Enthalpy- 9- chmeistry

9.1- 9.4

Define enthalpy and give its symbol. (2)

Enthapy is a measure if heat energy in a chemical system at constant pressure, H is its symbol.

Define chemical system.

The atoms, molecules or ions making up the chemicals in the reaction.

Define enthalpy change .

The heat energy change that occurs in a chemical system at constant pressure. It’s symbol is delta H.

Give the formula that calculate enthalpy change and give its unit.(2)

delta H (kj mol^-1)= H(products) - H(reactants)

What does the conservation of energy state and define the three mediums that heat energy is transferred? (4)

Conservation energy- energy cannot be created or destroyed, it can only be transferred.

1. System- chemicals (reactants and products)

2. Surroundings- Everything outside the system (eg. container, thermometer, air, lab)

3. Universe- Everything involve in the process- System and surrounding

Describe the enthalpy profile diagram for both exothermic and endothermic reactions in terms of arrows in enthalpy change and activation energy. (also draw a graphs- label x and y axis) (4)

y axis is enthalpy and x axis is progress of reaction

Exothermic- Enthalpy change decrease from reactant to product. Reactant is higher up while product lower.

Activation energy in exothermic reaction from reactant is smaller than in endothermic.

Endothermic- Enthalpy changes increase from reactants to products, reactant is lower than products.

Define activation energy with symbol and explain why activation energy in exothermic reactions are smaller than in endothermic reactions. (5)

Activation energy is the minimum energy required for reactant molecules to have successful collision to start a reaction. Symbol is E_a

Exothermic reactions require less activation energy because their reactants start at a higher energy state and closer to the peak of transition state, less energy is needed to break bonds for a reaction compared to endothermic reactions.

Explain the differences of exothermic and endothermic in terms of the amount of enthalpy in reactant or product, energy transfer in the mediums, whether chemical system and surrounding gain or lose energy, temperature in surroundings increase or decrease and the enthalpy change is negative or positive. (6)

In exothermic reactions:

Enthalpy is higher in reactants than products

In endothermic reactions:

Enthalpy is higher in products than reactants

In exothermic reactions:

Energy is transferred from the system to the surroundings

In endothermic reactions:

Energy is transferred from the surrounding to the system

In exothermic reactions:

Chemical system lose energy and surroundings gain energy

In endothermic reactions:

Chemical system gain energy and surroundings lose energy

In exothermic reactions:

Temperature of surroundings increases

In endothermic reactions:

Temperature of surroundings decreases

In exothermic reactions: enthalpy chnage is negative

In endothermic reactions: enthalpy chnage is positive

Define standard enthalpy changes and give symbol.

The enthalpy change for a reaction depends on conditions like temperature and pressure. Symbol = delta H^⦵

List 3 standard conditions.

1. Standard Temperature= 298K

2. Standard Pressure= 100kPa

3. Standard concentration = 1 mol dm^-3

Define standard state.

The physical state of a substance under standard conditions.

Define the standard enthalpy change of reaction.( 2 ways) Give its symbol

The enthalpy change when the reactants in the stoichiometric equation react to form the products, with all the species in their standard state.

Or when the reaction occurs in molar quantities shown in chemical equation, under standard conditions.

Symbol= delta rH^⦵

Define stoichiometric equations.

Balanced chemical eqution with exact mole to mole ratios between reactants and products in a chmeical reaction.

What kind or reaction would enthalpy change of reaction be?

Could be exothermic or endothermic.

What are the other 3 types of enthalpy changes? Give their symbols (3)

1. Enthalpy change of formation (delta fH)

2. Enthalpy change of combustion (delta cH)

3. Enthalpy change of neutralisation (delta neutH)

Define the enthalpy change of formation and give an example. (2)

The enthalpy change when 1 mole of compound is formed from its elements.

Mg_(s) + ½ O_{2 (g)} → MgO_(s)

Define the standard enthalpy change of formation.

Enthalpy change that take place when 1 mole of compound is formed from its element in their standard states under condition.

What type of reaction could enthalpy change of formation be?

Exothermic or endothermic

Define the enthalpy change of combustion and give an example. (2)

The enthalpy change when 1 mole of substance is completely burned in oxygen.

C4H10 _(g) + 6½ O_{2 (g)} → 4CO_2(g) + 5H₂0_(l)

Define the standard enthalpy change of combustion.

The enthalpy change when 1 mole of substance is completely burned in oxygen in standard states under standard conditions.

What type of reaction is enthalpy change of combustion?

exothermic

Define the enthalpy change of neutralisation and give an example. (2)

The enthapy chnage when 1 mole of water is made when acid and alkali react.

HCl _(aq) + NaOH _(aq) → H₂O_(l) + NaCl _(aq)

Define the standard enthalpy change of neutralisation .

The enthalpy change when 1 mole of water is made when acid and alkali react together in standard state under standard conditions.

What type of reaction is enthalpy change of neutralisation?

Exothermic

What is the equation that is used to determine the heat energy from experiments result. Explain which each symbol represent in the equation.

q = mcΔT

q= heat energy

m= mass (g)

c= specific heat capicity (usually 4.18 Jg^-1 K^-1 for water)

ΔT= temperature change (°C or Kelvin)

Describe the experiment of using spirit burner for the enthalpy change of combustion. (5)

1. Measure a certain volume of water and pour it into a calorimeter or a beaker, record the initial temperature.

2. Add fuel/ alcohol to a spirit burner and weigh the whole thing

3. Place the spirit burner under the calorimeter and light the sprirt burner. Stir the water with thermometer

4. Extinguish the spirit burner after about 3 mins or when the water reach to a certain temperature. Record the maximum temperature of water.

