Chemistry EPI revision - rate of reactions

What was the aim for this experiment?

to investigate the effect of hydrochloric acid concengtration on the rate of reaction between hydrochloric acid and sodium thiosulfate

what was the hypothesis for this experiment?

If the concentration of hydrochloric acid is increased, then the rate of reaction will increase, resulting in a shorter time for the cross to become obscured

Independent variable

concentration of hydrochloric acid (mol L-1)

Dependent variable

Time taken for the cross to become obscured (s) - used to determine the reaction rate

controlled variables

volume and concentration of sodium thiosulfate, volume of HCL and water, size/type of flask, position of X

What is the conclusion for this experiment?

The results showed that as the concentration of hydrochloric acid increased, the rate of reaction between hydrochloric acid and sodium thiosulfate generally increased, as shown by the shorter time for the cross to become obscured. Therefore, the hypothesis was supported. This supports collision theory, as a higher concentration of HCL provides more H+ particles per unit volume, increasing the frequency of successful collisions and therefore increasing the reaction.

How to improve reliability?

Repeat each concentration and calculate the average time, this reduces the effect of random errors and anomalies, making the results more consistent.

How to improve validity?

Keep all controlled variables constant, make sure that the only variable changing is the HCL concentration. This ensures that any change in reaction rate is due to the change in HCL concentration rather than other factors.

How to improve accuracy?

use a light sensor connected to data logging software to detect when the cross becomes obscured instead of relying on human observation and reaction time. This reduces errors caused by subjective judgement and delays when starting or stopping the stopwatch, producing more precise measurements of reaction time.

What is energy?

The capacity to do work or cause change

What unit is energy measured in

Joules (J)

what is enthalpy

The total energy of a system at constant pressure, including chemical potentional energy and particle kinetic energy

what is the law of conservation energy

energy cannot be created nor destroyed, only transferred or transformed. The total energy of the system and surroundings remains constant

what is enthalpy change

the heat absorbed or released by a reaction at constant pressure

exothermic reactions

• releases heat to the surroundings

• surroundings generally become warmer

• products have lower enthalpy then reactants

• delta H is negative

• example - combustion fuels

• energy is written on the products side

Endothermic reactions

• absorbs heat from the surroundings

• surroundings generally become cooler

• products have higher enthalpy than reactants

• delta H is positive

• example - thermal decomposition of calcium carbonate

• energy is written on the reactants side

endothermic reaction

endothermic reaction

exothermic and endothermic reactions

• the vertical axis is enthalpy/ energy (potential energy)

• the horizontal axis is reaction progress

• the peak represents activation complex

• activation energy is the energy needed to reach the peak from the reactants

• delta H is the difference between the energy of the products and reactants

Energy chnages when bonds break and form

• Bond break is endothermic - energy must be absorbed to overcome the attraction holding atoms together. Breaking stronger bonds requires more energy

• Bond making is exothermic - energy is released when new bonds form. Forming stronger bonds releases more energy

How do you determine whether a reaction is exothermic or endothermic from bonds?

Compare the energy absorbed breaking bonds with the energy released forming bonds.

• If more energy is released than absorbed, the reaction is exothermic.

• If more energy is absorbed than released, the reaction is endothermic.

physical changes vs chemical changes

physical change

• no new substance formed

• usually involved changes in intermolecular forces

• usually requires less energy

• examples - melting, boiling, sublimination

chemical change

• new substances are formed

• involved breaking and forming chemical bonds

• usually involves larger energy changes

• examples - combustion, decomposition, rusting

what is a fuel

a fuel is a substance that releases energy when it reacts, usually by combustion with oxygen - combustion is generally exothermic

fuel examples

• fossil fuels: coal, oil and natural gas

• biofuels: bioethanol, biodiesel and biogas

Fuel value

Fuel value is the energy released by the complete combustion of a given mass or volume of fuel. A higher fuel value means more energy is released per unit of fuel.

For a pure compound, fuel value can be calculated using its heat of combustion.

fuel value = energy releases/mass of fuel (kJ g-1)

carbon emissions

Combustion of carbon-containing fuels can produce carbon dioxide, CO2

• CO2 is a greenhouse gas.

• Greenhouse gases absorb and re-emit infrared radiation, contributing to the greenhouse effect.

• Increasing atmospheric greenhouse gas concentrations contribute to global warming and climate change.

