C5 More Exothermic and Endothermic Reactions (page 139)

Question 1

How can you measure the amount of energy released by a chemical reaction (in solution)?

Answer 1

by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction (see diagram on page 139)

Question 2

What is the biggest problem with energy measurements?

Answer 2

the biggest problem is the amount of energy lost to the surroundings.

Question 3

How can you reduce energy lost to the sorroundings?

Answer 3

you can reduce it a bit by putting the polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation.

This method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids.

You can also use this method to investage what effect different varibles have on the amount of energy transferred - e.g. the mass or concentration of the reactants used.

Question 4

How could you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH)?

Answer 4

1) Put 25cm³ of 0.25 mol/dm³ of hydrochloric acid and sodium hydroxide in separate beakers

2) Place the beakers in a water bath set to 25°C until they are both at the same temperature (25°C).

3) Add the HCl followed by the NaOH to a polystyrene cup with a lid - as in the diagram above (page 139)

4) Take the temperature of the mixture every 30 seconds, and record the highest temperature

5) Repeat steps 1-4 gusing 0.5 mol/dm³ and then 1 mol/dm³ of hydrochloric acid

Question 5

What are Reaction Profiles?

Answer 5

they are diagrams that show the relative energies of the reactants and products in a reaction and how the energy changes over the course of the reaction

Question 6

Reaction profiles are sometimes called what?

Answer 6

energy level diagrams

Question 7

On page 139, look at the diagram tat shows an exothermic reaction, what do it tell you?

Answer 7

the products are at the lower energy than the reactants. The difference in height represents the overall energy change in the reaction (the energy given out) per mole.

The initial rise in energy represents the energy needed to start a reaction. This is the activation energy (Ea)

Question 8

What is ‘The activation energy’?

Answer 8

it is the miniumu amount of energy the reactants need to colide with each other and react. The greater the activation enrgy, the more energy needed to start the reaction - this has to be supplied, e.g. by heating the reaction mixture

Question 9

The diagram at the bottom of page 139 shows an endothermic reaction, explain why it is an endothermic reaction?

Answer 9

This shows an endothermic reaction because the products are at a higher energy than the reactants.

The difference in height represents the overall energy change during the reaction ( the energy taken in) per mole

Question 10

In an exothermic reaction what do you need to remember?

Answer 10

that the particles release energy to their surroundings - even though the reaction mixture gets warmer, the particles themselves have lost energy

Question 11

Here is the equation for the combustion of methane in air.

CH⁴(g) + 2O²(g) &raquo_space;»> CO2(g) + 2H2O(g)

Draw a reaction profile for this reaction? (3 marks)

Answer 11

Check your diagram agains page 245, (the answer to p.139)

1 mark for correct axes, 1 mark for correct energy levels of reactants and products, 1 mark for correct shape of curve linking the reactants to the products