Exothermic, Endothermic reactions and Energy Transfer Flashcards
What is an exothermic reaction?
A reaction in which energy is transferred from the reacting substances to their surroundings
What is an endothermic reaction?
A reaction in which energy is transferred to the reacting substances from their surroundings
What always happens to energy in chemical reactions?
Energy is conserved
Two examples of exothermic reactions
Combustion (e.g., burning fuels like methane).
Neutralisation (e.g., acid + alkali reactions).
Two examples of endothermic reactions
Thermal decomposition (e.g., breaking down calcium carbonate into calcium oxide and carbon dioxide).
Photosynthesis (requires energy from sunlight).
What are the steps to investigate temperature change in a reaction using a polystyrene cup and a thermometer?
Measure initial temperature:
Use a thermometer to measure and record the starting temperature of the reactant placed into the polystyrene cup(e.g., an acid or alkali).
Add the second reactant:
Add the second reactant (e.g., alkali or solid) to the cup and quickly cover it with a lid, if available, to minimize heat loss.
Stir gently:
Stir the mixture carefully to ensure the reaction occurs evenly.
Measure temperature change:
Use the thermometer to record the highest or lowest temperature reached during the reaction.
Repeat for accuracy:
Repeat the experiment at least 3 times to calculate a reliable average temperature change.
Analyze results:
Calculate the temperature change by subtracting the initial temperature from the final temperature.
Explain the reaction that happens in a single-use handwarmer
The handwarmer uses the exothermic oxidation of iron. Iron turns to hydrated iron III oxide with sodium chloride as a catalyst.
Explain the reaction of a self-heating food container
The exothermic reaction of calcium oxide + water -> calcium hydroxide when a button is pressed at the bottom of a can that breaks a barrier and allows the calcium oxide and water to mix. However this takes up a lot of space in the can and takes a lot of time to heat the food or drink.
Explain the reaction in an immediate-use cooling pack
The endothermic reaction of ammonium nitrate kept separately in a cool pack from water in there too. When squeezed, the bag containing ammonium nitrate breaks allowing them to mix. When dissolving, the reaction takes in heat from it’s surroundings.
Explain the reaction that happens in a reusable handwarmer
A reusable handwarmer uses sodium ethanoate to form crystals after it has been dissolved in hot water and left to cool. The metal disc inside is clicked to scrape off some metal and start the crystallisation process.
Explain the trend of exothermic reaction on a reaction profile.
The reactants have more energy than the products. Over the progress of reaction, energy is released (exothermic) . The difference in energy between reactants and products is measured in KJ/mol.
Explain the trend of endothermic reaction on a reaction profile.
The reactants have less energy than the products. Over the progress of reaction, energy is taken in (endothermic) and the surroundings get colder. The difference in energy between reactants and products is measured in KJ/mol.
What is activation energy?
The minimum amount of energy needed before colliding particles of reactants have enough energy to make a reaction. On a reaction profile, the activation energy is the point of highest energy between reactants and products ( in the middle because allows reactants to form it’s products).
What are the energy trends of breaking and making chemical bonds in chemical reactions?
Breaking bonds absorbs energy (endothermic), Forming bonds releases energy (exothermic). It takes more energy to break a bond than to make one.
Why Breaking Bonds Is Endothermic
Breaking bonds requires energy to overcome the attraction between atoms. This energy input is necessary to separate the atoms, making the process endothermic.
What is bond energy?
The energy needed to break the bond between two atoms measured in KJ/mol.
How do you work out energy change for a chemical reaction?
Add up the KJ/mols of each bond for the bonds in the reactants. Add up the KJ/mols of each bond for the bonds formed in the products. The difference between the two is the energy change.
If the energy required to break bonds is greater than the energy transferred to the surroundings when bonds are made, will the reaction be exothermic or endothermic?
Endothermic, because more energy is absorbed to break bonds than is released.
Calculate energy needed to break all bonds in 0.0960 mol of oxygen gas.
Bond energy of O=O = 498 kJ/mol.
Energy for 0.0960 mol: 498×0.0960=47.81kJ
What happens to make an electrical cell?
When two metals are dipped in a salt solution and joined by a wire, the more reactive metal will donate electrons to the less reactive metal. The greater the difference in reactivity between the two metals, the higher the voltage produced by the cell.
What is a battery made of?
Two more more cells joined together to increase the voltage available
Describe an electrical cell made of zinc and copper.
There’s a zinc rod and a copper rod dipped in a salt solution (or dilute acid). A wire connects the rods. Zinc atoms donate electrons via the connecting wire to the copper ions, so zinc acts as the negative terminal of the cell. The flow of electrons is the electrical current. The current will flow in the circuit opposite until the copper or zinc is used up.
Why is copper rod the one to accept electrons from zinc rod in an electrical cell?
There is a tendency for any metal atom to give away electrons and form a positive ion. The greater the tendency to form their positive ion, the more reactive the metal is so copper ions accept the electrons and become copper atoms.
Explain what primary cells are
The first mass-produced batteries. They are not rechargeable. They are dry cells of zinc and carbon electrode rods. This is different to modern alkaline batteries. When the zinc and carbon runs out, the battery should be disposed.
What are the pros and cons of hydrogen fuel?
Advantages: does not need electrical recharging, no pollutants are produced, can be a range of sizes. Disadvantages: hydrogen is very flammable, it is produced for the cell by non-renewable sources, hydrogen is difficult to store.
What is a hydrogen fuel cell?
A battery cell fed hydrogen and oxygen and produces water. The energy released in the reaction is transferred to electrical energy.
Explain what happens inside of a hydrogen fuel cell.
Hydrogen Input (Negative Electrode):
Hydrogen gas (H₂) enters the negative electrode.
It reacts with hydroxide ions (OH⁻) to form water (H₂O) and electrons (e⁻).
Electrons Flow:
The electrons produced flow through an external circuit, creating electricity.
Oxygen Input (Positive Electrode):
Oxygen gas (O₂) enters the positive electrode.
It reacts with water (H₂O) and electrons (from the circuit) to form hydroxide ions (OH⁻).
Overall Reaction:
Adding the two electrode reactions cancels out intermediate particles (electrons and OH⁻),
This shows that the only product is water.
Key Features:
The fuel cell uses hydrogen and oxygen to produce electricity.
The only waste product is water.
This makes hydrogen fuel cells an environmentally friendly energy source.
How can you measure the the rate of a reaction?
By monitoring the amount of reactants used up over time. Or by measuring the amount of products made over time.
What does a gradient line graph for the rate of a reaction show?
It tells you the rate of reaction at that time. The steeper the gradient the faster the reaction.
