3.2.1 - Enthalpy Changes
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69 Terms
What is the system in enthalpy changes?
The chemicals involved in the reaction
What are the surroundings in enthalpy changes?
Everything outside the chemicals
What happens in an exothermic reaction?
Energy is transferred from the system to the surroundings
What is the sign of ΔH in an exothermic reaction?
Negative
Why is ΔH negative in an exothermic reaction?
Products have less energy than reactants
What happens in an endothermic reaction?
Energy is transferred from the surroundings to the system
What is the sign of ΔH in an endothermic reaction?
Positive
Why is ΔH positive in an endothermic reaction?
Products have more energy than reactants
What is activation energy?
Minimum energy particles need to collide and react
What does activation energy represent on a reaction profile?
Energy barrier between reactants and products
Give an example of an exothermic process
Combustion
Give an example of an endothermic process
Thermal decomposition of calcium carbonate
What are standard conditions for enthalpy changes?
100 kPa 298 K and solutions at 1 mol dm^-3
What temperature is used in standard conditions?
298 K
What pressure is used in standard conditions?
100 kPa
What concentration is used for solutions in standard conditions?
1 mol dm^-3
What symbol shows an enthalpy change measured under standard conditions?
°
What is the enthalpy change of reaction?
Enthalpy change when reactants react in the proportions shown in the equation
What is the symbol for enthalpy change of reaction?
ΔrH
What is the standard enthalpy change of formation?
Enthalpy change when 1 mole of compound forms from its elements in standard states
What is the symbol for standard enthalpy of formation?
ΔfH
What is the enthalpy of formation of an element in its standard state?
0 kJ mol^-1
What is the standard enthalpy change of combustion?
Enthalpy change when 1 mole of substance burns completely in oxygen
What is the symbol for standard enthalpy of combustion?
ΔcH
What are the products of complete combustion of hydrocarbons?
CO2 and H2O
What products can form in incomplete combustion?
Carbon monoxide carbon and water
Why is incomplete combustion less exothermic?
Less energy is released
What is the standard enthalpy change of neutralisation?
Enthalpy change when acid and alkali form 1 mole of water
What is the symbol for enthalpy change of neutralisation?
ΔneutH
What equation is used in calorimetry?
q = mcΔT
What does q represent in calorimetry?
Energy transferred in joules
What does m represent in calorimetry?
Mass in grams
What does c represent in calorimetry?
Specific heat capacity
What does ΔT represent in calorimetry?
Temperature change
What is the specific heat capacity of water?
4.18 J g^-1 K^-1
What density is assumed for aqueous solutions in calorimetry?
1 g cm^-3
Why is a polystyrene cup used in calorimetry?
To reduce heat transfer to surroundings
Why is the thermometer bulb immersed in solution?
To measure accurate temperature
Why is the reaction mixture stirred?
To keep temperature uniform
Why are temperatures measured before mixing reactants?
To obtain an accurate starting temperature
Why may extrapolation be needed in calorimetry?
Heat loss occurs during slow reactions
What are common errors in calorimetry?
Heat loss incomplete reaction and assuming solution properties equal water
Why is heat lost in calorimetry experiments?
Energy transfers to surroundings
What assumption is made about solution heat capacity?
It is the same as water
What assumption is made about solution density?
It is the same as water
What sign is given to exothermic enthalpy changes?
Negative
What sign is given to endothermic enthalpy changes?
Positive
What units are used for enthalpy changes?
kJ mol^-1
How do you calculate moles from concentration and volume?
Moles = concentration × volume in dm^3
How do you calculate moles from mass and Mr?
Moles = mass ÷ Mr
What mass is used in q = mcΔT for solution reactions?
Mass of the solution
What mass is used in q = mcΔT for combustion experiments?
Mass of water heated
What are common errors in combustion calorimetry?
Heat loss incomplete combustion and evaporation of fuel
Why are combustion calorimetry values often inaccurate?
Not all heat transfers to the water
What is bond enthalpy?
Enthalpy change when 1 mole of gaseous bonds is broken
Why are mean bond enthalpies used?
Bond energies vary slightly in different molecules
Is bond breaking exothermic or endothermic?
Endothermic
Is bond making exothermic or endothermic?
Exothermic
Why is bond breaking endothermic?
Energy is required to break bonds
Why is bond making exothermic?
Energy is released when bonds form
What is the equation for enthalpy change using bond enthalpies?
ΔH = Σ bond energies broken − Σ bond energies made
Why are bond enthalpy calculations less accurate?
Mean bond enthalpies are average values
What happens in exothermic reactions regarding bond energies?
More energy released making bonds than breaking them
What happens in endothermic reactions regarding bond energies?
More energy absorbed breaking bonds than making them
What is Hess’s Law?
Total enthalpy change is independent of route taken
What principle is Hess’s Law based on?
Conservation of energy
What is the formula using enthalpies of formation?
ΔHreaction = ΣΔfH products − ΣΔfH reactants
What is the formula using enthalpies of combustion?
ΔHreaction = ΣΔcH reactants − ΣΔcH products
Why do elements not appear in Hess cycles with ΔfH values?
Elements have ΔfH = 0