Measuring Enthalpy Change (DP IB Chemistry)
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State the Law of Conservation of Energy.
The Law of Conservation of Energy states that energy is neither created or destroyed so the total energy of a process remains the same.
Define the term system in the context of chemical reactions.
In the context of chemical reactions, the term system refers to the inside of a reaction vessel.
What is kinetic energy?
Kinetic energy is energy that a particle has due to its motion.
Define the term surroundings in the context of chemical reactions.
In the context of chemical reactions, surroundings are everything outside the reaction vessel.
What is a closed system?
A closed system is one in which energy can be exchanged with the surroundings but matter cannot.
True or False?
Most chemical reactions take place in open systems.
True.
Most chemical reactions take place in open systems.
What is the difference between heat and temperature?
Temperature is a measure of the average kinetic energy of the particles whereas heat is the a measure of the energy content of a substance.
True or False?
Chemical reactions involve a transfer of energy between the system and the surroundings with the total energy decreasing.
False.
Chemical reactions involve a transfer of energy between the system and the surroundings with the total energy being conserved.
Name the three types of system.
The three types of system are:
Open
Closed
Isolated
True or False?
Open systems can exchange energy and matter.
True.
Open systems can exchange energy and matter.
Define the term enthalpy.
Enthalpy is the total chemical energy inside a substance.
What is an exothermic reaction?
An exothermic reaction in a reaction where heat energy is given out, by the system, to the surroundings causing the temperature of the surroundings to increase.
What is an endothermic reaction?
In an endothermic reaction is a reaction where heat energy is taken in, by the system, from the surroundings causing the temperature of the surroundings to decrease.
What does ΔH represent?
ΔH represents the change in enthalpy.
The sign of ΔH for an endothermic reaction is .
The sign of ΔH for an endothermic reaction is positive.
What does it mean for a reaction to be kinetically controlled?
A reaction is kinetically controlled if it is thermodynamically possible but does not occur due to a slow rate.
True or False?
In an exothermic reaction, energy is transferred from the surroundings to the system.
False.
In an exothermic reaction, energy is transferred from the system to the surroundings.
During an endothermic reaction the temperature of the surroundings .
During an endothermic reaction the temperature of the surroundings decreases.
True or False?
Enthalpy change can be positive or negative.
True.
Enthalpy change can be positive or negative.
What happens to enthalpy in an exothermic reaction?
In an exothermic reaction, enthalpy decreases.
What is an energy profile?
An energy profile is a diagram showing the energies of reactants, transition state(s), and products of a reaction over time.
Define transition state.
A transition state is a stage during the reaction at which chemical bonds are partially broken and formed.
Define activation energy.
Activation energy is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.
What does Ea represent in an energy profile diagram?
Ea represents the activation energy.
In an exothermic reaction, how does the potential energy of the products compare to the reactants?
In an exothermic reaction, the products are lower in potential energy than the reactants
Explain what type of reaction is represented in this energy profile.
This energy profile represents an endothermic reaction because the energy of the products is higher than the reactants.
What does letter B represent?
Letter B represents the activation energy.
Which letter represents the enthalpy change, ΔH?
Letter D represents the enthalpy change.
Calculate the activation energy of the forward reaction.
The activation energy of the forward reaction is:
70 kJ mol–1 + 20 kJ mol–1 = +90 kJ mol–1
In an endothermic reaction, how does the energy of the products compare to the reactants?
In an endothermic reaction, the products are higher in energy than the reactants.
What does the highest point on an energy profile represent?
The highest point on an energy profile represents the transition state.
True or False?
The following reaction will have a negative ΔH.
False.
The following reaction will have a positive ΔH.
What should the labels be on the x and y axis on the following energy profile?
The labels on the x and y axis should be:
x-axis: reaction coordinate,
y-axis: potential energy.
Define standard enthalpy change.
Standard enthalpy change of reaction is the heat transferred at constant pressure under standard conditions and states.
What symbol is used to represent standard enthalpy change?
ΔHꝊ represents the standard enthalpy change for a chemical reaction.
