Topic 5: Energetics Flashcards
How do you convert celsius to kelvin?
Celsius+273= K
What is the definition of heat?
measure of the total kinetic energy of particles in a substance
What is the definition of temperature?
Measure of average kinetic energy of particles in a substance
Heat energy flows from___to___
High temp. object to low temp. obj.
The Kelvin scale is based on…Therefore, zero kelvin refers to…
Kinetic energy. 0K= no movement
Enthalpy can be calculated using this equation:
q=mc (delta)T. m= mass (mass of water unless specified), c= specific heat capacity (usually for water), Delta T= change in temp
What is specific heat capacity?
Energy required to raise the temp. of 1 g of substance by 1K
What is the specific heat capacity of water?
4.186 J/ (gC)
For q=mc(delta)T, how can you tell if the reaction is endo or exothermic?
if q is positive, endothermic, temp increased. If q is negative, exothermic, temp. decreased
What is enthalpy (H)?
The amount of heat energy stored in a substance within its chemical bonds.
What is an exothermic reaction? What does its enthalpy graph look like?
More energy is released than absorbed. Therefore, reactants have more energy than products, so reactants are higher on the enthalpy graph. Products are more stable.
Does an exothermic rxn have a positive or negative delta H (change in enthalpy)?
Negative. energy released is negative
What is an endothermic rxn? What does its enthalpy graph look like?
More energy is absorbed than released. Therefore, products are higher on the graph than reactants. Reactants are more stable.
What is standard enthalpy change?
Enthalpy change (delta H) of a reaction w/ standard conditions and standard states of the substances. Measured in kJ/mol.
What are the values for STP?
289 K, 101.3 kPa, 1 mol/L
What is standard enthalpy change of formation?
energy change when 1mol of compound is formed from its elements in standard states and conditions
How do you calculate standard enthalpy change of formation?
The sum of the delta H of products minus the sum of delta H of reactants.
What is the standard enthalpy change of formation for an element?
zero
How do you take the q=mc delta T equation and use it to find enthalpy change?
divide the q value by the number of moles
What is the standard enthalpy of neutralization?
enthalpy change when a strong acid and base react to form 1 mol H2O. Always exothermic.
What are five steps to calculate standard enthalpy of neutralization?
1) find # moles of acid/base
2) Find limiting reactant and # moles of H2O that can be produced
3) Add volumes of acid and base together, assuming that 1 cm^3 = 1 g, to get the mass
4) Use that mass for q=mcdeltaT
5) Use q/n
What is Hess’ law?
Regardless of the multiple steps of a rxn, the change in enthalpy is the sum of all the enthalpy changes. Therefore, you can manipulate reaction equations to find the change in enthalpy for the whole rxn.
What is calorimetry?
process of measuring amount of heat released/absorbed during a reaction. Change in heat can be measured by observing the temp of the H2O or surroundings.
How can calorimetry be performed?
bomb shell, cup/lid/thermometer/ flame combustion, etc.
What are 5 assumptions that can lead to errors in calorimetry?
all heat is transfered to H2O, All solution is dissolved, mass of H2O is constant, there is unlimited O2, there are standard conditions
What is average bond enthalpy?
energy required to break 1 mol of the same type of bond averaged across many similar compounds
Is bonds forming endo/exo?
Bonds forming=exothermic. Releases energy
Is bonds breaking endo/exo?
Bonds breaking=endo. Takes energy to break bonds
How do you use avg. bond enthalpy to calculate enthalpy change?
Sum of energy of bonds broken minus sum of energy of bonds formed. Formation is negative, but you don’t subtract a negative; just subtract the absolute value of formation.
What are two forms of oxygen in the atmosphere?
O2 and O3, ozone
What does ozone do?
It absorbs UV radiation
Why are O3 bonds easier to break than O2 bonds?
Double bond in O2 is stronger
What equation can you use to calculate the wavelengths required to break O3 bonds?
(h x c)/ wavelength = Photon energy
What wavelength breaks O2 bonds?
UV radiation w/ wavelength less than 242 nm
What wavelength breaks O3 bonds?
UV radiation w/ wavelengths less than 330 nm
What is the ozone-oxygen cycle?
Process by which O3 is continually regenerated in the stratosphere
What are the steps of the ozone-oxygen cycle?
High energy UV radiation breaks O2, O3 forms. Lower energy UV breaks O3. Significant because dangerous UV has been absorbed, so the stratosphere is warmer
