Understanding Points
Reactivity 1.1.1—Chemical reactions involve a transfer of energy between the system and the
surroundings, while total energy is conserved.
Reactivity 1.1.2—Reactions are described as endothermic or exothermic, depending on the
direction of energy transfer between the system and the surroundings.
Reactivity 1.1.3—The relative stability of reactants and products determines whether reactions are endothermic or exothermic.
Reactivity 1.1.4—The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.
Enthalpy: the amount of heat energy contained in a substance, which is stored in chemical bonds
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Represented as : ∆H°
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Measured in : kJ mol-1
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Standard enthalpy change ( ΔH⦵): the heat transferred at constant pressure under standard conditions and states
Exothermic Reaction | Endothermic Reaction |
Releases heat energy to the surroundings | Absorbs heat energy from the surroundings |
Enthalpy of reactants > Enthalpy of products | Enthalpy of products > Enthalpy of reactants |
Products are more energetically stable | Reactants are more energetically stable |
∆H<0 | ∆H>0 |
Combustion, neutralization | Melting Ice, evaporating water |
Enthalpy
Reactants
Products
Progress of reaction | Enthalpy
Reactants
Products
Progress of reaction |
Heat/enthalpy change calculation
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q=mcT
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q = enthalpy (amount of heat produced)
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m = mass of heated solution (H2O)
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c = specific heat capacity (4.18 J g-1 K-1 for H2O)
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∆T= temperature change
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If the temperature of the solution increases, the reaction is exothermic, and q is negative
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If the temperature of the solution decreases, the reaction is endothermic, and q is positive
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when question is asking for enthalpy change in moles, divide the q value by the no. of moles of the limiting reagent
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Assumptions made :
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1. Assume solution has the same density as water (1g cm-3)
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2. The solution has the same heat capacity as water (4.18 J g-1 K-1)
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3. No heat loss from the system
Calorimetry: a technique used to measure changes in enthalpy of chemical reactions
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When the reaction occurs, it is either going to release or absorb heat, which can be measured by the change in temperature of the surrounding water
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Heat released from exothermic reaction is absorbed by water → increase in temperature
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Heat absorbed from endothermic reaction is from water → decrease in temperature
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Experimental vs Literature values
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Heat loss to surroundings and copper calorimeter
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Incomplete fuel combustion due to limited oxygen supply (and fuel loss if spirit burner)
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Experiment conducted under standard conditions
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Random measurement error N/A
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Figure 5.1.3-1: Standard calorimetry setup with spirit burner
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To compensate for the heat lost by the water we can extrapolate a graph
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Allows us to calculate the temperature rise that would have taken place if the reaction had been instantaneous
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Figure 5.1.3-2. Extrapolation of graph to determine the temperature change