Enthalpy Changes Flashcards by Mia Williams

What is enthalpy?

The heat stored within a chemical system which cannot be measured directly

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What is enthalpy change?

What is its symbol?

The heat exchange with the surroundings during a reaction.

🔺H

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If heat is:
1. Lost
2. Gained
In a system what does it mean?

Heat gained to surroundings and temperature increase

2. Heat lost from surroundings and a temperature decrease

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How do we work out enthalpy change?

= H products- H reactants

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What happens in an exothermic reaction?

Heat is lost from the system to the surroundings, so the temperature increases.

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What type of reaction is making chemical bonds?

Exothermic

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Draw an enthalpy diagram for an exothermic reaction and label delta H:

Reactants higher than products
Arrow down is labelled as delta H
-Ve

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What is an example of an exothermic reaction?

Combustion of fuels

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What happened during an endothermic reaction?

Heat is gained to the system from the surroundings and the temperature decreases

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What type of reaction is breaking chemical bonds?

Endothermic

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Draw an enthalpy profile diagram for an endothermic reaction and label the delta H:

Reactants lower than products
Arrow up is delta H
+ Ve

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What is an example of an endothermic reaction?

Thermal decomposition

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What is the activation energy and its symbol?

The minimum energy required by reactants in order to form products
Ea

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Draw an enthalpy profile diagram including the activation energy for an exothermic reaction:

Activation energy line goes up from reactants down to products

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Draw an enthalpy profile diagram including the activation energy for an endothermic reaction:

Activation energy goes up and above products from reactants then down back to products

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What are the standard conditions? And for a standard enthalpy change to be measured what also must we make sure?

100kPa
298K 25 degrees Celsius
1 mol dm3

Substances are in their standard states

For an enthalpy change of reaction what must we need to specify?

An equation showing the amounts in moles of reactants and products

The states of every species

Enthalpy change in KJ mol-1

What is the enthalpy of reaction?

The heat change when the number of moles of reactants specified in a given reaction equation react

What is enthalpy of combustion?

The heat change due to the complete combustion of one mole of a substance

What is the enthalpy of formation?

The heat change when one mole of a compound is formed from its elements in their natural states

What is the enthalpy of formation of an element?

What is the enthalpy of neutralisation?

The heat change which accompanies the formation of 1 mole of water when an acid and base/alkali react under the conditions stated.

What is the average bond enthalpy? (E)

The breaking of one mole of covalent bonds in a gaseous molecule.
(Endothermic)

What is calorimetry a measure of?

Enthalpy change

What is the equation to work out heat energy exchanged the untis?

Q= mc(delta)T Mass (g) Specific heat capacity Temp change (degrees Celsius) Q- heat exchanged J

What is the equation that works out delta H? And units:

Delta H= Q/ n Delta H- kJmol-1 Q- kJ n- mol

Which mol do we use?

Use the mol of the substance that IS NOT in excess in Q/n

What steps do we use to work out enthalpy change of reaction!

Q= mc(delta) T Divide Q by 1000 to get kJ Work out moles Delta H= Q/n

What must we make sure we do when writing out a combustion equation? How do we do this?

The mole of reactant is 1! Balance C, H then the O

How do we work out enthalpy change of combustion?

Q= mc(delta) T Convert Q in kJ (divide by 1000) Work out mol n= m/M Delta H= Q/n

What are the experimental errors in a combustion reaction?

1. Heat loss- inefficient transfer of heat from flame to water 2. Incomplete combustion 3. Evaporation of substance combusted 4. Non-standard conditions

What must we do when writing out the equation for a neutralisation reaction?

Make sure there is 1 mole of water!

How do we work out enthalpy change for a beaut realisation reaction?

``` Q= mc(delta) T Divide Q by 1000 Work out mole of water- By working out mole of reactant which IS NOT IN EXCESS and working out ratio of this to water Then Q/ n (of water) ```

What are the steps to working out the delta H using bond enthalpies!

1. Write balanced equation 2. Draw out the molecules 3. Add up bond enthalpies (E values) for bonds broken 4. Add up bond enthalpies (E values) for bonds made 5. Delta H= total E(bonds broken) - total E (bonds made)

What does Hess' law state?

The total enthalpy change from reactants to products is independent of the route taken.

Draw a hess' cycle for a combustion reaction:

Arrows point down to combustion products

What is the equation we use when applying Hess' law in a combustion reaction and how do we re- arrange it?

Reaction H+ combustion H products= combustion H Reactants Reaction H= combustion H reactants- combustion H products

Draw Hess' cycle for a formation reaction:

Arrows point up from formation elements

What is the equation we use when applying Hess' law in a formation reaction and how do we re- arrange it?

Reaction H+ formation H reactants = formation H products Reaction H= formation H products- formation H reactants