C5 (energy changes)

Question 1

what is an exothermic reaction?

Answer 1

transfers energy from the reacting molecule to the surroundings. this means that during exothermic reactions, the temperature of the surroundings increases. energy changes are shown as negative.
- combustion (burning), certain oxidation reactions, neutralisation.

Question 2

describe the energy profile for an exothermic reaction:

Answer 2

• the products have less energy than the reactants, as energy has been transferred from the reaction to the surroundings.
• the difference between the energy of the reactants and the energy of the products tells us the energy that’s been released to the surroundings.

Question 3

what are some uses of exothermic reactions?

Answer 3

• hand warmers.
• self-heating cans (e.g. for food and drink).

Question 4

what is an endothermic reaction?

Answer 4

takes in energy from their surroundings. the temperature of the surroundings decreases. energy changes are shown as positive.
- thermal decomposition.

Question 5

describe the energy profile for an endothermic reaction:

Answer 5

• the products have more energy than the reactants, as energy has been taken in from the surroundings.
• the difference in energy between the reactants and products tells us the amount of energy taken in from the surroundings.

Question 6

what is the activation energy in energy profiles?

Answer 6

• in all energy profiles, the energy rises to a peak.
• reactions can only occur when particles collide with each other, and they have to do that with sufficient energy. the minimum amount of energy required for particles to react is the activation energy.
• the activation energy is from the reactants to the peak of the curve.

Question 7

describe the formation and breaking of bonds concerning endothermic and exothermic reactions:

Answer 7

• breaking a chemical bond requires energy, making it an endothermic reaction.
• forming a chemical bond releases energy, making it an exothermic reaction.
• every chemical bond has an energy value, telling us the energy required to break that bond.

Question 8

how would you carry out bond energy calculations?

Answer 8

1. understand which bonds are being broken, and which are being created (the reactants’ bonds are being broken, the products’ bonds are being made).
2. put the energy values in (they should be given to you in the question). remember that in endothermic reactions, bonds are positive, and in exothermic, bonds are negative.
3. calculate the final energy change for the reaction. subtract the product value from the reactant value, to get your final answer.

Question 9

what is a fuel cell?

Answer 9

a type of electrochemical cell. it converts energy between its electrical and chemical forms.
- it converts chemical energy (fuel and oxygen) into electrical energy, which we can use to power things.

Question 10

describe the hydrogen-oxygen fuel cell and its structure:

Answer 10

• combines hydrogen and oxygen to form water, at the same time as generating lots of electrical energy.
• in the centre is the electrolyte (e.g. potassium hydroxide), which is a solution that ions can move through.
• on either side of this are electrodes, with the negative anode on the left, and the positive cathode on the right.
• they’re connected by a wire on the top, allowing electrons to flow from the anode to the cathode.
• on the outside of the electrodes, are the anode and cathode compartments. each ha an inlet on the top.
  > on the left, hydrogen will enter into the
  anode compartment. on the right, oxygen
  will enter into the cathode compartment.
• at the end, all of the water and heat will leave via the outlet at the bottom of the cathode compartment.
• the cathodes are made of porous carbon, with many tiny holes. they also contain catalysts, which speed up the reaction.

Question 11

how does a hydrogen-oxygen fuel cell work?

Answer 11

1. hydrogen enters into the anode compartment, and is oxidised by the anode (each hydrogen atom loses an electron to become a hydrogen ion). this creates a potential difference across the cell, driving the next step.
2. the electrons pass around the wire into the cathode, and the hydrogen ions go through the electrolyte, and into the cathode.
3. the electrons and hydrogen ions can react with the oxygen that comes in through the cathode compartment.
4. they all combine to make water, however, this leaves us with a spare oxygen atom. this means we need two sets of hydrogen ions and electrons to react with each oxygen molecule.
5. the water then leaves the fuel cell via the outlet, and the reaction is complete.

Question 12

what are half equations?

Answer 12

equations that purely show the loss or gain of electrons in a reaction.
> in fuel cells, it shows the oxidation of
hydrogen and the reduction of oxygen.
combining the two equations would create
an overall equation, showing the reaction
that occurs at a fuel cell.

Question 13

where does the electrical energy come from in a fuel cell?

Answer 13

the electrical energy comes from the movement of electrons through the wire at the top, which is powered by the chemical reaction.

Question 14

what are the pros and cons of hydrogen-oxygen fuel cells?

Answer 14

MAIN IDEA:
- could replace power stations.
- the main idea is to replace the fossil fuel engines and batteries used in vehicles, both of which are polluting.

PROS:
- only requires hydrogen and oxygen, and doesn’t produce any carbon dioxide or other pollutants.
- relatively simple devices; last longer than batteries, less polluting to dispose of.

CONS:
- hydrogen is a gas, so it takes up more space to store than fossil fuels or batteries.
- it’s explosive when mixed with air, making storing it dangerous.
- making the hydrogen in the first place requires energy, and that energy usually comes from fossil fuels.