4.5 Energy changes - flashcards
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40 Terms
Q: What is an exothermic reaction?
A: A reaction that releases energy to the surroundings, usually shown by a rise in temperature.
Q: Give 3 examples of exothermic reactions.
A: Combustion, neutralisation (acid + alkali), oxidation reactions.
Q: Name two everyday uses of exothermic reactions.
A: Hand warmers (iron oxidation), self-heating cans (hot chocolate/coffee).
Q: What is an endothermic reaction?
A: A reaction that takes in energy from the surroundings, usually shown by a fall in temperature.
Q: Give 3 examples of endothermic reactions.
A: Reaction between citric acid and sodium hydrogencarbonate, thermal decomposition, electrolysis.
Q: What happens if products have more energy than reactants?
A: Energy is absorbed from the surroundings (endothermic reaction).
Q: What happens if products have less energy than reactants?
A: Excess energy is released to the surroundings (exothermic reaction).
Q: What is collision theory?
A: Particles must collide with sufficient energy and the correct orientation for a reaction to occur.
Q: Name three factors that affect collisions in chemical reactions.
A: Energy, orientation, number of collisions per second.
Q: What is activation energy?
A: The minimum energy required for reactants to collide and react.
Q: Draw the general shape of an exothermic reaction profile.
A: Reactants at higher energy than products; initial rise = activation energy; overall drop = energy released.
Q: Draw the general shape of an endothermic reaction profile.
A: Products at higher energy than reactants; initial rise = activation energy; overall rise = energy absorbed.
Q: Explain why breaking bonds is endothermic.
A: Energy must be supplied to break existing bonds.
Q: Explain why forming bonds is exothermic.
A: Energy is released when new bonds are formed.
Q: When is a reaction overall exothermic or endothermic?
A: Exothermic: energy released forming new bonds > energy used to break old bonds.
Q: What is an electrochemical cell?
A: A system of two different electrodes in contact with an electrolyte that produces electricity.
Q: What is the function of the electrolyte in a cell?
A: A liquid that contains ions which react with the electrodes to allow charge flow.
Q: Why are the electrodes in a cell made of metals?
A: Metals conduct electricity and can release electrons during reactions.
Q: In a zinc-copper cell, which electrode is negative and why?
A: Zinc is negative because it is more reactive and loses electrons more easily.
Q: How does a wire connected between electrodes produce electricity?
A: Electrons flow from the negative electrode (more reactive) to the positive electrode, creating a current.
Q: What factors affect the voltage of a cell?
A: Type of metals, difference in reactivity, and type of electrolyte.
Q: What is the difference between a battery and a single cell?
A: A battery connects two or more cells in series to increase voltage.
Q: What happens in non-rechargeable batteries over time?
A: Reactants are used up, products cannot revert to reactants, so the cell cannot produce electricity.
Q: What is a rechargeable battery?
A: A battery where the reaction can be reversed by applying an external electric current.
Q: What is a fuel cell?
A: An electrical cell supplied with fuel and oxygen that produces energy from the reaction efficiently.
Q: What type of fuel cell uses hydrogen and oxygen?
A: Hydrogen-oxygen fuel cell.
Q: What is the reaction at the anode (negative electrode) in a hydrogen-oxygen fuel cell?
A: H2 → 2H+ + 2e⁻ (oxidation)
Q: What is the reaction at the cathode (positive electrode) in a hydrogen-oxygen fuel cell?
A: O2 + 4H++ 4e⁻ → 2H2O (reduction)
Q: What is the overall equation for a hydrogen-oxygen fuel cell?
A: 2H2 + O2 → 2H2O
Q: List three advantages of hydrogen fuel cells.
A: Produce no pollution, more energy per kg than petrol/diesel, no moving parts so no energy lost in transmission.
Q: List three disadvantages of hydrogen fuel cells.
A: Expensive materials, high-pressure storage tanks needed, less efficient in low temperatures.
Q: How does a fuel cell produce electricity continuously?
A: As long as fuel (hydrogen) and oxygen are supplied, the reaction continues.
Q: Why are hydrogen fuel cells better for the environment than batteries in electric vehicles?
A: No toxic battery disposal, fewer pollutants, higher energy efficiency.
Q: What is the difference in energy change between exothermic and endothermic reactions?
A: Exothermic: ΔE is negative (energy released). Endothermic: ΔE is positive (energy absorbed).
Q: In terms of bond energies, when is a reaction exothermic?
A: When the energy released forming bonds > energy needed to break bonds.
Q: In terms of bond energies, when is a reaction endothermic?
A: When the energy needed to break bonds > energy released forming bonds.
Q: Give an example of an endothermic reaction used in sports injury packs.
A: Sodium hydrogencarbonate + citric acid reaction.
Q: Give an example of an exothermic reaction used in hand warmers.
A: Oxidation of iron in air with a salt solution catalyst.
Q: What is thermal decomposition?
A: A reaction where a compound breaks down into simpler substances when heated (endothermic).
Q: What is electrolysis?
A: Using electricity to break down compounds into elements (endothermic).