Chemical Energetics Flashcards
(18 cards)
Define the enthalpy of a substance.
The energy content of a substance.
State the symbol and unit for enthalpy charge.
Symbol: ΔH or ΔHr
Unit: kJ mol ^-1
State the differences between endothermic and exothermic reactions, in terms of:
1. energy absorption or release
2. temperature of surroundings
3. enthalpy of the system
4. enthalpy content of products VS reactants
5. energetic stability of products VS reactants
6. sign of ΔH
7. transfer of energy stores
- Endothermic reaction: The reacting system absorbs heat from the surroundings.
Exothermic reaction: The reacting system releases heat to the surroundings.
2.
Endothermic reaction: The temperature of the surrounding decreases.
Exothermic reaction: The temperature of the surrounding increases.
3.
Endothermic reaction:
The enthalpy of the system increases.
Exothermic reaction:
The enthalpy of the system decreases.
4.
Endothermic reaction:
The enthalpy content of products is higher than that of reactants.
Exothermic reaction:
The enthalpy content of products is lower than that of reactants.
5.
Endothermic reaction: The products are less stable than the reactants.
Exothermic reaction: The products are more stable than the reactants.
6.
Endothermic reaction: positive
Exothermic reaction: negative
7.
Endothermic reaction: thermal energy transferred to chemical potential energy
Exothermic reaction: chemical potential energy transferred to thermal energy
Define activation energy.
The minimum energy which the reacting particles must possess in order to overcome the energy barrier before the formation of the products.
A reaction does not take place if reacting particles have less energy than the activation energy.
True/False? Explain.
True.
Reacting particles that have less energy than the activation energy cannot break their bonds during collision with other reacting particles and thus no reaction takes place.
Categorise the physical changes below into endothermic and exothermic processes.
1. boiling
2. melting
3. sublimation
4. freezing
5. condensation
6. dissolution of some ionic compounds (e.g NaCl and NH4Cl) in water
7. dissolution of acids and alkali in water
Endothermic reactions:
1, 2, 3, 6
Exothermic reactions:
4, 5, 7
Categorise the chemical changes below into endothermic and exothermic processes.
1. respiration
2. photosynthesis
3. combustion
4. neutralisation
5. electrolysis of water
6. thermal decomposition of metal carbonate
7. reaction between ammonium salt and alkali
Endothermic reactions:
2, 5, 6, 7
Exothermic reactions:
1, 3, 4
The presence of a catalyst decreases the activation energy and speeds up the reaction.
True/False?
True
In which process is energy released?
1: photosynthesis
2: fractional distillation of crude oil
3: electrolysis of water to form H2 and O2
4: forming a hydrogen molecule from 2 hydrogen atoms
Answer: D
Option B: provides heat
Sodium hydroxide reacts with hydrochloric acid according to the equation below.
NaOH (aq) + HNO3 (aq) → NaNO3 (aq) + H2O (l)
ΔHr = –57.3 kJ mol^–1
Calculate the energy change when 50.0 cm^3 of 1.00 mol dm^-3 HNO3 is added to 25.0 cm^3 of 1.00 mol dm^-3 sodium hydroxide solution.
ηNaOH = (25.0÷1000) × 1.00 = 0.02500 mol
ηHNO3 required = 1 × 0.02500 = 0.02500 mol
ηHNO3 added = (50.0÷1000) × 1.00 = 0.0500 mol > 0.02500 mol
NaOH is the limiting reagent.
Energy change = 0.02500 mol × (–57.3 kJ mol^–1) = –1.43 kJ
What type of process is bond breaking, and what is the sign of energy change?
Bond breaking is an endothermic process, energy change for bond breaking is positive.
What type of process is bond forming, and what is the sign of energy change?
Bond forming is an exothermic process, energy change for bond forming is negative.
State the equation of the enthalpy change of reaction, in terms of bond breaking/forming.
Enthalpy change of reaction, ΔHr = Σbond energy (bonds broken) – Σbond energy (bonds formed)
Note: Σ means ‘sum of’
q = mc△T
State what each symbol represents, and their respective units.
q: quantity of heat/energy transferred (Unit: J)
m: mass of solution (Unit: g)
c: specific heat capacity of solution (Unit: J g^-1 K^-1)
△T: change in temperature of solution (Unit: K)
List the assumptions made when conducting a calorimeter experiment.
- Specific heat capacity of solution approximates that of water (4.2 J g^-1 K^-1).
- The total volume of the solution is used in the calculation of mass of solution.
- Density of the solution approximates that of water (1.00g cm^-3).
- The calorimeter is perfectly insulated, with all the chemical energy released transformed into heat energy.
- Heat capacity of the polystyrene cup is negligible.
State the equation of enthalpy change of reaction, △Hr, in terms of heat transferred.
△Hr = q/n, where q is quantity of heat transferred and n is number of moles of limiting reagent.
Explain why a temperature correction is needed in a calorimeter experiment.
Heat loss to the surroundings is more significant if the reaction is slow.
A calorimeter experiment is conducted to determine the enthalpy change of combustion of ethanol. Give 2 reasons why the value obtained from experiment, -725kJ mol^-1, is different from the literature value of ethanol, -1368kJ mol^-1.
1: Incomplete combustion may occur due to an insufficient supply of oxygen.
2: Energy is lost to the surrounding air / metal container.
