Chemistry Chapter 5 "Energetics"
What is Absolute temperature?
0 kelvin (-237.15°C) - the temperature at which the thermodynamic system has the lowest energy - the temperature at which all the particles have the least amount of kinetic energy
What are the two types of oxygen in the atmosphere?
1. di - oxygen (O2) 2. tri - oxygen or ozone (O3)
In a school lab, which apparatus can be used to determine enthalpy changes (ΔH)?
"a coffee cup calorimeter"
A reaction is carried out in a total of 50.0 cm3 of aqueous solution. The temperature of the reaction mixture reaches a maximum, showing an increase of 20.0 °C above the initial temperature. The amount of the limiting reagent that reacted was 0.10 mol. What is the enthalpy change (in kJ mol-1) for the reaction? Assume the specific heat capacity of the solution = 4.18 J g-1 K-1 and the density of the solution is 1.0 g cm-3.
- (50.0 × 4.18 × 20.0) / (0.10 × 1000) Firstly, you calculate the heat given out in the experiment using q = mcΔT = (50.0 × 4.18 × 20.0). This is heat given out and so a negative sign is placed in front. The value is then divided by 0.10 to get the value per mole; and converted to kJ by dividing by 1000.
What are the standard conditions for enthalpy?
- A pressure of 100 kPa - A temperature of 25°C (298 K)
Who published Hess' Law?
- Germain Henri Hess - 1840
What is reaction mechanism?
- How reactions happen at a molecular level by suggesting a sequence of bond breaking and bond making - The microscopic path by which reactants are transformed into products
What is a "coffee cup calorimeter" made up of?
- a polystyrene cup - a lid - thermometer
what happens during the thermal decomposition of calcium carbonate?
- calcium carbonate is heated - decomposes to form calcium oxide and carbon dioxide
What are some examples of exothermic reactions?
- combustion - neutralisation reactions
What are the advantages of the simple polystyrene cup as a calorimeter?
- good heat insulator, so it reduces heat loss to the surroundings - the expanded polystyrene cup absorbs very little heat itself, so there is no need to calculate the heat absorbed by the calorimeter
What are some forms that energy takes?
- heat - light - sound - electrical energy
What are the limitations to calculating enthalpy changes in a school lab?
- heat loss in the surroundings and heat absorbed by the calorimeter - incomplete combustion of the fuel - assumptions made about the specific heat capacity and density of aqueous solutions
what can the difference in the values in enthalpy of combustion?
- not all the heat produced by the combustion reaction is transferred to the water. Some is needed to heat the copper colorimeter and some has passed to the surroundings - the combustion of the ethanol is unlikely to be complete owing to the unlimited oxygen available, as assumed by the literature value - the experiment was not preformed under standard conditions
Why is Hess' Law useful?
- not always possible to directly measure the enthalpy change we want - it may be an endothermic reaction that needs a constant heat supply - it may be that the reaction doesn't 'stop' when you need it to - it may be that the reaction is too slow - Hess cycles allow us to measure enthalpy changes directly
What are the types of systems?
- open - closed - isolated
What does Hess' Law state?
- the enthalpy change of a reaction is independent of the pathway of that reaction - the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same
What does the increase in temperature when an object is heated up rely on?
- the mass of an object - the heat added - the nature of the substance
25 cm3 of 1.0 mol dm-3 NaOH is added to 25 cm3 of 1.0 mol dm-3 HCl. The temperature increases by 6.0°C. Determine the enthalpy change for the reaction, in kJ mol-1. Assume the specific heat capacity of the solution and its density are the same as that of water (4.18 J g-1 oC and 1 g cm-3). #1
-50.2 kJ mol-1
What is an endothermic?
A reaction in which heat is absorbed (transferred from the surroundings to the system)
what is an exothermic reaction?
A reaction in which heat is released (transferred from the system to the surroundings)
Which of the following is correct about a neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH)? - A decrease in the temperature of the solution is recorded and ΔH = positive - An increase in the temperature of the solution is recorded and ΔH = negative - A decrease in the temperature of the solution is recorded and ΔH = negative - An increase in the temperature of the solution is recorded and ΔH = positive
An increase in the temperature of the solution is recorded and ΔH = negative A neutralisation reaction is always exothermic - the temperature of the solution (the surroundings) increases and the enthalpy change is negative.
What is the word Enthalpy derived from?
Ancient Greek word -- "to warm in"
what is the thermochemical equation for the combustion of ethanol?
C2H5OH (l) + 3O2(g) → 2CO2(g) + 3H2O(l)
What is the chemical equation of the thermal decomposition of calcium carbonate?