5. Re- weigh the spirit burner

Explain the steps to calculate the enthalpy change of combustion through experimental results. (4)

1. Use the equation q = mcΔT to get heat energy change in J

2. Change the unit J into kj by dividing it by 1000

3. Calculate the number of moles of fuel/ alcohol that burnt (mass/ Mr)

4. Calculate enthalpy change of combustion by dividing kj by moles.

Why might experimental enthalpy change of combustion not be accurate and what are their solutions? (4)

1. Heat loss to the surroundings from calorimeter - put more insulating layer and lid

2. Incomplete combution which produce impurties intead of carbon dioxide, not releasing maximum amount of energy- give enough supply of oxygen

3. Evaporation of alcohol before combustion- keep the lid of the spirit burner on until starting to combust.

4. Non- standard conditions where conditions for the actual experiment are unlikely to be identical to standard conditions.

What is the equation of percentage uncertainty for single readings?

percentage uncertainty = absolute uncertainty/ mean results x 100

What is the equation of percentage uncertainty for multiple readings?

percentage uncertainty = ½ x range / mean x 100

What alternative container is used in lab experiments to hold solutions and what are its qualities to make it work?

Polystyrene cups - light, cheap, waterproof and good insulator.

List the steps how the enthalpy change of reaction between a solution and a solid or enthalpy change of neutralisation between two solutions could be measured. (4)

1. Use the equation q = mcΔT to get heat energy change in J

2. Change the unit J into kj by dividing it by 1000

3. Calculate the number of moles of the limited reactant using (n= cxv/1000)

4. Calculate enthalpy change of combustion by dividing kj by moles.

Deacribe the method how a solid and a solution react together to measure the maximum temperature for the enthalpy change of reaction. (4)

1. Pipette some solution in the polystyrene cup, also weigh out the solid

2. Use a stop watch and record the temperature of the pure solution in the polystyrene cup every 30 secs until temperature is constant

3. Add the solid powder to the solution and stir the mixture with the thermometer. Record temperature every 30 secs until it has fallen for several minutes.

4. Plot a graph with the result and draw the line of best fit.

Explain how a extrapolating graph will be drawn and how could it estimate the true maximum temperature. (2)

1. A dashed line should be drawn, extending along the cooling section until it meets another dashing line extending from the end of the intial constant temperature (where solid is added to the solution).

2. The meeting point where two dashed lines meet is the true value of maximum temperature.

Explain why the measured temperature in the experiment is not the true maximum temperature and why an extrapolating graph could help to estimate the real maximum temperature. (2)

The measured temperature is not the real maximum temperature because as soon as the solid is being add, heat will immediately dissipate to the surroundings.

An extrapolating graph helps by estimating the temperature outside the range of know data, extending the known trend, beyond existing plotted trends.

Define bond enthalpy.

Energy require to break 1 mol of bonds in gaseous molecule.

Why must bond enthalpy be in gaseous state?

To ensure the energy measured is strictly for breaking chemical bonds between isolated atoms, unaffected by intermolecular forces present in liquid or solid state.

Explain why bond enthalpies are always endothermic.

Because energy is required to break bonds/ overcome the force holding the bonds.

Explain how average bond enthalpy is different from actual bond enthalpy (2)

Average bond enthalpy is measured by taking an average value/ mean from a specific type of bond in different envirnoment.

While the actual bond enthalpy is affected and depends on the environment, each reaction have different conditions.

How would you determine if the overall reaction is exothermic or endothermic in terms of enthalpy changes associated with bonds breaking and bonds forming? (2) (draw graph to help see it)

The overall reaction would be exothermic if energy release when making bonds is greater than energy required when breaking bonds.

The overall reaction would be endothermic if the energy required when breaking bonds is greater than energy release when making bonds.

What is the equation for calculating enthalpy changes of reaction from average bond enthalpies?

ΔrH = (bond enthalpies in reactant) - (bond enthalpies in products)

What does hess’ law and the conservation energy suggests? (2)

It states that the enthalpy change for a chemical reaction is independent of the route taken.

Which means the enthalpy change only depends on the starting and final product, it does not matter which path is taken, the total energy should remain the same.

Why is Hess’ law used instead of the direct experimental measurements? (5)

• Because some reactions are hard to measure

• Producing multiple products

• Reactions are too slow or fast

• Or when some elements can’t react under standard conditions

• Hess’ law provide an indirect and alternative way to calculate the enthalpy change

Describe the Hess’ law construction of enthalpy cycles to calculate enthalpy changes for a reaction from enthalpy chnages of combustion(in terms or direction of arrows and what would dbe inviolved in the process of indirect pathway)? (2) (better to see draw out)

Using the enthalpy change of combustion in enthalpy cycles, the arrows are facing downwards, with CO2 and H2O forming during the indirect process.

Describe the Hess’ law construction of enthalpy cycles to calculate enthalpy changes for a reaction from enthalpy chnages of formation (in terms or direction of arrows and what would dbe inviolved in the process of indirect pathway)? (2) (better to see draw out)

Using the enthalpy change of formation in enthalpy cycles, the arrows are facing upwards, with elements that involve needs to be written down during the indirect process.

Why is the standard enthalpy change of formation zero for elements?

Because there would be no reaction forming from an element to an element and the element is already in its stable form so no formation reaction is needed.