Topic 5 Flashcards
What is heat?
the transfer of energy between objects of different temperature
According to the second law of thermodynamics, heat will spontaneously flow from an object of higher temperature to an object of lower temperature
Once the two objects reach the same temperature, which is known as thermal equilibrium, no more energy will be transferred.
What is temperature
Measure of the average kinetic energy of particles
- absolute temperature in kelvin is directly proportional to the average kinetic energy of particles in a substance
What determines which way heat will flow?
Temp differences determine which way heat will flow.
Heat always flows from higher to lower temperature.
The higher the enthalpy of a substance…….
The higher the enthalpy of a substance, the less energetically stable it is.
system
all the reactants, products and solvents
surroundings
includes apparatus for the reaction, thermometers, measuring devices, lab, everything external to the reacting substances
calorimeter
is any apparatus used to measure the amount of heat being exchanged with the surroundings
Law of conservation of energy
energy can be converted from one form to another and the total amount of energy for a given system will remain constant
Exothermic reactions
heat is transferred from the system to the surroundings (-∆H)
- Bond formation is an exothermic process
- products are more energetically stable than reactants
- metal replacement reaction between zinc and copper sulfate
Endothermic reactions
chemical reactions that absorb heat from the surroundings (+∆H)
- Bond breaking/ bond disassociation is an endothermic process
- products are less energetically stable than reactants
- ammonium nitrate dissolving in water
What is ∆H?
∆H is the change in enthalpy (heat) - the heat transferred by a closed system during a chemical reaction (kJ)
What is the the standard enthalpy change of combustion?
the heat evolved upon complete combustion of 1 mol of substance
What is the standard enthalpy change of formation?
the energy change upon the formation of 1 mol of a substance from its constituent elements in their standard state.
- can be either positive or negative.
- – negative value means that under standard conditions the compound is energetically more stable than its elements
- – positive value means that it is less energetically stable
- The enthalpy change of formation of an element in its most stable form in the standard state is zero by definition.
- e.g, O2
heat of the substance is dependent on?
heat of the substance is dependent on: mass, type of substance, and temperature change
what is specific heat capacity?
the amount of energy (J) required to raise the temperature of 1g of a substance by 1 K. It can also be in kJ and for 1kg of a substance - JK-1g-1 or kJK-1kg-1
- Specific heat of water: 4.18 JK-1g-1or 4.18 kJK-1kg-1
- Specific heat of copper: 0.39 JK-1g
- therefore, copper is a better conductor
What is the main systematic error in a calorimetry experiment?
heat loss to the surroundings
What is the unit for q?
J Joules
what is the unit for m?
g
what is c?
specific heat capacity
deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.
the most stable state is where all energy has been released. When going to a more stable state, energy will be released, and when going to a less stable state, energy will be gained (from the surroundings).
- On an enthalpy level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if it is higher, heat is gained (less stable, ΔH is +ve).
What formula is used to calculate the heat energy change when the temperature of a pure substance is changed.
q = mcΔT
What is average bond enthalpy?
The energy required to break a bond in standard conditions
- Bond Enthalpy is the energy required when one mole of bonds is broken in the gaseous state (bond enthalpies are valid only in the gaseous state). Bond breaking needs energy and is an endothermic process. Bond making releases energy and it is an exothermic process. Average bond energy is the average energy required to break a mole of the same type of bonds in the gaseous state in a variety of similar compounds.
What are the standard conditions?
298K, 100kPa and everything in their standard states
Are enthalpy changes in combustion pos or neg & why?
always negative as heat is released during
combustion processes.
what is the standard enthalpy of combustion of carbon?
The standard enthalpy of combustion of carbon is the heat released when one mole of solid carbon (graphite) is completely combusted to form carbon dioxide gas under standard conditions
What is the relationship between the enthalpy of combustion of the alkanes and the number of carbon atoms?
- a linear relationship between the enthalpy of combustion of the alkanes & the number of carbon atoms bc the compounds in the homologous series have an extra –CH2– group between successive members
Hesses Law
if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same
Where can stored chemical energy come from?
- attractions between protons & electrons
- repulsions between atomic nuclei
- repulsions between electrons
- electron movement
- vibrations & rotations around bonds
What is the specific heat capacity of water?
4.18Jg−1K−1.
the higher something is in specific heat capacity….
the smallest increase in temp it will have
State the region of the electromagnetic spectrum that provides the energy required to break molecular oxygen into oxygen atoms in the upper atmosphere.
UV light
Law of conservation of energy
energy can’t be created or destroyed. So, if a reaction can happen in two different ways the overall energy change for the first way must be equal to the overall energy change for the second.
What happens when heat is transferred to an object?
when heat is transferred to an object, it increases in the average kinetic energy of its particles & therefore causes an increase in its temperature or a change in phase
Why is the change in heat enthalpy positive in endothermic reactions?
bc the amount of energy in the bonds which is stored has increased
what is standard enthalpy change of reaction (ΔHƟ).
when the enthalpy change is measured under standard conditions
Standard conditions
pressure: 100kPa
temp: 298