What are the standard conditions used when determining enthalpy change?
The standard conditions are:
A pressure of 100kPa
A concentration of 1 mol dm-3 for all solutions
A temperature of 298K
One mole of water is formed from hydrogen and oxygen releasing 286 kJ
H2 (g) + ½O2 (g) → H2O (l) ΔHꝊr= -286 kJ mol-1
Calculate ΔHr for the reaction below:
2H2 (g) + O2 (g) → 2H2O (l)
Two moles of water are formed, which means that ΔHr for the reaction is:
2 x (-286 kJ mol-1) = - 572 kJ mol-1
True or False?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions is known as the standard enthalpy change of combustion.
False.
The enthalpy change when one mole of a compound is formed from its elements under standard conditions is known as the enthalpy of formation.
Define standard change change of neutralisation.
The standard change change of neutralisation is:
The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions.
What symbol is used to denote standard conditions?
The symbol Ꝋ is used to denote standard conditions,
The standard enthalpy change of combustion is always .
The standard enthalpy change of combustion is always exothermic.
Define standard enthalpy change of combustion.
The standard enthalpy change of combustion is:
The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
True or False?
The standard enthalpy change of formation can be endothermic or exothermic.
True.
The standard enthalpy change of formation can be endothermic or exothermic.
What is the ΔHӨf for elements in their standard state?
The ΔHӨf for elements in their standard state is 0 kJ mol-1.
What type of enthalpy change is shown in this reaction?
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
The type of enthalpy change is shown in this reaction is the enthalpy change of combustion.
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
What type of enthalpy change is shown in this reaction?
2Na (s) + ½O2 (g) → Na2O (s)
The type of enthalpy change is shown in this reaction is the enthalpy change of formation.
2Na (s) + ½O2 (g) → Na2O (s)
What is calorimetry?
Calorimetry is a technique used to measure changes in enthalpy of chemical reactions.
Define the term specific heat capacity.
Specific heat capacity is the energy needed to raise the temperature of 1 g of a substance by 1 °C.
State the equation for calculating heat energy change, Q.
The equation for calculating heat energy change is:
Q = m × c × ΔT
What is the main source of error in combustion calorimetry experiments?
The main source of error in combustion calorimetry experiments is heat loss.
True or False?
In calorimetry experiments, the specific heat capacity of the solution is assumed to be the same as pure water.
True.
In calorimetry experiments, the specific heat capacity of the solution is assumed to be the same as pure water, i.e. 4.18 J g−1 K−1.
What does the symbol ΔT represent in the equation for heat energy change?
The symbol ΔT represents the temperature change.
What does the symbol Q represent in the equation for heat energy change?
Q represents the heat energy change.
What does the symbol m represent in the equation for heat energy change?
m represents the mass of the substance being heated.
How can heat loss be reduced when determining the enthalpy of a solution using calorimetry?
Heat loss can be reduced by using a polystyrene cup / vacuum flask or metal can as a calorimeter and placing a lid on top.
What does the symbol c represent in the equation for heat energy change?
c represents the specific heat capacity.
What is a temperature correction graph used for?
A temperature correction graph is used to determine the maximum enthalpy change for reactions that are not instantaneous.
Give the two equations used to calculate the enthalpy of combustion from calorimetry.
Two equations used to calculate the enthalpy of combustion from calorimetry are:
q = m x c x ΔT
ΔH = q ÷ n
What does n represent in the following equation?
ΔH = q ÷ n
n represents the number of moles.
True or False?
There is no need to convert the temperature from oC to K in calorimetry.
True.
There is no need to convert the temperature from oC to K in calorimetry because the temperature change in oC is equal to the change in temperature in K.
Give two assumptions made when calculating the enthalpy changes for reactions in solution.
Assumptions made when calculating the enthalpy changes for reactions in solution include:
That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
That the density of the solution is the same as pure water, i.e. 1 g cm-3
The specific heat capacity of the container is ignored
The reaction is complete
There are negligible heat losses