Energetics

every chemical reaction involves…

energy transfer.

exothermic

chemical reactions in which heat energy is given out to the surroundings.

endothermic

chemical reactions in which heat energy is taken in from the surroundings.

examples of exothermic reactions

• burning fuels

• respiration

examples of endothermic reactions

• melting

  (heat energy is taken in by a substance as it changes from solid → liquid)

• photosynthesis

describe a simple calorimetry experiment for a combustion reaction

1. measure 100cm3 water in measuring cylinder.

2. pour water into steel can and record temperature.

3. choose spirit burner/fuel.

4. record mass of whole burner.

5. put fuel burner underneath steel can.

6. light wick of fuel burner.

7. watch temp. of water rise until water is 20 degrees warmer.

8. record new mass of fuel burner.

9. repeat experiment with different fuel burners.

describe a simple calorimetry experiment for a displacement reaction

1. pipette 25cm3 copper (II) sulfate solution into polystyrene cup.

2. put thermometer though hole in the the lid, stir, and record temp. every 30 seconds for 2 ½ minutes.

3. at 3 minutes add zinc powder.

4. stir.

5. record temp. every 30 seconds for 6 minutes.

enthalpy change of a displacement reaction

mole = volume x concentration

describe a simple calorimetry experiment for dissolving

1. Place polystyrene cup in a beaker.

2. 50cm³ of water into the cup and measure the initial temperature.

3. Add a known mass of the solid solute into the water in the cup and stir the solution to dissolve the solid.

4. Once the solid has dissolved, measure and record the final temperature reached of the water.

describe a simple calorimetry experiment for a neutralisation reaction

neutralisation is an exothermic reaction - they release energy to the surroundings.

1. Measure 25cm³ of the acid solution with a known concentration into a polystyrene cup. Place the cup into a glass beaker.

2. Measure and record the initial temperature of acid.

3. Measure out 25cm³ of alkali solution. with the same concentration as the acid solution (to make sure that the number of moles of acid and alkali used are equal.)

4. Add the 25cm³ of alkali solution into the polystyrene cup and stir the solution.

5. Measure and record the highest temperature reached by the solution.

energy transferred

E = specific heat capacity (4.18) x mass of water x temp. difference

enthalpy value/energy change

enthalpy value = energy transferred/moles fuel burnt

enthalpy of fuel

energy transferred/moles used

bond breaking - endo or exo?

endothermic process

bond making - endo or exo?

exothermic process

low pressure

fewer moles/particles of reactants compared to products

higher pressure

more moles/particles of reactants compared to products

why will changing the temperature have less effect on the yield compared to (….)?

• the reactions is not very exothermic

• therefore, there will be a smaller enthalpy change.

reasons why something was a good fuel

• produced most heat energy per gram of fuel.

• produced most heat energy per mole of fuel.

suggest two reasons why student’s result is lower than the data book value

• incomplete combustion.

• copper/steel can absorbs heat.

• evaporation of water

explain, in terms of bond breaking and bond making, why this reaction gives out energy

• breaking bonds is an endothermic reaction. Therefore, the endothermic reaction takes in heat energy from the surroundings.

• making bonds is an exothermic reaction. An exothermic reaction gives out heat energy to the surroundings.

• this is an exothermic reaction because more heat energy is given out, then given in.