Carbon emission intensity measures the mass of carbon dioxide released per unit of energy produced, usually in CO2 MJ-1

A lower value means less CO2 is produced for the same amount of energy.

comparing fuels

• coal - advantages - abundant and relatively cheap disadvantage - high carbon emissions

• natural gas advantages - lower carbon emissions than coal per unit of energy disadvantages - non renewable; produces CO2

• LPG - advatnages - lower carbon emissions than petrol per unit of energy disadvantages - lower fuel value per litre than petrol

• bioethanol - advantages- renewable can reduce net lifecycle emissions disadvantages - production uses land, water and energy

Why might natural gas be preferred to coal for electricity generation?

Natural gas generally produces less carbon dioxide per unit of energy released than coal, so switching from coal to natural gas can reduce direct combustion emissions. However, natural gas is still a fossil fuel, and methane leaks can increase its overall climate impact.

What are bio fuels

Biofuels are fuels produced from biomass, meaning organic material from plants or other biological sources.

examples - Bioethanol: Produced through fermentation of sugars.

• Biodiesel: Can be produced from plant oils or waste animal fats.

• Biogas: Produced when microorganisms break down organic matter

why are biofuels considered renewable

biofuels can be produced from biomass that can be replenished, unlike fossil fuels that take millions of years to form. Plants absorb atmospheric CO2 through photosynthesis. When biofuels are burned, some of this carbon returns to the atmosphere. However, biofuels are not automatically carbon neutral because growing crops, manufacturing fuel and transporting materials can produce additional greenhouse gas emissions.

advantages and disadvantage of biofuels

Advantages

• Renewable when biomass is sustainably sourced.

• May reduce lifecycle greenhouse gas emissions compared with fossil fuels.

• Can make use of agricultural and organic waste.

• Usually have low sulfur content compared with many conventional fossil fuels.

Disadvantages

• Growing crops may compete with food production for land and water.

• Manufacturing and transport require energy.

• The overall environmental benefit depends on how the fuel is produced.

• Some biofuels may require substantial resources to produce.

reaction rate

reaction rate is the amount of reactant consumed or product formed per unit of time.

rate = amount used or produced/ time taken

• A faster reaction has a higher reaction rate.

• A slower reaction has a lower reaction rate.

• Reaction rate can be measured by monitoring the disappearance of a reactant or the formation of a product.

our experiment

• Solid sulfur forms, making the solution cloudy.

• The cross eventually becomes obscured.

• The shorter the time taken for the cross to disappear, the faster the reaction.

collision theory

for a chemical reaction to occur, reacting particles must collide with sufficient energy to overcome the activation energy and with the correct orientation.

• Particles must collide.

• They must collide with energy equal to or greater than the activation energy (\(E_a\)).

• They must have a suitable orientation.

a successful collision meets these requirements

why does increasinf HCL concentration increase reaction rate?

Higher HCL concentration - more H+ particles unit volume - increased frequency of collisions - more successful collisions per second - higher reaction rate and shorter reaction time

memorisable answer - increasing the concentration of HCL increases the number of H+ partciles per unit volume, increasing the frequency of collisions between reacting particles. This increases the number of successful collisions per second, resulting in a faster reaction rate and shorter time for the cross to become obscured. (collision only increases the collision frequency not the kinetic energy)

factors affecting reaction rate

• concentration - higher concentration = faster reaction - more particles per unit volume, increasinf collision frequency

• temperature - higher temperature = faster reaction - particles have greater average kinetic enerhu - more collisions exceed

• pressure - higher pressure - faster reaction - gas particles ate closer together, increasinf collision frequency

• surface area - greater surface area = faster reaction - more particles are exposed and available to collide

• catalyst = faster rction - provides an alternative pathway with lower activation energy

activation energy

Activation energy (\(E_a\)) is the minimum energy required for a successful reaction to occur.

Catalyst

A substance that increases reaction rate without being consumed overall in the reaction.

enzyme

Enzymes are biological catalysts, usually proteins, that speed up specific reactions.

• The substrate is the substance an enzyme acts on.

• The active site is where the substrate binds.

• In the lock-and-key model, the substrate fits the enzyme's specific active site.

• Products leave the enzyme, which remains available to catalyse another reaction.

possible sources of error

• Human reaction time when starting and stopping the stopwatch.

• Different judgements about exactly when the cross disappears.

• Delays between mixing the reactants and starting the timer.

• Inconsistent temperature between trials.

• Inaccurate measurement of solution volumes.

what we can do to improve

Improvements:

• Use a light sensor to detect cloudiness objectively.

• Start timing immediately when the acid is added.

• Maintain a constant temperature.

• Measure volumes consistently and accurately.

• Repeat trials and average the results.