topic 8: energetics I
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what is enthalpy change
the amount of heat energy taken in or given out during any change in a system provided the pressure is constant
what happens in exothermic change
energy is transferred from the system (chemicals) to the surroundings. the products have less energy than the reactants
is the delta H negative or positive for exothermic reaction
negative
what are some examples of exothermic processes
the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration
draw how a reaction profile of exothermic reaction looks like
progress of reaction = x axis energy = y axis product line lower than reactants as energy is lost curve going over to the products the part above the reactants line is activation energy and the part below is delta H
what happens in endothermic change
energy is transferred from the surroundings to the system (chemicals) the products have more energy than the reactants
what is an example of an endothermic reaction
thermal decomposition of calcium carbonate
is the delta H positive or negative in a endothermic reaction
positive
draw the reaction profile of an endothermic reaction
the curve that joins the reactants and products the gap between the reactants and products are delta H the activation energy goes from the reactants to the top of the curve progress of curve=x axis energy = y axis
what is the standard enthalpy change of formation
the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states symbol deltafH
what are some examples of standard enthalpy change of formation using MgCl2 and Fe2O3 as examples
Mg(s) + Cl2(g) —>MgCl2(s) 2Fe(s) + 1.5O2(g) —> Fe2O3(s)
what is the standard enthalpy change of combustion
the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states Symbol deltacH
what are the standard conditions
• 100 kPa pressure • 298 K (room temperature or 25oC) • solutions at 1mol dm-3 • all substances should have their normal state at 298K
what is an example of standard enthalpy of combustion using CH4
CH4(g) + 2O2(g) —> CO2(g) + 2 H2O (l)
what is the enthalpy change of reaction
the enthalpy change when the number of moles of reactants as specified in the balanced equation react together
what is the standard enthalpy change of neutralisation
the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
are enthalpy change of neutralisation always exothermic or endothermic
always exothermic
what does Hess law state
total enthalpy change for a reaction is independent of the route by which the chemical change takes place
how if the deltaH worked out using Hess’s law
deltaH reaction = ΣdeltafH products - ΣdeltafH reactants
how do you draw and use Hess law to work out the enthalpy change of combustion
reactants in the top left, products in the top right and the combustion products at the bottom the arrow from the reactants and products to the combustion products goes downwards
how do you draw and use Hess Law to work out the enthalpy change to form a hydrate salt from an anhydrous salt
have the anhydrous salt(s) and water in the top left corner and have the hydrated salt in the top right corner have the aqueous anhydrous salt at the bottom have the arrows going from the reactants and products to the bottom
what are Hess law cycles used to measure
to measure the enthalpy change for a reaction that cannot be measured directly by experiments. instead alternative reactions are carried out that can be measured experimentally
why can the formation of a hydrated salt from an anhydrous salt not be done experimentally
because it is impossible to add the exact amount of water and it is not easy to measure the temperature change of a solid
how do you draw and use the Hess law to work out the thermal decomposition of calcium carbonate
have calcium carbonate in the top left corner, have the CaO(s) + CO2(g) in the top right corner and have CaCl2(aq) + H2O(l) + CO2(g) the arrows should go down towards the bottom box