2.1 thermochemistry Flashcards
for exothermic reactions, the products have (more/less) potential energy than the reactants?
less
what are exothermic reactions?
reactions that give out heat
(changes potential energy into kinetic energy)
for endothermic reactions, the products have (more/less) potential energy than the reactants?
more
what are endothermic reactions?
reactions that absorb heat
(changes kinetic energy into potential energy)
what is enthalpy? what symbol does it have?
the energy at a certain pressure
H
enthalpy change definition
heat (or energy) change at constant pressure)
the change in chemical potential energy during a chemical reaction
∆H
if the enthalpy change is negative, the reaction is (endothermic/exothermic)?
exothermic
if the enthalpy change is positive, the reaction is (endothermic/exothermic)?
endothermic
standard enthalpy change definition
enthalpy changes measured under standard conditions
∆H ^θ
what does the enthalpy change of a reaction depend on?
the reaction conditions
what are the standard conditions for the measurement of enthalpy changes?
- 1 atm (or 100kPa)
- 25°C (or 298K)
- constant pressure
what are enthalpy changes always measured in?
KJ mol ^-1
what are the 3 standard enthalpy changes you need to know?
- standard enthalpy of reaction ∆rH ^ θ
- standard enthalpy of combustion ∆cH^θ
- standard enthalpy of formation ∆fH^θ
what is the standard enthalpy of reaction?
the general name for any reaction with an enthalpy change
( the amount of heat produced by the reaction)
what is the standard enthalpy of combustion? (definition)
- the enthalpy change when 1 mole of substance is completely burned in oxygen under standard conditions, with all reactants and products in their standard states
what are the standard conditions for the standard enthalpy of combustion?
- 298K
- 100KPa or 1 atm
what is a standard state?
the state (solid, liquid or gas) of that substance at standard conditions
at a level what are usually the products for standard enthalpy of combustion reactions?
carbon dioxide or water or both
what is the standard enthalpy of formation? (definition)
the enthalpy change when 1 mole of substance is formed from its constituent elements in their standard states under standard conditions
(the enthalpy change when 1 mole of a compound is producrd from its elements in their standard states)
what are the 3 ways to calculate an enthalpy change for a reaction?
- indirectly - using Hess’ Law
- using bond enthalpy data
- directly - by experiment
what is an example of when it is not possible to determine enthalpy change directly by experiment?
when many reactions are occuring at once and it becomes difficult to directly look at one specific reaction
what does Hess’ law state?
the enthalpy change for a chemical reaction is the same, independent of the route taken (from reactants to products)
(this is a consequence of the first law of thermodynamics which states that energy cannot be created or destroyed)
which way does the arrows go in a standard enthalpy of formation?
arrows up
(from the elements in their standard states that make up the reaction)
which way does the arrows go in a standard enthalpy of combustion?
(if CO2 and H2O not given,) put in a box below
arrows down
why is the enthalpy change of formation for elements in their standard states 0?
because they are in their standard states, requiring no energy to form
bond breaking is (exothermic/endothermic)?
endothermic
bond making is (exothermic/endothermic)?
exothermic
overall enthalpy change equation? (bonds)
∆Hθ = total bond enthalpy of the reactants - total bond enthalpy of the products
(breaking - making)
average bond enthalpy definition
the energy required to break 1 mole of a bond in a gaseous substance at 298K (standard conditions)
why is the value of an enthalpy change obtained by using bond enthalpy values not always accurate?
because the data used for each type of bond is an average bond enthalpy; the actual value for each bond will depend on its environment
how can you directly calculate enthalpy changes from experimental data?
using calorimetry (measure by experiment)
(a colorimeter is set up)
when carrying out the process of directly calculating enthalpy changes from experimental data, what is the aim?
to ensure that the temperature change measured is as accurate as possible
why is a stirrer used in a calorimeter?
to ensure the reaction mixture is fully mixed in order to get the maximum temperature change
why is a lid used in a calorimeter?
to minimise heat loss or absorption
why is the cup well insulated in a calorimeter?
so that the heat is not lost to, or absorbed from the surroundings
what is the equation used to measure the enthalpy change of any reaction?
