C7 - Energy changes

Question 1

Give 3 examples of exothermic reactions, and 2 specific products that utilise them:

Answer 1

-combustion
-neutralisation
-many oxidation reactions

-hand warmers
-self-heating cans

Question 2

Give 2 examples of endothermic reactions, and 1 product that utilises them:

Answer 2

-thermal decomposition
-reaction of citric acid and sodium hydrogencarbonate

-sports injury packs

Question 3

RP4 - Describe how you could investigate certain factors affecting the temperature change of a neutralisation reaction:

Answer 3

-pour HCl into polystyrene cup and record initial temperature
-add some NaOH
-put lid with hole on cup, use the thermometer to stir it
-record temperature when it stays fairly constant, calculate ΔT

-repeat experiment with changing factors (eg concentration of HCl, volume of NaOH)
-plot ΔT vs other factor

This should be an exothermic reaction

Question 4

RP4 - Why is a polystyrene cup used instead of a glass beaker?

Answer 4

-polystyrene is a better thermal insulator than glass
-reduces heat loss to surroundings

Question 5

What are reaction profiles used for?

Answer 5

used for showing:
-relative energies of the reactants and products
-activation energy
-overall energy change of a reaction

Question 6

Draw a labelled diagram for the general reaction profile of both exothermic and endothermic reactions:

Answer 6

label the x-axis with progress of the reaction

enthalpy change just means overall energy change

Question 7

What causes energy to be released/taken in during a chemical reaction?

Answer 7

-bonds being broken/formed

-breaking bonds requires energy (endo)
-forming bonds releases energy (exo)

Energy is released when bonds are formed to decrease the atom’s potential energy, and thus increase its stability.
Full explanation

Question 8

Why might a particular reaction be exothermic?

Answer 8

-more energy released from forming bonds than the energy taken in to break bonds
-energy is released to the surroundings

If set in a particular calculation context, refer to the actual energy values released/taken in

Question 9

What is meant by the term bond energy?

Answer 9

the amount of energy required to break 1 mole of a particular bond

Question 10

How do you calculate energy changes given a particular reaction?

Answer 10

-draw out all reactants and products as molecules
-count number of bonds of each type on either side
-calculate the amount of energy taken in on the reactants side, and the amount of energy released on the products side
-use formula below to get overall energy change, and if the answer is negative, the reaction is exothermic

Question 11

Explain how to make a simple electrochemical cell, and a battery:

Answer 11

-put 2 electrodes of different metals in contact with an electrolyte, and connect the electrodes into a circuit (add voltmeter to measure voltage)
-chemicals react to generate electrical current

-multiple cells can be connected in series to make a battery

Question 12

What factors determine the voltage produced by an electrochemical cell?

Answer 12

-the difference in reactivity of the 2 metal electrodes
-type of ions in the electrolyte
-concentration of electrolyte

The higher the difference in reactivity, the larger the PD

Question 13

Describe the differences between rechargeable and non-rechargeable cells/batteries:

Answer 13

non-rechargeable cells - chemical reactions stop when one of the reactant electrodes have been used up (eg alkaline batteries are non-rechargeable)

rechargeable cells - can be recharged because the chemical reactions are reversed when an external electrical current is supplied

Question 14

Why can’t alkaline batteries be recharged?

Answer 14

-alkaline batteries are non-rechargeable
-the reaction is not reversible

Question 15

What is a fuel cell?

Answer 15

-cells that are supplied by an external source of fuel (eg hydrogen) and oxygen/air
-fuel is oxidised electrochemically in the fuel cell to produce a PD

Fuel cells are alternatives to rechargeable cells

Question 16

For fuel cells, what charges do the cathode and anode have?

Answer 16

-cathode is positive
-anode is negative

Other way around in electrolysis

Question 17

Describe how a hydrogen fuel cell operates:

Answer 17

-H₂ pumped in, oxidised, and the e⁻ is dropped off at the negative electrode
-electrons flow through circuit (due to a PD created between the 2 electrodes), creating a current
-H⁺ ions diffuse through electrolyte
-O₂ pumped in through other side, reacts with the H⁺ and e⁻ at the positive electrode to form water, which leaves through an outlet

Question 18

Give the 2 half equations that happen at the electrodes in a hydrogen fuel cell:

Answer 18

negative electrode: H₂ → 2H⁺ + 2e⁻

positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O

Overall reaction is 2H₂ + O₂ → 2H₂O

Question 19

Name some pros of hydrogen fuel cells:

Answer 19

-abundant fuel source (hydrogen and oxygen)
-no harmful GGs emitted, only water vapour
-less polluting to dispose of than electrochemical cells which contain toxic chemicals

Question 20

Name some cons of hydrogen fuel cells:

Answer 20

-expensive
-hard to store the explosive hydrogen gas
-energy-consuming process to make the hydrogen (which may come from fossil fuels)