Topic 4- Energetics Flashcards Preview

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Flashcards in Topic 4- Energetics Deck (8)
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1

What is the first law of thermodynamics?

What is an exothermic reaction?

What is an example of an exothermic reaction?

What is an endothermic reaction?

What is an example of an endothermic reaction?

What is enthalpy a measure of?

What can't you measure concerning enthalpy?

What can you instead measure?

What is enthalpy change represented by?

What is the formula for calculating enthalpy change?

Energy can neither be created nor destroyed but it can be converted from one form to another

Energy is given out

eg Combustion of fuels

Energy is absorbed

eg Photosynthesis

Enthalpy is a measure of the heat energy of a substance at constant pressure

You can't measure the actual enthalpy of a substance

Instead you can measure an enthalpy change

Delta H (basically triangle and capital H)

Enthalpy change (delta H) = Enthalpy of products - Enthalpy of reactants.

2

Describe an exothermic reaction on a graph (three parts)?

What does an exothermic reaction graph show concerning the reactants and products?

Describe an endothermic reaction on a graph (three parts)?

What does an endothermic reaction graph show concerning the reactants and products?

What do enthalpy values vary with?

What is a substance under these conditions said to be?

What is the pressure for this?

What is the temperature for this described as and what is it usually?

What is standard enthalpy shown as?

What are any solutions of what concentration?

- Enthalpy of reactants > Products
- Delta H = Negative
- Exothermic = heat given out

Shows the reactants going down to the products

- Enthalpy of reactant < Products
- Delta H = Positive
- Endothermic = heat absorbed

Shows the reactants going up to the products

According to the conditions

In its standard state

100 kPa (1 atmosphere)

A stated temperature (usually 298 K, 25 oC)

Delta H, then London underground symbol on top and 298 on the bottom

Concentration 1 mol dm-3.

3

What is the definition of temperature?

What is the definition of heat?

What does calorimetry involve?

What two things does this usually involve?

What are the two common types of calorimetry?

What is the formula for calculating calorimetry (word)?

What are the units for each part of the formula?

What is the formula for calculating calorimetry (symbol)?

What is the important conversion at the end?

The average kinetic energy of the particles in a system

A measure of the total energy of all the particles present in a given amount of substance

Involves the practical determination of enthalpy changes

Usually involves heating (or cooling) known amounts of water

- Water is heated up, reaction is exothermic
- Water cools down, reaction is endothermic

Heat energy= Mass x Specific Heat Capacity x Change in Temperature

- Heat energy= Joules
- Mass= Grams
- Specific Heat Capacity= J K -1 g -1
- Change in Temperature= Kelvin (K)

q = m x c x delta T

This formula gives you a heat energy change in J (joules) whereas standard enthalpy changes are quoted in kJ so you need to divide the number by 1000 to convert it to kJ.

4

What must then be done at the end of a calorimetry practical?

What is therefore the equation for calculating the enthalpy change for an exothermic reaction?

What is used to represent an exothermic reaction?

What is used to represent an endothermic reaction?

You must divide by the moles used, to obtain the enthalpy change (kJ mol-1)

Delta H= q / n

A minus (-) for an exothermic reaction

A plus (+) for an endothermic reaction.

5

What is the specific heat capacity of water?

What is the definition of the enthalpy of neutralisation?

What type of value is this?

What is the equation of enthalpy of neutralisation?

What is the value obtained when strong acids react with strong alkalis?

4.18

The enthalpy change when one mole of water is formed from its ions in dilute solution

Exothermic values

H+ (aq) + OH- (aq) ---> H2O (l)

A value of -57 kJ mol-1.

6

What is the definition of Hess's Law?

What happens if you go in the opposite direction of an arrow in Hess's Law?

What is the definition of standard enthalpy of formation?

What is this reaction usually?

What is there on the right hand side of the equation?

What do elements in their standard states have?

What does the enthalpy of reaction from enthalpies of formation state?

What is the formula for calculating enthalpy of formation?

The enthalpy change is independent of the path taken

You subtract the value of the enthalpy change

The enthalpy change when one mole of a compound is formed from its elements, all reactants and products in their standard states, and under standard conditions

Usually, but not exclusively, exothermic

Only one mole of product on the Right Hand Side of the equation

Zero enthalpy of formation

If you formed the products from their elements you should need the same amounts of every substance as if you formed the reactants from their elements

Enthalpy of products - enthalpy of reactants.

7

What is the definition of standard enthalpy of combustion?

What are all reactants and products in?

What is this reaction always?

What is there always only on the left hand side of the equation?

What does the enthalpy of reaction from enthalpies of combustion state?

What is the formula for calculating the standard enthalpy of combustion?

What is a mnemonic for remembering calculating enthalpies of combustion and formation?

The enthalpy change when one mole of a substance undergoes combustion in excess oxygen

In their standard states

Always exothermic

Always only one mole of what you're burning on the Left Hand Side of the equation

If you burned all the products you should get the same amounts of oxidation products such as CO2 and H2O as if you burned the reactants

Enthalpy of reactants - enthalpy of products

CRAP FAPR!

8

What is the definition of mean bond dissociation enthalpy?

How are bond dissociation enthalpies averaged?

What does this mean?

Describe an exothermic reaction concerning bonds?

Describe an endothermic reaction concerning bonds?

What is the first step for calculating bond enthalpies?

What is the second step for calculating bond enthalpies?

How do we calculate bond enthalpies applying Hess's Law?

The enthalpy change, averaged over many compounds, required to break a covalent bond into gaseous atoms

Averaged over lots of compounds

They are not as accurate as values from combustion or formation data

Energy released making bonds > energy used to break bonds

Energy used to break bonds > energy released making bonds

1. Energy is put in to break bonds to form separate, gaseous atoms

2. The gaseous atoms then combine to form bonds and energy is released. It's value will be equal and opposite to that of breaking the bonds

Delta Hr= Step 1 + Step 2.