∆H = -mc∆T / n
∆H = the enthalpy change
m = total mass of the solution (g) (we assume 1cm^3=1g)
c = a constant 4.18 Jg^-1K^-1
∆T = the temperature change
n = moles
once we know the energy given out (by q = mc ∆T), what equation can be used to convert it to an enthalpy?
∆H = - q/n
q = heat energy (J or KJ)
n = number or moles (always lowest no of moles)
neutralisation reactions are (endothermic/exothermic)?
exothermic
displacement reactions are (endothermic/exothermic)?
exothermic
why isn’t your value when doing the calorimetry experiment exactly right?
due to heat loss to the surroundings
how can you minimise heat loss to the surroundings when doing thr calorimeter experiment?
- insultate the polystyrene cup
- add a windshield
- add a lid to the calorimetrr
how can you obtain thr maximum value of ∆T when doing the calorimeter experiment?
if you have a solid you are adding to a solution, you need to make sure the solid is crushed so the reaction can happen immediately as well as ensure the heat loss is minimised
- crush into POWDER
what is the unit for the density of water?
gcm^-3
how do you measure the temperature change of a reaction by a graph?
extrapolate both lines to the point where the two products were added together
why is the temperature of the solution measured for 3 minutes before adding thr (e.g magesium)?
to ensure that the (initial) temperature is constant
explain why extrapolation gives a more accurate tempetature change than using the maximum temperature recorded in the experiment?
extrapolation gives the temperature that would’ve been reached if the reaction occured instantly (/to allow for heat loss during the experiment)
explain why you don’t need to accurately weigh the (e.g powderd magnesium) in a calorimeter experiment?
because it is in excess
at what pressure does changes in enthalpy occur?
constant pressure
what is the first law of thermodynamics? (or the law of conservation of energy)
energy cannot be created or destroyed only transformed from one form to another
what is the combined equation for working out the enthalpy change?
∆H = -mC∆T / n
m = mass (usually liquid)
C = specific heat capacity
∆T = temperature change
n = no of moles
what is the unit for enthalpy changes?
kJ mol^-1
bond enthalpy definition
the amount of energy that is needed to break a bond
standard molar enthalpy change of formation definition
the enthalpy change when 1 mole of the substance is formed from its constituent elements in their standard states under standard conditions
why are energy change reactions carries out in an insulated container?
to prevent heat energy loss to the surroundings
give two reasons for the changing amounts of CO2 in the atmosphere after 1900? [2]
more burning of fossil fuels / more industries / more transportation / deforestation
(any two)
use the temperature rise from the graph (9.6°C) to calculate the amount of heat given out during the experiment. (assume that the density of the solution is 1.00gcm^-1 and that its specific heat capacity is 4.18JK^-1g^-1)
(mass of 50 cm^3 of substance)
heat = 50 x 4.18 x 9.6
= 2006J
ethanol, C2H5OH is a liquid at room temp. an equation to represent the standard molar enthalpy change of formation of ethanol is:
2C (g) + 3H2 (g) + 1/2O2 (g) -> C2H5OH (l)
suggest why this enthalpy change cannot be measured directly? [1]
(if these elements were reacted together) other products would form/carbon does not react with hydrogen AND oxygen under standard conditions
in terms of bond strengths, explain why enthalpy changes of combustion are negative [1]
negative enthalpy change means energy in bonds broken is less than that in bonds made
explain why the enthalpy change of combustion of propanol is more negative than that of ethanol [1]
more bonds broken and made in propanol and therefore more energy released
explain why the temperature falls on continuing to add HCl AFTER the maximum temperature has been reached? [2]
- no further reaction occurs
- the excess acid cools the solution
state what is meant by molar mass [1]
the mass of one mole of compound
CO(g) + 2H2(g) —> CH3OH(l) is more exothermic than CO(g) + 2H2(g) —> CH3OH(g).
state why these two values are different, explaining your answer [1]
in the reaction, methanol is produced as a liquid - it is more exothermic because heat is given out when a gas becomes a liquid
