12. Thermochemistry

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exothermic reactions? i studied them before they were cool!

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24 Terms

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thermochemistry

the study of heat changes that accompany chemical reactions

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exothermic reaction

a chemical reaction that gives out heat, temperature goes up

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endothermic reaction

a chemical reaction that takes in heat, temperature goes down

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exothermic Δh value

negative

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endothermic Δh value

positive

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heat of reaction (Δh)

the heat change when a reaction takes place according to a balanced equation

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heat of reaction standard unit

kJ mol^-1

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bond energy

the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other

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heat of combustion of a substance

the heat change when ONE mole of a substance is completely burned in excess oxygen

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to write combustion equation

write formula of substance, add O2, -> show CO2 and H2O as products, balance equation

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calorimeter

any container used for the determination of heat changes

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how does a calorimeter work

known mass of substance whose heat of combustion is being measured is placed inside the crucible in the bomb. filled with oxygen under pressure. bomb placed in a known quantity of water in calorimeter. substance ignited using an ignition coil of wire. rise in temperature is measured and used to calculate heat produced.

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formula to calculate heat produced

m x c x Δt

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kilogram calorific value of a fuel (kJ/kg)

the heat energy produced when 1kg of the fuel is completely burned in oxygen

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heat of formation

the heat change that takes place when ONE mole of a compound in its standard state is formed from its elements in their standard state

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to write formation equation

write formula of substance on rhs, write reactants in standard state on lhs, balance

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hess's law

if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage

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heat of neutralisation

the heat change when one mole of H+ ions from an acid reacts with one mole of OH- ions from a base

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what kind of heat of reaction would heat of neutralisation reactions be expected to have?

negative

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to calculate heat of neutralisation

heat given out = m x c x Δt

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density of solution

same as density of water

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what value do you use if the acid and base are at two different temperatures at the beginning of experiment?

use the average value

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if the acid is dibasic (sulfuric acid)…

divide by 2

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if given heat capacity (capital C)…

use c x Δt