Chemical Energy Flashcards
are combustion reactions exo- or endothermic
exothermic - fuels release energy
problems with exothermic reactions
may incur costs in supplying heat to keep the reaction rate
problems with endothermic reactions
may require heat to be removed to prevent the temperature from rising. could cause an explosion
4 main ways to calculate chemical energy
- proportion
- Eh=cm delta T
- bond breaking/ bond making
- Hess’s law
what is enthalpy
a measure of chemical energy in a substance
what is the enthalpy of combustion
the energy released when one mole of substance burns completely in oxygen
complete combustion
plentiful supply of oxygen
products - carbon dioxide and water (unless only carbon or hydrogen is being burned)
incomplete combustion
not enough O2
when there is a poor supply of oxygen, the hydrocarbon cannot burn completely and so the products are carbon monoxide, carbon and water.
steps for calculating the enthalpy of combustion from proportion
- rewrite the information in the question as a proportion
- now input the mass of one mole (gfm)
- multiply diagonal numbers and divide by number in corner
* negative sign to show energy is released*
set up used to calculate the energy released by a fuel
a thermometer in glass beaker on top of alcohol in the spirit burner
measurements taken in energy released by fuel experiment
- mass of burner before + after burning
2. temperature of water before and after (temperature rise)
specific heat capacity equation
Eh=cm delta T
specific heat capacity definition
the energy required to raise the temperature of 1kg of a substance by 1-degree Celcius
c of water
4.18 kJ per kg per degree
general rule of thumb for decimal places
100s/1000s - no dp
10s - 1 dp
units - 2 dp
calculating enthalpy of combustion
- work out energy released (no need for negative sign)
2. use proportion to work out enthalpy of combustion
why are experimental and theoretical values different
heat lost to the surroundings
ways to improve enthalpy of combustion experiment
- use a heat shield to prevent heat escaping
2. use a copper can around the glass beaker as copper is a good conductor of heat
other sources of error in enthalpy of combustion experiment
- not enough oxygen for complete combustion
- some of the fuel evaporates (Mass decreases but fuel not burned to release energy)
- no lid on container
- if water has not been stirred
- some heat energy absorbed by beaker/ other apparatus
the bomb calorimeter
can be used to improve experiment
- heat no longer lost to surroundings
- complete combustion occurs
- diagram in notes and camera roll*
why is bond breaking positive
bonds form to allow atoms to achieve stability so to break chemical bonds, energy needs to be put in to destroy stability (endothermic process)
unstable atoms
high in energy - reactive
stable atoms
low in energy - less reactive
why is bond making negative
when chemical bonds are formed, the new product is more stable and therefore lower in energy than the reactants
excess energy is released to surroundings as heat
(exothermic process)
bond breaking…
energy required
endothermic
enthalpy change = positive
bond making…
energy released
exothermic
enthalpy change = negative
molar bond enthalpy
the energy required to break one mole of chemical bonds in the gaseous state and from two moles of gaseous atoms
bond enthalpy table in data booklet
gives us bond enthalpies of common diatomic molecules
these bonds only exist in these diatomic molecules so values are very accurate
why are values for triple bonds higher than single/double
it takes more energy to break a triple bond than a single or double bond as it is stronger
mean bond enthalpy table in data booklet
these bonds can be found in more than one substance
eg: C-H found in alkanes, alkenes, alcohols, aldehydes…
the mean (average) value of the C-H bond in all compounds is calculated and displayed in the table
bond breaking, bond making equation
enthalpy change = sum of all bonds broken + sum of all bonds made
(break up AND make up)
bonds made will have a negative value
steps when using bond breaking, bond making
- draw full structural formula to show all bonds
- make a list of all bonds which are broken in reactants
- fill in values using page 10 of db
- repeat process for bond making
- calculate total energy in and total energy given out.
- calculate enthalpy change
what is enthalpy of formation?
the energy change one mole of a compound from its elements in their standard states.
what is Hess’s law?
Hess’s law states that the enthalpy change for a chemical reaction is independent of the route taken
steps for basic Hess’s law question
- find starting point
- follow alternative route and change signs if arrows are in wrong direction
- enthalpy change is the sum of all steps
applications of Hess’s law
Hess’s law can be used to calculate enthalpy changes, which are difficult to determine by experiment.
Each of the substances shown in the equation can be burned and their enthalpies of combustion found by experiment.
what is enthalpy of formation
quantity of energy taken in or given out when 1 mole of a substance is formed
alternative method when it is difficult to determine by the direct route (enthalpy of formation)
use enthalpy of combustion to help work out the energies of the different steps (pg10)
Hess’s law calculations steps
- write out target equation
- write out balanced equations for the enthalpy of combustion for each substance
- compare and rearrange the known equations to get to target equation
- cancel out and add equations and enthalpy change values to get the target.
enthalpy of combustion equations for C and H
C(s)+02–>CO2
H2(g)+1/2O2(g)–> H20(g)
