Chemistry Unit 2- Energy

(d) explain that catalysts: (i) affect the conditions that are needed, often requiring lower temperatures and reducing energy demand and CO2 emissions from burning of fossil fuels,

Because catalysts work to reduce the activation energy required for a reaction to take place, less energy is required. As a result, lower temperatures are required and with a reduced energy demand, less fossil fuels will be burnt to generate the electricity and less CO2 will be released as a consequence

(k) use Hess' law to construct enthalpy cycles and carry out calculations to determine: (i) an enthalpy change of reaction from enthalpy changes of combustion,

CRMP

Define dynamic equilibrium

Dynamic equilibrium is the equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction.

(b) explain why an increase in the pressure of a gas, increasing its concentration, may increase the rate of a reaction involving gases;

If the pressure of a gas is increased, the rate of reaction also increases. This is because; -the same number of molecules occupy a smaller volume so, as there are more collisions in a given time, more collisions will have greater energy than activation energy so the rate of reaction will increase.

(d) construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products;

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(d) explain that catalysts: (ii) enable different reactions to be used, with better atom economy and with reduced waste,

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(e) explain, using enthalpy profile diagrams, how the presence of a catalyst allows a reaction to proceed via a different route with a lower activation energy, giving rise to an increased reaction rate;

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(f) explain qualitatively the Boltzmann distribution and its relationship with activation energy;

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(h) interpret the catalytic behaviour in (e), in terms of the Boltzmann distribution;

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(i) explain that a dynamic equilibrium exists when the rate of the forward reaction is equal to the rate of the reverse reaction;

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(k) use Hess' law to construct enthalpy cycles and carry out calculations to determine: (iii) an enthalpy change of reaction from an unfamiliar enthalpy cycle.

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(i) define and use the term average bond enthalpy (H positive; bond breaking of one mole of bonds);

-Average bond enthalpy is the average enthalpy change required to break 1 mole of a given bond by homolytic fission in the molecules of a gaseous species. -for bonds in polyatomic molecules like CH4, the bond energies for successive broken bonds differ. An average is therefore taken.

(k) apply le Chatelier's principle to deduce qualitatively (from appropriate information) the effect of a change in pressure, on a homogeneous system in equilibrium;

A change in pressure will only affect equilibria involving gases Increase in pressure- An increase in pressure will shift equilibrium to the side with fewer gas molecules to reduce the pressure. Decrease in pressure- A decrease in pressure will shift the equilibrium to the side with more gas molecules to raise the pressure again.

(e) explain qualitatively, using enthalpy profile diagrams, the term activation energy;

Activation energy is the minimum energy required to start a reaction by the breaking of bonds.

(g) describe qualitatively, using the Boltzmann distribution, the effect of temperature changes on the proportion of molecules exceeding the activation energy and hence the reaction rate;

An increase in temperature, increases the rate of reaction and this is because; At an increased temperature, the particles have greater kinetic energy. This means, more of the particles have more energy than the activation energy of the reaction. This causes more frequent, successful collisions to occur. It terms of the Boltzman distribution, the maximum curve is lower and moves to the right. This shows that the number of particles with at least activation energy is greater, thus causing an increase in the rate of reaction.

(j) calculate an enthalpy change of reaction from average bond enthalpies;

BRMP enthalphy of reaction can be calculated by bond enthalpies using the equation; bond enthalpy of reactants - bond enthalpy of products

(a) explain that some chemical reactions are accompanied by enthalpy changes that can be exothermic (H, negative) or endothermic (H, positive);

Enthalpy is the energy stored in a chemical system. Exothermic enthalpy change is negative as the enthalpy of products is smaller than enthalpy of reactants, there is heat loss from the chemical system to the surroundings. The negative sign represents how heat was lost from the system. Endothermic enthalpy change is positive as the enthalpy of the products is greater than the enthalpy of the reactants, there is heat gain to the chemical system from the surroundings. The positive sign represents the heat gain in the chemical system.

(d) explain that catalysts: (iii) are often enzymes, generating very specific products, and operating effectively close to room temperatures and pressures,

Enzymes are a form of biological catalysts. They operate in conditions close to room temperature and pressure so are useful in industry as they reduce the need to produce high temperature and pressure which requires lots of energy to create. As they are proteins the are more specific than inorganic catalysts. This property can be useful as they can select one molecule from a mixture and cause that to react but not others.

(k) use Hess' law to construct enthalpy cycles and carry out calculations to determine: (ii) an enthalpy change of reaction from enthalpy changes of formation,

FPMR

(a) describe qualitatively, in terms of collision theory, the effect of concentration changes on the rate of a reaction;

If the concentration of the reactants are increased, the rate of reaction also increases and this is because; -an increased concentration means there are more molecules in the same volume -the molecule will be closer together so there is a greater chance of them colliding -collisions will be more frequent so, as there are more collisions in a given time, more collisions will have greater energy than activation energy so the rate of reaction will increase.

