Energetics

Question 1

What is an exothermic reaction?

Answer 1

A reactions which gives out energy (heat) to its surrounds and is shown by a rise in temperature of the surroundings

Question 2

What is an endothermic reaction?

Answer 2

A reaction which takes in energy (heat) from the surroundings and is shown by a fall in temperature of the surroundings

Question 3

What is the enthalpy change?

Answer 3

The overall change in energy in a reaction is called the enthalpy change. The symbol is ^H
- the units of ^H are kJ/mol
- if a reaction is exothemric, the value is negative as it is giving out energy
- if the reaction is endothermic, the value is positive as it is taking in energy

Question 4

Calorimetry - dissolving, displacement and neutralisation

Answer 4

1) put 25cm^3 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 temp
3) Add HCL followed NaOH to a polystyrene cup with a lid
4) take the temp of the mixture every 30 seconds + record highest temperature

Question 5

Calorimetry - combustion

Answer 5

• put 50g of water in the copper can and record its temp
• weigh the spirit burner and lid
• put the spirt burner underneath the can and heat the water until it reaches about 50°C
  -put out the flame using the burner lid, and measure the final temp
• weigh the spirit burner and the lid again
• calculate enthalpy change

Question 6

How to calculate heat energy transferred

Answer 6

Q = m x c x ^T

heat energy transferred = mass of liquid being heated x specific heat capacity x change in temperature of liquid

Question 7

How to calculate molar enthalpy change?

Answer 7

^H = Q/ mol

Molar enthalpy change = heat energy change /moles

Question 8

Why could calculated figure be lower than actual figure?

Answer 8

• taken the temperature too long after heat was taken off - cooled down
• no/little insulation
• transferred to the surroundings
• flame moving about due to drafts
• object absorbed heat

Question 9

How to improve accuracy of test?

Answer 9

• more insulation
• thicker insulation
• set up heatproof mats at each side to cut down on draughts

Question 10

What process is bond breaking?

Answer 10

Endothermic as energy much be supplied to break existing bonds

Question 11

What process is bond making (formation)?

Answer 11

Exothermic as energy is released when new bonds are formed

Question 12

How to calculate enthalpy change?

Answer 12

Total energy absorbed to break bonds - total energy released in making bonds

Question 13

Bond breaking (endothermic) example

Answer 13

HCL —> H + Cl

Question 14

Bond forming (exothermic) example

Answer 14

C + O —> CO