10 - Enthalpy Flashcards by Ben Chapman

Define enthalpy

How much heat is stored in a system

symbol: H
unit :
kJ / mol
or
kJ mol^-1

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Define system

The atoms, ions and particles in the chemical involved in the reaction

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Define exothermic and endothermic

An exothermic reaction is one which releases heat energy from the system into the surroundings

An endothermic reaction is one which takes in heat energy from the surroundings into the system

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Give an example of an exothermic reaction

Combustion reactions

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Give an example of an endothermic reaction

Thermal decomposition of calcium carbonate

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What is the difference in enthalpy change for exothermic or endothermic reactions?

Exothermic reactions give a negative enthalpy change

Endothermic reactions give a positive enthalpy change

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What is the enthalpy change of the thermal decomposition of calcium carbonate?

+ 178 kJ mol^-1

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Define activation energy

Minimum energy required to start a reaction by breaking bonds

symbol: E~a

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What equation do we use to find out how much energy is released by / stored in a given amount of substance?

E = nH

Energy = moles * enthalpy

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What 3 things is enthalpy affected by?

Temperature

Pressure

Concentration

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What are the standard conditions for measuring enthalpy change?

298K temperature

101kPa pressure

1 mol dm^-3 concentration

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Define standard enthalpy change of formation

Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with substances in standard states

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How do you calculate the equations of standard formation?

You write the product (like H~2 0) on the right of the reaction arrow

You then decide how many moles of the elements are needed to make that 1 mole of product

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What is the standard formation equation of ammonia

0.5 N~2 (g)
\+
1.5 H~2 (g)
-->
NH~3 (g)

You would then probably have to write the standard enthalpy change of formation afterwards

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What is the enthalpy of formation of just an element?

Always 0 kJ/mol for just elements

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What is the first way of measuring enthalpy change?

Transferring heat to and from water or surrounding air

You then measure the temperature of the water / air

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Define specific heat capacity

The amount of energy needed to heat 1g of a substance by 1K

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What is the specific heat capacity of water?

4.18 J g^-1 K^-1

How do you calculate the heat energy a fuel has released?

Q = mc delta T

Heat energy = mass of surroundings specific heat capacity temperature change in surroundings

What is the density of water / a solution?

1 g cm^-3

Remember that it is grams and centimetres

How do you calculate enthalpy change from heat energy?

Delta H = Q / n

Enthalpy change = heat energy released / number of moles which released that energy

What is direct determination?

When enthalpy change is calculated from one experiment

What are the 4 steps for calculating enthalpy change from experiment results?

Calculate Q using Q = mc Delta T

Calculate n of the surroundings

Do Delta H = Q / n

Make enthalpy change +ve / -ve if reaction is endothermic or exothermic respectively

Define standard enthalpy change of combustion

Enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with substances all in standard states

Define standard enthalpy change of neutralisation

Enthalpy change that accompanies the reaction of an acid and a base to from 1 mole of water under standard conditions with substances all in standard states

What is the other type of standard enthalpy change?

Standard enthalpy change of reaction

When is standard enthalpy change of reaction used?

When the reaction isn't any of the others (formation/combustion)

What are the 3 reasons that you can't measure enthalpy change for a reaction?

There are too many possible products that will be made (we can't choose a select one) Rate of reaction is too slow Activation energy is too high

When do we use Hess's law?

when dealing with reactions where we can't measure enthalpy change

State Hess's law

If a reaction can take place by 2 routes and the start and end conditions are the same, the enthalpy change for each route is the same

Describe what Hess's law means

If we can do multiple reactions to get the same result as our original reaction (which we can't do) We will get the same overall enthalpy change over the multiple alternative reactions than we would doing the original reaction

Describe enthalpy cycles

Reaction A goes straight from reactants to products but we can't use this reaction Reaction B and then reaction C get us from the reactants to the same products and we can do these reactions respectively A = B + C is a rule we need to know

What happens when you go in the opposite direction of an arrow on an enthalpy cycle?

You change the sign of the enthalpy change of that reaction

For combustion reactions, where do the enthalpy cycle arrows point? How do you remember this?

Both B and C arrows point downwards to the combustion products which are H~2 O and CO~2 combustion=point down

For formation reactions, where do the enthalpy cycle arrows point? How do you remember this?

Both B and C point upwards formation=point up

What do you do for a Hess law enthalpy change of reaction?

You are given a table of reactions and their enthalpy changes You have to assign the reactions to the letters like A, B etc

What happens in terms of energy when bonds are broken and when bonds are formed? How does this relate to exothermic / endothermic?

Forming bonds releases energy Breaking bonds requires energy Making is exothermic and breaking is endothermic

Define average bond enthalpy

Energy required to break one mole of bonds in a gaseous molecule

What is the formula for enthalpy change using bond enthalpy?

Enthalpy change = Sum of enthalpy changes of bonds broken - Sum of enthalpy change of bonds formed

Why are bond enthalpies always positive?

They are the enthalpy change when bonds are formed | Forming bonds releases energy

Describe the practical experiment for determination of enthalpy change of combustion

Measure a known volume of water into a beaker Weigh the burner containing fuel Take initial temp of water Burner is lit and water heated until temperature has risen by a reasonable amount Final temperature is taken The burner is weighed to find mass of fuel burnt

When assigning reactions to letters in an enthalpy change of reaction Hess question What do you need to do in order to see which reaction you are looking for?

You have to cancel out any substances which are in reactants and products in order to find the reaction in the table BUT DO NOT CROSS THEM OUT because you will need them later

Define standard enthalpy change of reaction

Enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation under standard conditions with all substances in standard states