Enthalpy (delta H)
State the sign of ΔH for an endothermic reaction
+ΔH
Which is higher in energy for an exothermic reaction: reactants or products?
reactants
In Hess's Law, If a chemical equation is reversed, what happens to the value of ΔH?
The sign flips.
What is calorimetry?
The process of measuring the heat released or absorbed during chemical reactions.
Define activation energy (Ea).
minimum energy that colliding particles need for a successful collision leading to a reaction.
or energetic barrier that reactants must overcome to transform into products.
What does the symbol ΔH represent in a chemical reaction?
-- ΔH represents the change in enthalpy of a system during a chemical reaction.
-- the total heat (energy) absorbed or released and is measured by the difference in enthalpy of the products and reactants.
For both endothermic and exothermic reactions:
state which is more stable, reactants or products.
Exothermic: Products are more stable
Endothermic: reactants are more stable
According to Collision theory, what 2 things are needed for successful collision between reactant particles?
sufficient energy and proper orientation.
In calorimetry, if the temperature of the water increases, is the reaction exothermic or endothermic?
Exothermic
How do catalysts increase the rate of reaction?
Bonus +50: What are biological catalysts called?
By lowering the activation energy through an alternative pathway.
Enzymes are biological catalysts
A solute is dissolved in water and the reaction beaker gets colder. Is the reaction endo or exothermic?
Endothermic
What are the x and y axis labels for an energy profile diagram?
X: reaction coordinate
Y: potential energy (kJ)
What is the general formula of a combustion reaction?
CxHy + O2 --> CO2 + H2O
The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C.
How many joules of heat energy were added? (Specific heat of Al = 0.90 J g–1K–1)
9.0 J
Sketch an energy profile with and without a catalyst. label both axes.
Catalyst lowers Ea.
No effect on delta H, Reactants or Products.
x: reaction coordinate
Y: potential energy
Bond breaking _______ energy and is an ________________ process.
Bond forming ________ energy and is an ___________ process.
Bond breaking absorbs energy and is an endothermic process.
Bond forming releases energy and is an exothermic process.
BARF = BREAK ABSORB, RELEASE FORM
State the activation energy (Ea) and delta H for a reaction with the following energetic values:
Reactants: 300 kJ
Transition State: 450 kJ
Products: 100 kJ
Ea: 150 kJ (T.S. - reactants)
delta H: -200 kJ (Products - reactants)
Balance the following chemical equation:
__C₇H₈O₂(l) + __O₂(g) → __CO₂(g) + __H₂O(l)
1 C₇H₈O₂(l) + 8 O₂(g) → 7 CO₂(g) + 4 H₂O(l)
Why do calorimetry experiments typically measure a smaller change in temperature than is expected from theoretical values?
Some heat is lost to the surroundings rather than being absorbed by the water or solution, so the measured temperature change is smaller.
The calorimeter itself (cup, thermometer) absorbs some heat, which is usually not accounted for in calculations.
incomplete combustion
State 4 changes that will increase the rate of reaction. Be specific about how that variable is changed (increase/decrease/add/remove)
increase temperature
increase surface area/decrease particle size
add a catalyst
increase concentration
Use bond enthalpies to calculate delta H for the following reaction:
C2H5OH(l)+3O2(g) → 2CO2(g)+3H2O(l)
-1236 kJ/mol
Sketch a diagram with the following information:
Reactants (forward): 300 kJ
Transition State: 450 kJ
Products (forward): 100 kJ
What is the activation energy Ea and delta H of the reverse reaction?
Ea reverse = +350kJ
delta H reverse = +200 kJ
Which combination of ΔH1, ΔH2, and ΔH3 would give the enthalpy of the reaction?
CS2 (l) + 3O2 (g) → CO2 (g) + 2SO2 (g)
ΔH1 C (s) + O2 (g) → CO2 (g)
ΔH2 S (s) + O2 (g) → SO2 (g)
ΔH3 C (s) + 2S (s) → CS2 (l)
ΔH = ΔH1 + 2(ΔH2) − ΔH3
The energy from 5.70g of ethanol (C2H5OH) was used to heat up 250 g of water. The temperature of the water rose from 298K to 310K (the specific heat capacity of water is 4.18 J g-1 K-1). The molar mass of ethanol is 46.1 g/mol.
Calculate the enthalpy of combustion ΔHc of ethanol in kJ mol–1.
12.54 kJ / 0.1236 mol = - 101.5 kJ/mol
(energy is released)
Construct two Maxwell–Boltzmann energy distribution curves to explain the effect of temperature on the probability of successful collisions. Why does a small decrease in temperature lead to a large decrease in the rate of a reaction?
include x and y axis labels.
Kinetic energy is on the x-axis, number of molecules is on the y-axis.
Both curves begin at origin. Warmer sample peak is lower and to the right and ends above the colder sample.
A small decrease in temperature shifts the Maxwell–Boltzmann distribution to the left, reducing the average kinetic energy of particles. Because the distribution is skewed right rather than even/symmetrical, this small shift causes a large decrease in the amount of particles with enough energy to react, as activation energy is in the high-energy tail end of the distribution.