Evaluating Energy Changes in Chemical Reactions

 

1. An exothermic reaction gives off heat energy to the surrounding. The temperature of surrounding increase
2. An endothermic reaction absorbs heat energy from the surrounding. The temperature of the surrounding drops.
3. The energy content of reactants is higher than that of the product in exothermic reaction
4. The energy content of product  is higher
5. The different of the energy contents between product and reactant is the heat of reaction.
6. Heat of reaction, ¦¤H has a negative sign for exothermic reaction and a positive sign for endothermic reaction.
7. In endothermic reaction, the total energy released to form new bonds in the products is greater than the total energy absorbed to break old bonds in the reactants.
8. In endothermic reaction, the total energy released to form new bonds in the products is less than the total energy absorbed to break old bonds in the reactants.
9. A thermochemical equation is an equation that shows the heat of reaction together with a balanced chemical equation.
10. Cold pack and hot packs are application of endothermic and exothermic reaction

 

Heat of Precipitation

 

1. Heat given off in the reaction = heat absorbed in the reaction mixture = mcӨ

Where m is the mass of reaction mixture (g)

C is the specific heat capacity of reaction mixture (Jg^-1oC^-1)

Ө is the temperature change of reaction mixture (^oC)

2. Heat of precipitation is the amount of heat given off when one mole of precipitate is formed when mixing the aqueous solution of the ion of insoluble salt under standard condition.

 

 

Heat of Displacement

 

1. Heat of displacement is the amount of heat energy given off when one mole of metal is displaced from its aqueous solution ( by the more electropositive metal) under standard condition.

 

13.4 Heat of Neutralisation

 

1. Heat of neutralization is the amount of heat given off when one mole of water is produced from the neutralization reaction between an acid and alkali under standard condition.

 

 

Heat of Combustion

 

1. Heat of combustion is the amount of heat given off when one mole of a substance is completely burnt in excess oxygen
2. The amount of energy released when 1 g of a substance burns completely is known as the fuel value of the fuel
3. Fuels used for industry are preferably of

a)     Greater fuel value

b)     Lower energy cost

c)     Easily available / accessible

d)     Environmentally friendly

4. There are other sources of energy that need to be harnessed for future needs

a)     Nuclear energy

b)     Wind energy

c)     Geothermal energy

d)     Hydroelectric energy

e)     Biomass energy

 

EXERCISE

 

1.  
1. State the meaning of heat of displacement
2. Figure show the energy level diagram for a reaction between magnesium and iron (II) sulphate solution

                                        Mg + FeSO₄

 

                                                                      ∆H = -65 kJ mol-1

                                                                   

                                                                    MgSO₄ +  Fe 

 

                                                              i.      State one observation of this reaction

                                                             ii.      State information that can be obtained from the energy level diagram above

                                                           iii.      Predict the heat of displacement if the reaction is repeated using calcium to replace magnesium. Explain your answer.

3. Describe an experiment to determine the heat of precipitation of lead(II) sulphate using the following apparatus and material: 50 cm³ of 0.2 mol dm^-3 lead (II) nitrate solution, 50 cm³ of 0.2 mol dm^-3 zinc sulphate solution, polystyrene cup and thermometer
1.  
1. Two experiment were carried out to determine the strength of an acid will affect the heat of neutralization

 

Experiment I

100 cm³ of 0.5 mol dm^-3 hydrochloric acid was added to 100 cm³ of 0.5 mol dm^-3 potassium hydroxide solution in a polystyrene cup. The temperature of the reaction mixture increases by 3.3 ^oC

 

Experiment II

100 cm³ of 0.5 mol dm^-3 ethanoic acid was added to 100 cm³ of 0.5 mol dm^-3 potassium hydroxide solution in a polystyrene cup. The temperature of the reaction mixture increases by 3.3 ^oC

 

                                                              i.      Calculate the heat of neutralization for both experiment

                                                             ii.      Compare the value of the heat of neutralization in Experiment I and of Experiment II. Explain your answer.

2. Explain why the heat of neutralization between nitric acid and sodium hydroxide is the same as the heat of neutralization between hydrochloric acid and potassium hydroxide
2.  
1. Suggest four characteristic of a good fuel in terms of economy and environment
2. Table 3 below show the equation and value of heat of combustion for ethanol and propanol

 

                                                              i.      Explain why the heat of combustion of propanol is different from heat of combustion of ethanol

                                                             ii.      Which alcohol is a better fuel? Why?

3. Describe how you can carry an experiment to determine the heat of combustion of ethanol