CaCO3 (s) → CaO (s) + CO2 (g)
The following data was collected for a reaction taking place in an aqueous solution: ΔT of solution = -5.00 oC What describes the change in temperature and the enthalpy change, ΔH?
Decrease in temperature and positive ΔH The decrease in temperature shows that the reaction is endothermic. An endothermic reaction is one where energy is absorbed from the surroundings to the system resulting in a decrease in temperature of the surroundings (the aqueous solution) and is shown by a positive sign for the ΔH.
What is planck relation?
E = hv = hc/λ
What does the E stand for in the planck relation?
Energy
What is the heat content of a system known as?
Enthalpy (H)
Which of the following is correct about the following reaction? CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) - Heat is gained by the system and ΔH = negative - Heat is lost from the system and ΔH = negative - Heat is gained by the system and ΔH = positive - Heat is lost from the system and ΔH = positive
Heat is lost from the system and ΔH = negative A combustion reaction is always exothermic. In an exothermic reaction, heat is lost from the system to the surroundings and ΔH = negative
Which of the following is/are correct for an exothermic reaction? I. the enthalpy change, ΔH, is negative II. the products are less energetically stable than the reactants III. heat is released from the system to the surroundings
I and III only
Which of the following processes is/are endothermic? I. H2O (s) → H2O (l) II. CO2 (g) → CO2 (s) III. Br2 (l) → Br2 (g)
I and III only the melting (s) → (l) of ice is endothermic as energy needs to be supplied to overcome the hydrogen bonds between the water molecules holding the solid together. Deposition (g) → (s) is exothermic - energy is released as the CO2 molecules change from the gaseous phase to the solid phase. Evaporation (l) → (g) is endothermic as energy is required to overcome the intermolecular forces between the bromine molecules.
Which of the following is/are correct about two objects of different temperatures? I. The particles in the object of higher temperature have higher average kinetic energy than the particles in the object of lower temperature. II. When the two objects come into contact, heat will flow spontaneously from the higher temperature object to the lower temperature object. III. At thermal equilibrium, both objects have the same temperature.
I, II and III
Which of the following could result in the experimental value for an enthalpy change in aqueous solution being different to the theoretical, or accepted, value? I. Heat loss to the surroundings II. Assuming that the density of the aqueous solution is the same as that of water III. Assuming that the specific heat capacity of the solution is 4.18 J g-1 K-1 #1
I, II and III.
Chemical reactions involve changes in enthalpy. What sign is given to the enthalpy change for an exothermic reaction? Answer with positive or negative.
Negative Enthalpy changes are defined in terms of whether heat is transferred to or from the system to the surroundings. In an exothermic reaction heat is given out (lost) by the system to the surroundings, hence the sign is negative. The temperature of the surroundings, as measured by the thermometer, increases.
What happens to energy in thermal equilibrium?
No more energy is transferred
What is known as thermal equilibrium?
Once the two objects reach the same temperature
why is the perpetual motion machine impossible?
Such a machine is impossible as it would violate the first or second law of thermodynamics
Why are bonds in ozone and oxygen broken by UV of different wavelengths?
The are all different bond lengths and strengths. Single bonds are easier to break then triple bonds.
What is temperature?
The average kinetic energy of the particles in a substance
Which statement is correct for an endothermic reaction? - The products are more energetically stable than the reactants - The products have lower enthalpy than the reactants - Heat is released to the surroundings - The enthalpy change is positive
The enthalpy change is positive
When the two solids hydrated barium hydroxide and ammonium nitrate are mixed and stirred together a solution is produced and the temperature of the surroundings decreases. Which statement about the enthalpy change for this reaction is correct? - The reaction is endothermic and ΔH is negative. - The reaction is exothermic and ΔH is negative. - The reaction is endothermic and ΔH is positive. - The reaction is exothermic and ΔH is positive.
The reaction is endothermic and ΔH is positive.
what is specific heat capacity (c) measured in?
