CH2 - Thermochemistry

• Created by: zopetre_
• Created on: 01-06-17 09:46

Describe exothermic reactions

One that releases energy to the surroundings, temperature rise, ΔH is negative and bonds are made

Acids with metals

In hand warmers

Thermite reactions

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Describe endothermic reactions

One that takes in energy from the surroundings, there's a temperature drop, ΔH is positive and bonds are made

Melting ice

In cold packs

Thermal decomposition

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Difference between enthalpy and enthalpy change

Enthalpy is the heat content of a system at constant pressure

Enthalpy change is the heat added to a system at constant pressure

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How do you calculate enthalpy?

ΔH = Hproducts - Hreactants

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What is the principle of conservation of energy?

Energy cannot be created or destroyed, only changed from one form to another

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What are the standard conditions for measuring ent

All substances in their standard states

A pressure of 1atm (101kPa)

A temperature of 298K (25 degrees celcius)

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Describe enthalpy change of formation/combustion

The enthalpy change of formation is the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard condtions

The enthalpy change of combustion is the enthalpy change when one mole of a substance is completely combused in oxygen under standard conditions

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What is Hess's Law?

States that the total enthalpy change for a reaction is independent of the route taken from reactants to products

ΔH1 = ΔH2 + ΔH3

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Calculation for reaction, formation and combustion

Reaction and formation = products - reactants

Combustion = reactants - products

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Difference between bond enthalpy and average bond

Bond enthalpy is the enthalpy required to break a covalent X-Y covalent bond into X and Y atoms, all in the gas phase

Average bond enthalpy is the average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species

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How do you calculate bond enthalpy?

1) Draw out each molecule to show bonds

2) Calculate energy to break all bonds (endothermic, positive value)

3) Calculate energy to make new bonds (exothermic, negative value)

bonds broken + bonds formed

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How do you measure enthalpy change?

q = mcΔT

m = mass of solution

c = specific heat capacity

ΔT = change in temperature

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ΔH = -q/n

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