Enthalpy Flashcards
What is enthalpy?
Heat content thats stored in chemical systems and measured in enthalpy change (^H) and measured using thermometre.
What is exothermic reaction and give examples?
Energy releasees to their surrounding (loses energy) so has a negative enthalpy change ( enthalpy products lower than reactants.) and a rise in temp of the surrounding but loss of heat in mixture. Identify by detecting a rise in temp of reaction mixture.
E.g. Oxidation reaction such as combustion of ethanol. Respiration in plants/animals. Acids reacting with metals such as Mg with HCL.
What is endothermic reactions and give examples of them?
Heat taken in from surrounding so reacting chemicals gain energy. Heat gained by chemicals and loss heat by surrounding. Positive enthalpy change ( enthalpy products higher than reactants.) and identify by a fall in temp in reaction vessel.
E.g. decomposition of calcium carbonate. Photosynthesis.
What is activation energy?
The minimum amount of energy required to start a reaction by breaking of bonds.
(In exothermic, Ea energy arrow always points up and enthalpy change points down)
(In endothermic, Ea energy arrow always points up and enthalpy change points up.)
What are the standard enthalpy changes conditions in order to measure enthalpy change consistently?
Temp= 298K (25’C)
Pressure= 100KPa (1atm)
Solutions have conc of 1 moldm-3
(For a standard enthalpy changes conditions, any substance must be in its standard state. E.g. Mg =solid )
What is the standard enthalpy of formation (/\Hf)?
The enthalpy change that takes place when 1 mole of a compound is formed from its elements.
E.g. H2(g) + 1/2O2 —> H2O (l)
2C(s) +3H2 (g) +1/2O2 —> C3H5OH (l)
What is the standard enthalpy of combustion (/\Hc) ?
The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions.
E.g.
Whats the standard enthalpy change of neutralisation? (/\neutH)
The enthalpy change that takes place when 1 mole of water is formed in a neutralisation reaction.
What is the standard enthalpy of a reaction? (/\Hr)
The enthalpy change when the number of moles of reactants as specified in the balanced chemical equation react together under standard conditions.
How do you calculate enthalpy change from experimental results?
Q=mc/\T Q=heat change (Joules). m= mass of substance heated. c=specific heat capacity. /\T=change in temp.
If temp increases, reaction is exothermic.
1) find heat change in KJ ( J-KJ = divide 1000)
2) find moles reacted (n=M/mr)
3) find enthalpy change ( heat change in KJ / moles reacted)
If it’s exothermic enthalpy change would be negative.
What experiment is used to carry of the enthalpy of combustion?
.weigh known mass of water into beaker + record initial temp.
. Fill spirit burner with know mass of fuel. Light burner and burn for 5 mins.
. Record max temp reached by water
. Calc mass fuel burned
. Calc temp change.
Use enthalpy change equations to find enthalpy change.
Why do simple experiments to determine enthalpy change of combustion often produce a result less exothermic than accepted value?
Combustion may not be complete.
Some energy may have been lost to the surrounding.
What factors could affect not being able to measure enthalpy change directly and whats used instead?
.high activation energy
.slow reaction rate
.more than one reaction taking place
Use Hess’s law instead.
What is Hess’s law?
States that A reaction can take place by more than one route and the initial and final condition are the same, the total enthalpy will be the same.
What used to calculate enthalpy change if you have all enthalpies of combustion and formation?
If have enthalpies of combustion: enthalpy change of reactants - products.
If have enthalpies of formation: enthalpy change of products - reactants.
What is average bond enthalpy?
The average enthalpy change that takes place when 1 mole of a given bond is broken in a molecule in the gas phase to form gaseous atoms.
E.g. bond enthalpys of H-H and H-Cl bonds are:
H-H(g) —> H(g) + H(g)
H-Cl(g)—> H(g) + Cl(g)
Whats bond enthalpy ?
All chemical bonds contain energy
Bond energy gives indication of strength of a bond
Bond energies always endothermic (as energy required to break bond)
When bond form is exothermic as energy released.
Why do bond enthalpies have a + value?
Energy is required to break bonds (endothermic)
Explain in terms of bond breaking/making why a reaction is exothermic?
Breaking bonds requires energy to be taken in by reactants. making new bonds releases energy. The reaction is exothermic if more energy released than taken in.
Explain in terms of bond breaking/making why a reaction is endothermic.
Breaking bonds requires energy to be taken in by reactants. making new bonds releases energy. The reaction is endothermic if more energy taken in than released.
How do you find bond enthalpy?
Find bond enthalpy of breaking - bond enthalpy of making
(Remember GCSE chemistry.)
