FORM 5 CHEMISTRY KSSM CHAPTER 3: THERMOCHEMISTRY

Is this an energy level diagram for an exothermic reaction?

• TRUE

• FALSE

Answer explanation

The reaction between nitrogen gas, N₂ and oxygen gas, O₂ forming nitrogen monoxide gas, NO is an endothermic reaction.
During the reaction, the temperature of the mixture decreases.

Which statement best describes an endothermic reaction?

• Endothermic is a chemical reaction that releases heat to the surroundings.

• Endothermic is a chemical reaction that absorbs heat from the surroundings.

• In this reaction, chemical energy is converted to heat energy.

• In this reaction, temperature of the surroundings increases.

Answer explanation

In this reaction, heat energy is converted to chemical energy. Thus, surroundings lose heat energy and temperature of the surroundings decrease.

Choose the correct example of an exothermic reaction.

• Freezing process

• Melting of ice

• Photosynthesis

• Dissolving

  ammonium salts

  in water

Answer explanation

Fun Fact :)

During extreme cold exposure, the human body starts to lose heat rapidly and FREEZE!

Hence, freezing is considered an exothermic reaction.

The following thermochemical equation shows an exothermic reaction.

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) ΔH = - 467 kJ mol^-1

Choose the correct energy level diagram for the reaction.

• 
• 
• 
• 

Answer explanation

Heat of reaction, (ΔH) is the heat change of one mole of reactant that reacts or one mole of product that is formed.

When 10 g of fuel is burnt in excess oxygen, it raised the temperature of 300 cm³ water by 55°C.

Calculate the heat of combustion of the reaction in kJ/mol.

[Specific heat capacity of water = 4.2 J g⁻¹ °C⁻¹, Density of water = 1.0 g cm⁻³,

Molar mass of X = 20 g mol⁻¹]

• • + 138.6 kJ mol^-1

• 69.3 kJ mol^-1

• -138.6 kJ mol^-1

• -69.3 kJ mol^-1

Answer explanation

The heat of combustion is the heat released when one mole of substance is completely burnt in excess oxygen.

When 100 cm³ of 2.0 mol dm^-3 of sodium hydroxide is added to 100 cm³ of 2.0 mol dm^-3 of hydrochloric acid, the temperature rises by 10°C. Which of the following mixtures will produce an increment of 20°C in temperature?

• 200 cm³ of 2.0 mol dm⁻³ sodium hydroxide and 200 cm³ of 2.0 mol dm⁻³ hydrochloric acid.

• 100 cm³ of 2.0 mol dm⁻³ of sodium hydroxide and 100 cm³ of 4.0 mol dm⁻³ hydrochloric acid.

• 50 cm³ of 2.0 mol dm⁻³ of sodium hydroxide 50 cm³ of 2.0 mol dm⁻³ of hydrochloric acid.

• 50 cm³ of 4.0 mol dm⁻³ of sodium hydroxide and 50 cm³ of 4.0 mol dm⁻³ of hydrochloric acid.

Answer explanation

Volume of Solution and Concentration of Reactants affects the rate of temperature.

When the volume of solution is larger, the heat is distributed over a greater mass. Hence, temperature decreases.

When the concentrations of the acid and base are higher, more frequent collisions between ions will occur. Thus, temperature increases.

The thermochemical equation for the decomposition reaction of calcium carbonate, CaCO₃ is as follows:

CaCO₃(s) → CaO(s) + CO₂(g) ΔH = +540 kJ mol^-1

Select the statement(s) that can be obtained from the thermochemical equation of the reaction.

(I) The decomposition of calcium carbonate, CaCO₃ is an endothermic reaction.

(II) When 1 mole of calcium carbonate, CaCO₃ is decomposed, 540 kJ energy is absorbed from the environment.

(III) The energy content of the products is lower than the energy content of the reactant.

(IV) The surrounding temperature will rise when the decomposition reaction take place.

• I and II

• I and IV

• II and III

• III and IV

Answer explanation

Thermochemical equation provides several key pieces of information.

The thermochemical equation below shows the formation of lead(II) sulphate, PbSO₄ precipitate.

PbNO₃(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq) ΔH = -50 kJ mol^-1

The reaction above is an ___________ reaction.

Answer explanation

In a thermochemical equation, an exothermic reaction is represented with a negative enthalphy change, indicating that energy is released during the reaction.

Heat energy is absorbed during bond breaking while heat energy is released during bond formation.

• TRUE

• FALSE

Answer explanation

Bond breaking is an endothermic reaction while bond formation is an exothermic reaction.

An excess of magnesium powder, Mg is added into 50 cm³ of 0.25 mol dm^-3 of iron (II) sulphate, FeSO₄ solution. The temperature of the mixture increases.

Based on above statement, determine what type of heat of reaction involved.

• Heat of Precipitation

• Heat of Displacement

• Heat of Neutralization

• Heat of Combustion

Answer explanation

Heat of displacement is the heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal.