Topic 5 Energy changes

Question 1

what is the definition of an exothermic reaction?

Answer 1

An exothermic reaction is a reaction that transfres energy to the surroundings/ a reaction in which the reactant store more energy than the products

Question 2

Are neutralistaion reactions exothermic or endothermic?

Answer 2

Neutralisation reactions are exothermic.

Question 3

Give one example of an everyday use of an exothermic reaction and one example of an everyday use of an endothermic reaction.

Answer 3

exothermic: self heating cans/ hand warmers
endothermic; sports injury packs

Question 4

Sodium hydrocarbonate reacts with hydrochloric acid in an endothermic reaction. Do the reactants or the products of this reaction store more energy? Explain your answer.

Answer 4

The products store mor energy. In an endothermic reaction, energy is taken in from the surroundings. That means the total energy stored by the products is equal to the total energy stored by the reactants, plus the energy taken in during the reaction.

Question 5

Suggest in terms of storage and transfer why melting is an endothermic process.

Answer 5

Melting is an endothermic process because a substance in the liquid state stores more energy than the same substance in the solid state, so energy is taken in during the melting process.

Question 6

spontaneous reactions are reactions that occur without any external input. suggest why most spontaneous reactions are exothermic.

Answer 6

Energy is taken in during endothermic reactions, so most endothermic reactions need the reactants to be heated to supply enough energy for the reaction to take place. So endothermic reactions are less likely to be spontaneous than exothermic reactions.

Question 7

The sketch on the righ shows a reaction profile. Does it represent an exothermic reaction or an endothermic reaction?

Answer 7

endothermic - this is because the energy of the reactants is lower than the energy of the products.

Question 8

When you measure the temperature change of a reaction where the temperature increases, why is the value you measure usually less than the actual temperature change?

Answer 8

Some energy is always lost to the surroundings.

Question 9

Give two ways to insulate a reaction mixture when measuring a temperature change.

Answer 9

• mix the reagents in a polystyrene cup
• surround the reaction vessel with cotton wool
• put a lid on the reaction vessel

Question 10

What distance on a reaction represents the activation energy of the reaction?

Answer 10

The activation energy is the difference in height between the energy of the reactants and the highest point of the curve on the reaction profile.

Question 11

the reaction between magneium and sulphuric acid is exothermic.A student is investigating the effect of acid concerntration on the energy change of this reaction. She repeats the reaction several times with different concerntrations of acid. give two things she should do to make the experiment a fair test.

Answer 11

• use the same volume of acid, mass of magnesium or amount of insulation each time.

Question 12

if you calculate the overall change in bond energy for an exothermic reaction, would you expect it to +ive or -ive?

Answer 12

negative - in an exothermic reaction more energy is given out forming the bonds in the products then is taken in breaking the bonds in the reactants.

Question 13

‘’If the products of a reaction contain more bods than the reactants, then the products must have a higher total bond energy.’’ Explain why this statement is not true.

Answer 13

The total bond energy does not just depend on the number of bonds. Different bonds have different bond enegies depending on the strength of the bond, so a small number of strong bonds could have a higher total bond energy than a large number of weak bonds.

Question 14

Bromine and iodine form iodine monobromide as follows: I₂ + B_r ⇒ 2IBr

Use the following approximate bond energies to find the overall energy change of this reaction:

I-I: +150 KJ/mol, Br-Br: +190 KJ/mol, I-Br: =175KJ/mol

Answer 14

Energy supplied to break bonds: (1 x I-I) + (1 x Br-Br) = 150 + 190 340KJ/mol

Energy released from forming bonds: 2 x I-Br = 2x175 =350KJ/mol

Overall energy change: energy supplied energy released: 340 - 350 = -10kJ/mol

Question 15

4) The equation for the reaction between hydrogen and oxygen is: 2H₂ + O₂ → 2H₂O The overall energy change for this reaction is approximately* -500 kJ/mol. The approximate* bond energies in H₂ and O₂ are: H-H: +400 kJ/mol, O=O: +500 kJ/mol. Find the approximate* bond energy of the H-O bond in H₂O.

Answer 15

Question 16

What property of metals makes them suitable for use as electrodes?

Answer 16

They conduct electricity. All parts of a cell must be able to conduct electricity or the charge won’t be able to flow.

Question 17

What is the difference between a cell and a battery?

Answer 17

A cell is a system made up of a single pair of electrodes which are connected by a wire and in contact with an electrolyte.

a battery is a group of two or more cells connected in a series.

Question 18

Explain how an electrochemical cell produces electricity.

Answer 18

Chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes. if the electrodes are then connected by a wire, the charge is able to flow and electricity is produced.

Question 19

Explain why alkaline batteries are non-rechargeable.

Answer 19

The reactions in alaline batteries are irreversible. so once the reactants in the battery are used up, it can’t produce any more charge.

Question 20

Answer 20

0.15 + 0.40 = +0.55V

The voltage of the copper/zinc cell is the sum of the voltages of the copper/iron and the iron/zinc cell, because copper is less reactive than iron, and iron is less reactive than zinc.

Question 21

What is the uel in a hydrogen-oxygen fuel cell?

Answer 21

Hydrogen. Fuel cells work by reacting a fuel with oxygen to produce energy.

Question 22

What process takes place to create a potential difference in a fuel cell?

Answer 22

The fuel is oxidised. The potential difference is the voltage in the cell which is created when the fuel gives up electrons.

Question 23

Give two advantages and two disadvantages of using hydrogen-oxygen fuel cells rather than using rechargeable batteries to power vehicles.

Answer 23

Advantages:

fuel cells are less polluting than batteries because batteries contain toxic compounds

fuel cells are cheaper to make than batteries

fuel cells need to be replaced less often than batteries (even rechargeable batteries wear out eventually).

Diadvantages:

fuel cells require the sorage of hydrogen, while batteries don’t, and hydrogen requires a lot of storage space.

storing hydrogen can be dangerous because it is explosive when mised with air, so batteries can be safer.

fuel cells can use more fossil fuels than batteries because the production of hydrogen often consumes fossil fuels.

Question 24

Write down the half equations that represent what happens at the electrodes in a hydrogen-oxygen fuel cell, stating which occurs at the anode and which at the cathode.

Answer 24

4)

At the anode/negative electrode:

H₂ → 2H⁺ + 2e^-

At the cathode/positive electrode:

O₂ + 4H⁺ + 4e^- → 2H₂O

• in electrolysis, the anode is the positive electrode and the cathode is the negative electrode. But in fuel cells, it’s the other way around.