reaction between citric acid and sodium hydrogencarbonate
Photosythsis
What is energy transferred as in these reactions?
Mostly as heat energy.
What are some everyday uses of exothermic reactions?
Hand warmers
Self-heating cans
What are some everyday uses of endothermic reactions?
Instant cold packs (used in sports injuries)
In terms of bonds, when is energy released?
Energy is released when bonds are formed.
In terms of bonds, when is energy taken in?
Energy is taken in when bonds are broken.
What happens to temperature in an exothermic reaction?
The temperature of the surroundings increases.
What happens to temperature in an endothermic reaction?
The temperature of the surroundings decreases.
What is activation energy?
The minimum amount of energy needed for a reaction to start.
: What does a reaction profile show?
The energy levels of reactants and products during a chemical reaction.
In a reaction profile, how can you tell if a reaction is exothermic?
The products are at a lower energy level than the reactants.
In a reaction profile, how can you tell if a reaction is endothermic?
The products are at a higher energy level than the reactants.
What is meant by bond energy (or bond enthalpy)?
The energy needed to break 1 mole of a specific type of bond in a gaseous molecule.
Is bond breaking endothermic or exothermic?
Endothermic – energy is taken in to break bonds.
Is bond forming endothermic or exothermic?
Exothermic – energy is released when new bonds form.
What is the formula to calculate the overall energy change in a reaction?
Energy change = Total energy in (bonds broken) – Total energy out (bonds formed)
What does it mean if the energy change is negative?
The reaction is exothermic – more energy is released forming bonds than is taken in to break them.
What does it mean if the energy change is positive?
The reaction is endothermic – more energy is taken in to break bonds than is released when forming them.
What units are bond energies measured in?
kJ/mol (kilojoules per mole)
Why do we use average bond energies in calculations?
Because actual bond energies vary depending on the compound they’re in, so we use mean values.
How do you calculate the energy needed to break bonds?
Multiply the bond energy value (from data) by the number of bonds broken.
If a reaction has higher bond energies in the reactants than the products, what type of reaction is it?
Endothermic, because more energy is required to break bonds than is released forming them.
What is a chemical cell?
A device that produces a voltage from a chemical reaction between two different metals in an electrolyte.
What three components are needed to make a simple cell?
Two different metals (electrodes)
An electrolyte
Wires to complete the circuit
What causes the voltage in a cell?
The difference in reactivity between the two metals. The bigger the difference, the higher the voltage.
What is the function of the electrolyte?
It allows ions to move, enabling the chemical reactions that produce electricity.
What affects the voltage of a cell?
The types of metals used
The electrolyte used
The concentration of the electrolyte
What is a battery?
A group of two or more cells connected in series to produce a higher voltage.
What is a non-rechargeable battery?
A battery where the chemical reactions are irreversible – once the reactants are used up, it can’t be recharged (e.g. alkaline batteries).
What is a rechargeable battery?
A battery where the chemical reactions can be reversed by applying an external current, allowing it to be used multiple times (e.g. lithium-ion batteries).
Why do batteries go flat?
The reactants are used up, so the cell can no longer produce a voltage.
What are the advantages of rechargeable batteries?
Reusable – reduce waste
Cheaper over time
Less environmental impact than disposing of many single-use batteries
What is a fuel cell?
A device that produces electricity through a chemical reaction between a fuel (often hydrogen) and oxygen, without combustion.
What is the most common type of fuel cell in the GCSE spec?
The hydrogen-oxygen fuel cell.
What is the only product of a hydrogen fuel cell?
Water (H₂O).
What are the half-equations at the electrodes in a hydrogen-oxygen fuel cell?
Anode (−): H₂ → 2H⁺ + 2e⁻
Cathode (+): O₂ + 4H⁺ + 4e⁻ → 2H₂O
What is the overall equation for the hydrogen-oxygen fuel cell?
2H₂ + O₂ → 2H₂O
Is the reaction in a fuel cell exothermic or endothermic?
Exothermic – energy is released as electricity and heat.
What are the advantages of hydrogen fuel cells?
Only product is water (no CO₂)
Continuous supply of energy if fuel is provided
Lightweight and compact
No recharging required
What are the disadvantages of hydrogen fuel cells?
Hydrogen is flammable and difficult to store
Hydrogen production may involve fossil fuels
Expensive to manufacture the fuel cell system
What are fuel cells commonly used for?
Spacecraft
Electric vehicles
Backup power systems
How is a hydrogen fuel cell different from a battery?
Fuel cells need a constant supply of fuel and oxygen
Batteries store a limited amount of chemicals and need recharging
Fuel cells don’t run down if fuel is supplied
