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

what is the main idea behind exothermic and endothermic reactions?

Answer 2

different chemicals/molecules store different amounts of energy in their bonds (chemical energy stores)
- we’re interested in the total energy of the products compared to that of the reactants

Question 3

what do the y and x axes on a reaction profile represent?

Answer 3

y axis = energy
x axis = progress of reaction

Question 4

describe the energy profile for an exothermic reaction:

Answer 4

• 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 5

what are some uses of exothermic reactions?

Answer 5

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

Question 6

what is an endothermic reaction?

Answer 6

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

Question 7

describe the energy profile for an endothermic reaction:

Answer 7

• 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 8

what is the activation energy in energy profiles?

Answer 8

• 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 collide with each other and react is the activation energy.
• the activation energy is from the reactants to the peak of the curve.

Question 9

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

Answer 9

• 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 10

define bond energy:

Answer 10

the amount of energy required to break one mole of a particular covalent bond

Question 11

what is a mole?

Answer 11

6.02 x 10^23 molecules of something (in this case, bonds)
- this is Avogadro’s constant

Question 12

how would you carry out bond energy calculations?

Answer 12

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).
3. calculate the final energy change for the reaction. subtract the product value from the reactant value, to get your final answer.

Question 13

how can you tell if a reaction is exothermic or endothermic after a bond energy calculation?

Answer 13

• the final answer will be negative if the reaction is exothermic, as it’s losing energy to the surroundings
• a positive answer indicates an endothermic reaction

Question 14

what happens during a chemical reaction?

Answer 14

1. the bonds of the reactant molecules are broken
2. the atoms rearrange themselves
3. the bonds of the product molecules are formed

Question 15

what is a fuel cell?

Answer 15

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 16

how can a cell be made?

Answer 16

connect two different electrodes with a wire, place them in contact with an electrolyte (a liquid through which charged ions can flow). there’s also a voltmeter to measure the voltage of the cell

Question 17

what is the difference between a cell and a battery?

Answer 17

batteries are very similar to cells, but consist of two or more cells connected together in series to provide a greater voltage

Question 18

what factors affect the voltage of a cell?

Answer 18

1. the metals used for the two electrodes. the greater the difference in reactivity between the two metals, the greater the voltage will be
2. the type and concentration of electrolyte used
3. the conditions, such as temperature

Question 19

describe the hydrogen-oxygen fuel cell and its structure:

Answer 19

• in the centre is the electrolyte (e.g. potassium hydroxide)
• 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 has 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.

Question 20

what is the role of the hydrogen-oxygen fuel cell?

Answer 20

combines hydrogen and oxygen to form water, at the same time as generating lots of electrical energy

Question 20

describe the electrodes in a hydrogen-oxygen fuel cell:

Answer 20

the cathodes are made of porous carbon, with many tiny holes. they also contain catalysts, which speed up the reaction

Question 21

how does a hydrogen-oxygen fuel cell work?

Answer 21

1. hydrogen enters into the anode compartment, and is oxidised by the anode (each hydrogen atom loses an electron to become a hydrogen ion).
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 22

what are half equations?

Answer 22

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 23

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

Answer 23

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

Question 24

what are the pros of hydrogen-oxygen fuel cells?

Answer 24

- only requires hydrogen and oxygen (abundant), and doesn't produce any carbon dioxide or other pollutants. - relatively simple devices; last longer than batteries, less polluting to dispose of. - don't need to be electrically recharged

Question 25

what are the cons of hydrogen-oxygen fuel cells?

Answer 25

- 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.

Question 26

describe chemical cells:

Answer 26

- in the metal displacement reaction, one metal is oxidised (loses electrons). these metals are transferred to another metal, which gains the electrons and is reduced. - by using a chemical cell to conduct this reaction, the electron's movement provides a current, and can therefore power a component (e.g. LEDs). - the greater the difference in reactivity between the two metals in the cell, the greater the p.d. produced.

Question 27

describe rechargeable batteries:

Answer 27

an external electric current is supplied, which reverses the reaction, meaning the battery can be used again. - e.g. laptops and mobile phones

Question 28

describe non-rechargeable batteries:

Answer 28

- e.g. alkaline batteries, smoke alarms, tv remotes - the reaction isn't reversible - once the reactants are used up, the chemical reaction stops and no more potential differences are released

Question 29

what is the half equation for the reaction at the anode in a hydrogen-oxygen fuel cell?

Answer 29

H2 = 2H+ + 2e- - oxidation of hydrogen (loss)

Question 30

what is the half equation for the reaction at the cathode in a hydrogen-oxygen fuel cell?

Answer 30

O2 + 4H+ + 4e- = 2H20 - reduction of oxygen (gain)

Question 31

what is the overall equation in a hydrogen-oxygen fuel cell?

Answer 31

O2 + 2H2 = 2H20