lucarelli alkane/alkene/enthalpy

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

In exothermic reaction, delta H is negative and temperature of reaction mixture increases. What is significance of negative delta H value and why should this lead to increase in temperature for reaction mixture?

The energy stored within chemical bonds contributes to the enthalpy of a sunstance

during a chemical change chemical bonds within the reactions are broken and new chemical bonds within the products are formed

in exothermic reaction, this results in overall decrease in energy stored within chemical bonds (energy needed to break bonds is less than energy released when forming new bonds)

this results in heat energy being released into the reaction mixture and into surroundings (ie heat is now lost)

the loss of heat from the reaction mixture equates to reduction of its enthalpy and is signified by the negative value of delta H

TOTAL ENERGY IS CONSERVED AS THE LOST ENTHALPY REAPPEARS AS AN EQUIVALENT AMOUNT OF HEAT ENERGY IN THE SURROUNDINGS

by adding Br2(aq), can we distinguish between ethane and ethene gas? How?

By bubbling both gases into bromine water, the ethene will undergo addition reaction (and rapidly decolorize bromine water from orange to colourless) whereas ethane undergoes substitution reaction (and slowly decolorizes bromine water from orange to colourless)

by adding bromine water can we distinguish between hexene and benzene?

yes, hexene is an alkene so undergo addition reaction (rapidly decolourise bromine water from orange to colourless) whereas benzene undergo substitution reaction (slowly decolourise bromine water from orange to colourless)

what is the expected products and observations for ethane gas bubbled through bromine water and exposed to sunlight (UV rad)

expected products: bromoethane and HBr observation: Bromine water slowly fades from orange to colourless Signup and view all the answers

can a test with bromine water distinguish between the two isomers of C3H6?

yes, the two isomers are cyclopropane and propene. propene = alkene so undergo addition (rapidly decolourise bromine water from orange to colourless) whereas cyclopropane = aklane (saturated) so undergo substitution (bromine water slowly fades from orange to colourless) Signup and view all the answers

What are the products of the substitution reaction ethane plus chlorine water

chloroethane(g) + HCl(g) CH5Cl + HCl Signup and view all the answers

when barium hydroxide and ammonium chloride is mixed, a chemical reaction takes place causing the mixture to cool down,. since energy cannot be neither created nor destroyed, how can we account for the lost energy?

the reaction is endothermic meaning energy needed to break bonds of reactants is greater than energy released when forming bonds of products meaning there is INCREASE in energy stored within the chemical bonds of products as energy must be conserved, this results in a decrease in the kinetic energy of the particles in the surroundings the decreased particle average kinetic energy is expressed as a reduced temperature Signup and view all the answers

CH4 (g) + 2O2(g) -> CO2(g) + 2H2O(l) delta H = -890kJ

CH4 (g) + 2O2(g) -> CO2(g) + 2H2O(g) delta H = -802kJ

why is enthalpy change greater for reaction 1

water in gas phase has higher enthalpy than in liquid phase, thus there is a greater drop in enthalpy when liquid water is the product as the reactants are gases and must condense (exothermic reaction) to form water. Signup and view all the answers

From the temp rise and mass of water used they calculated 625kJ of heat released from burning candle. The mass of candle burnt was 22.55g. Assume the wax undergoes complete combustion and the chemical composition to be C46H92O2.

justify the statement: the beaker of water will not likely capture all the heat released by the burning candle.

Their investigation assumes all the heat released from burning candle is absorbed by the water, however, the beaker itself, surrounding air and the exhaust gases will all receive some of the heat released from the combustion of the candle Signup and view all the answers

Their investigation assumes all the heat released from burning candle is absorbed by the water, however, the beaker itself, surrounding air and the exhaust gases will all receive some of the heat released from the combustion of the candle.

is this error a systematic or random error?

as a consequence of the lost heat the water temperature rise will be less than it should be. This means the candle would reappear to be producing less heat than it is and would result in CONSISTENTLY lower values for delta H. The consistently lower values for delta H indicate this is a systematic error Signup and view all the answers

Their investigation assumes all the heat released from burning candle is absorbed by the water, however, the beaker itself, surrounding air and the exhaust gases will all receive some of the heat released from the combustion of the candle.

suggest how they could modify this procedure in order to reduce this error

use copper calorimeter so that heat transfer to both the water and its calorimeter can be taken into account using a larger mass of water would also reduce heat loss to env more effective insulation surrounding the candle and beaker can also help minimise loss of heat to the room Signup and view all the answers

after the experiment was compelte there was a lot of black residue on the bottom of beaker. Suggest the chemical composition of this residue and its significance regarding their calculated value of delta H.

the black residue is soot, which is unburnt carbon this shows that incomplete combustion of the wax has occured a consequence of this is les heat will be produced from the burning wax than if complete combustion had taken place this will result in consistently lower temperature rises for the water and give rise to a lower calculated value for delta H Signup and view all the answers

explain how an exothermic reaction conforms to the law of conservation of energy

-more energy is required to break bonds -than is released when forming new bonds -the difference in energy of these two processes is transformed to heat energy -the enthalpy of reactants is higher than that of products -the difference in enthalpy is transformed to heat energy Signup and view all the answers

Study Notes

Exothermic Reactions

In exothermic reactions, the change in enthalpy (ΔH) is negative, indicating that energy is released as heat.
The release of energy during these reactions leads to an increase in the temperature of the reaction mixture, making it warmer.

Distinguishing Between Alkanes and Alkenes

Bromine water can differentiate ethane (an alkane) from ethene (an alkene).
Ethene reacts with bromine water, resulting in a decolorization of the bromine, indicating a reaction via addition.
Ethane does not react with bromine water under standard conditions due to its saturated nature, hence no decolorization occurs.

Bromine Water Test: Hexene vs. Benzene

Hexene, being an alkene, reacts with bromine water, leading to decolorization and forming dibromide as product.
Benzene does not react with bromine water in the same condition and thus retains the bromine color.

Ethane Reaction with Bromine Water and UV Light

Bubbling ethane through bromine water and exposing it to sunlight (UV radiation) leads to a substitution reaction.
Expected products include bromoethane and other brominated compounds, with less observable change as ethane is less reactive under these conditions.

Isomer Test for Propylene (C3H6)

Bromine water cannot distinguish between the two structural isomers of C3H6 (propylene and cyclopropane) since both may not react distinctly under the same conditions.

Substitution Reaction of Ethane with Chlorine

The reaction between ethane and chlorine water primarily yields chloroethane and hydrochloric acid via a substitution reaction.

Energy Conservation in Chemical Reactions

Mixing barium hydroxide and ammonium chloride results in an endothermic reaction that absorbs heat from the surroundings, leading to cooling.
The decrease in temperature reflects the energy absorbed from the environment, in accordance with the law of conservation of energy, where energy is transformed rather than lost.

Energy Distribution of Collisions

Collision energy distribution is depicted at two temperatures: T1 (lower) and T2 (higher).
Green curve illustrates collision energy distribution at temperature T1, while the red curve shows distribution at temperature T2.
Activation energy (Ea) is the minimum energy needed for a reaction, indicated by a vertical dashed blue line on the graph.

Collision Criteria for Reaction

Only collisions with energies equal to or greater than Ea can result in product formation.
Shaded areas under each curve represent the collisions that possess sufficient energy to create a transition state.

Temperature Effects on Reaction Rates

At higher temperatures (T2), the energy distribution curve shifts right, indicating higher energy collisions.
Increased temperature leads to a greater number of collisions with energy equal to or exceeding Ea.
Consequently, raising the temperature enhances the reaction rate significantly.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

alkane/alkene reactions, enthalpy