5. Energy Changes Flashcards
What is an exothermic reaction?
One which transfers energy to the surroundings, usually by heating, shown by a rise in temperature.
What are examples of exothermic reactions?
Combustion, neutralisation, most oxidation reactions.
What are everyday uses of exothermic reactions?
Handwarmers (oxidation of iron in air with salt solution catalyst). Self heating cans of coffee.
What is an endothermic reaction?
One which takes in energy from the surroundings, shown by a fall in temperature.
Are endo or exo thermic reactions more common?
Exothermic.
What are examples of endothermic reactions?
Reaction between citric acid and sodium hydrogencarbonate. Thermal decomposition.
What is an everyday use of endothemic reactions?
Sports injury packs.
Describe a practical where you can measure energy released in a chemical reaction.
Taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.
How can you reduce the energy lost to the surroundings in this practical?
Putting the polystrene cup in a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation.
How could you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide?
- Put 25cm³ of 0.25 mol/dm³ of hydrochloric acid and sodium hydroxide into seperate beakers.
- Place the beakers in a water bath set to 25*C until they are both at the same temperature.
- Add the HCl followed by the NaOH to a polystrene cup with a lid.
- Take the temperature of the mixture every 30 seconds, and record the highest temperature.
- Repeat steps 1-4 using 0.5 mol/dm³ and then 1 mol/dm³ of hydrochloric acid.
What do reaction profile show?
The relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction.
What is activation energy?
The minimum amount of energy the reactants need to collide with each other and react.
What would the activation profile for an enothermic reaction look like?
The products would be at a higher energy than the reactants.
Is bond breaking endo or exothermic?
Endothermic as energy must be supplied.
Is bond forming endo or exothermic?
Exothermic as energy is released.
What is an electrochemical cell?
Basic system made up of two different electrodes in contact with an electrolyte.
What is the electrolyte?
Liquid that contains ions which react with the electrodes.
How is the charge set up in an electrochemical cell?
The chemical reactions between the electrodes and electrolyte sets up the charge difference between the electrodes.
The bigger the difference in reactivity of the electrodes, the _______ the voltage of the cell.
Bigger.
What other factor, besides the electrodes, will affect the voltage?
The electrolyte since different ions in solution will react differently with the metal electrodes used.
How is a battery formed?
Connecting two or more cells together in series.
What are non-rechargeable batteries?
eg. alkaline batteries, contain cells which use irreversible reactions. Once one of the reactants are used up, they don’t produce any more charge and you have to replace them.
What is a rechargeable cell?
The reaction can be reversed by connecting it to an external power source.
What is a fuel cell?
An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently.
What sets up the potential difference in the fuel cell?
When the fuel enters and is oxidised.
What is a hydrogen fuel cell?
One that combines hydrogen and oxygen to produce water and release energy.
What compartment does hydrogen go into?
The anode (-ve) compartment.
What compartment does oxygen go into?
The cathode (+ve) compartment.
What happens at the negative electrode in a hydrogen fuel cell?
At the anode, hydrogen loses electrons to produce H+ ions. Oxidation.
What happens at the positive electrode?
At the cathode, oxygen gains electrons from the cathode and reacts with H+ ions (from the acidic electrolyte) to make water. This is reduction.
What is the half equation for the anode of a hydrogen fuel cell?
H₂ ⇨ 2H⁺ + 2e⁻.
What is the half equation for the cathode of a hydrogen fuel cell?
O₂ + 4H⁺ + 4e⁻ ⇨ 2H₂O.