The units of specific heat capacity are either joules per gram per degree Celsius (J g−1 °C−1) or joules per gram per kelvin (J g−1 K−1)
What is Enthalpy?
a term used for the heat released or absorbed by a system at constant pressure
What does an exothermic reaction in an aqueous solution cause?
an increase in the temperature of the reaction mixture
Why is perpetual motion impossible to practice?
because of friction and other sources of energy loss
How can you demonstrate Hess' Law?
by drawing enthalpy circles
What can be measured in terms of enthalpy?
changes in enthalpy
What happens in an isolated system?
does not exchange energy or matter with its surrounding
What is needed to separate the atoms in a bond?
energy
What happens when bonds are broken?
energy is absorbed
what happens when bonds are formed?
energy is released
Are combustion reactions generally endothermic or exothermic?
exothermic ∆Hc values are generally negative
What does the v stand for in the planck relation?
frequency
what is a word equation for finding heat change?
heat change (q) = mass (m) x specific heat capacity (c) x temperature change (∆T)
How much heat is released when 10.0g of copper with a specific heat capacity of 0.385J g-1°C−1 is cooled from 85.0°C to 25.0°C?
heat change = m x c x ∆T heat change = 10.0g x 0.385g-1°C−1 x -60.0°C (the value is neg. as the Cu has lost heat) = -231 J
What happens to heat according to the second law of thermodynamics?
heat will spontaneously flow from an object of higher temperature to an object of lower temperature
what does oxygen and ozone do in the atmosphere?
help protect life on the Earth's surface and in the lower atmosphere by absorbing harmful ultraviolet (UV) radiation from the sun
Does water have a high or low specific heat capacity?
high
what is the rate of a reaction?
how quickly a reaction happens
What is a perpetual motion machine?
hypothetical machine that can do work indefinitely without an energy source
what is standard enthalpy change of reaction (ΔHƟ)?
if enthalpy change is measured under standard conditions
what does it mean for water to have a specific heat capacity?
it takes a lot of heat energy to increase the temperature of water and it also retains that heat for a longer period of time
why do substances with higher specific heat capacity require?
it takes a lot of heat energy to increase the temperature of water and it also retains that heat for a longer period of time
What is perpetual motion?
motion that continues indefinitely without any external source of energy
What does the word kinetics refer to?
movement
Is the ∆H for exothermic reactions negative or positive?
negative
When can the enthalpy change of a reaction be recorded from a calorimeter experiment?
once maximum and minimum temperatures of the reaction mixture are known
What happens in a closed system?
only energy is able to move between the system and surroundings
what are open systems?
places where matter and energy can move freely between the system and surroundings
What does the h stand for in the planck relation?
planck constant
Is the ∆H for endothermic reactions negative or positive?
positive
What is the equation used to calculate enthalpy changes?
q = mc∆T
What does the c stand for in the planck relation?
speed of light
What is Absolute temperature measured on?
the Kelvin scale (°C + 273)
What is specific heat capacity (c)?
the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius or one kelvin
What is Absolute temperature directly proportional to?
the average kinetic energy of the particles in a sample
what does it mean if temperature is increasing?
the average kinetic energy of the particles is increasing
what do chemical reactions require (bonds)?
the breaking of old bonds and the making of new bonds
what does the ∆T stand for in q = mc∆T?
the change in temperature in oC or K
What is a system?
the chemical reaction itself
What is a covalent bond due to?
the electrostatic attraction between the shared pair of electrons and the positive nuclei of the bonded atoms
what is bond enthalpy?
the energy needed to break one mole of bonds in gaseous molecules under standard conditions
What do thermochemical equations show?
the enthalpy change (∆H of a reaction
What is the standard enthalpy change of formation, ∆HfƟ?
the enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states
What is the molar enthalpy of combustion, or the standard enthalpy of combustion (ΔHƟc) defined as?
the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
what happens when heat is released from the system to the surroundings?
the enthalpy of the system decreases, so ΔH is negative
What is a surrounding?
the entire universe around the chemical reaction
What does q stand for in q = mc∆T?
the heat absorbed or released in Joules
What does m stand for in q = mc∆T?
the mass of solution in g
What does c stand for in q = mc∆T?
the specific heat capacity of the solution in J g−1 °C−1 or J g−1 K−1
What is an example of an endothermic reaction?
the thermal decomposition of calcium carbonate
What can not be measured in terms of enthalpy?
the total enthalpy of a system
What is heat?
the transfer of energy between objects of different temperature
Why are bond enthalpy values always positive?
they refer to bonds being broken (bond breaking is endothermic)
What is the assumption about water in enthalpy changes of a reaction?
water has a density of 1.00 g cm-3
What does the λ stand for in the planck relation?
wavelength
What are changes in enthalpy represented as?
ΔH
How do you calculate enthalpy changes from bond enthalpy values?
ΔH = ΣE(bonds broken) − ΣE(bonds formed) ΔH = Σ(bond enthalpies of reactants) - Σ(bond enthalpies of products)
what is the equation of Hess' Law?
ΔH1 = ΔH2 + ΔH3