(c) state what a catalyst does

a catalyst speeds up a reaction without being consumed by the overall reaction and they work by providing an alternative reaction pathway of lower activiation energy.

(l) explain, from given data, the importance in the chemical industry of a compromise between chemical equilibrium and reaction rate.

In industry, there is a comprimise between the producing the highest yield but doing that as cheaply as possible. A key example of this is the Haber process. Conditions favoured- -the forward reaction produces fewer gas molecules so is favoured by high pressure -the forward reaction is exothermic so is favoured by low temperature However, although low temperature would produce a high yield, the rate of reaction would be very slow as the activation energy is high to break the strong N bonds. High pressures are costly to generate and also very dangerous. The compromise- -a temperature of 400-500C is used as this is high enough to allow the reaction to occur at a realistic rate, whilst still producing an acceptable yield. -a high pressure is used but its not too high so a dangerous environment is prevents, usually 200atmospheres are used -an iron catalyst is used to increase the rate of reaction without needing to produce such high temperatures. Less energy is used to generate the heat and cost is therefore reduced.

(k) apply le Chatelier's principle to deduce qualitatively (from appropriate information) the effect of a change in concentration, on a homogeneous system in equilibrium;

Increase in concentration- If the concentration of reactants is increased, the equilibrium will shift to the right to make more product. If the concentration of products is increased, the equilibrium will shift to the left to remove the excess product. Decrease in concentration- If the concentration of reactants is decreased, the equilibrium will shift to the left to make more reactants. If the concentration of products is decreased, the equilibrium will shift to the right produce more product.

(k) apply le Chatelier's principle to deduce qualitatively (from appropriate information) the effect of a change in temperature, on a homogeneous system in equilibrium;

Increase in temperature- The equilibrium shifts in the endothermic direction to lower the temperature and oppose the change Decrease in temperature- The equilibrium shifts in the exothermic direction to increase the temperature and oppsoe the change.

(d) explain that catalysts: (iv) have great economic importance, eg iron in ammonia production, Ziegler- Natta catalyst in poly(ethene) production, platinum/palladium/rhodium in catalytic converters (see also 2.4.1.i);

Iron in ammonia production- The Haber process The production of ammonia from N2 and H2 requires a lot of energy input. This is because there is a very high activation energy required to break the triple bond between the N2. The use of an iron catalyst however, helps to reduce activation energy and the energy inout required. As a result, the process is much cheaper and less dangerous and more product with be formed with less energy input. The Ziegler and Natta process- This requires the use of specialised catalysts (Ziegler and Natta catalyst) which help to make more polyethene and give is more favourable characteristics- it is made more dense, more rigid and has a higher melting point with the use of this catalyst. Platinum/palladium/rhodium in catalytic converters- These provide a surface for the reaction of harmful by products of combustion in car engines into less harmful products. This is how they work- -the CO and NO gas molecule pass over the catalytic surface of the metal. They are held onto the surface of the metal by adsorbtion. -temporary bonds form between the catalytic surface and gas molecules -these bonds allow the molecules to react together -after the reaction, CO2 and N2 are desorbed from the surface and diffuse away from the catalytic surface.

(f) define and use the terms: (i) standard conditions,

Standard conditions are- -a pressure of 100kPa -a temperature of 298K (25C) -a concentration of 1 mol dm-3 (for reactions with aqueous solutions) -all substances must be in their standard state

(h) explain exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds;

The breaking of bonds is endothermic and has a positive enthalpy change, when making bonds however, it is exothermic giving a negative enthalphy change. An endothermic reaction is a reaction where more energy is required to break the bonds than form them and an exothermic reaction is where the is less energy to break the bond compared to forming them.

(f) define and use the terms: (ii) enthalpy change of reaction,

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

(f) define and use the terms: (iv) enthalpy change of combustion;

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.

(f) define and use the terms: (iii) enthalpy change of formation,

The enthalpy change that takes place when one mole of compound in its standard state is formed from its constituent elements in their standard states under standard conditions

(c) describe that endothermic processes require an input of heat energy, eg the thermal decomposition of calcium carbonate and photosynthesis

There are two key examples of endothermic reaction; Thermal decomposition of calcium carbonate- CaCO3(s) ---> CaO(s) + CO2(g) Photosynthesis- 6CO2(g) + 6H2O(l) --->C6H12O6(aq) + 6O2(g)

(b) describe the importance of oxidation as an exothermic process in the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration;

There are two key examples of exothermic reaction; Combustion of fuels- CH4(g) + 2O2(g) ---> CO2(g) + 2H2O(l) Oxidation of carbohydrates such as glucose in respiration- C6H12O6(aq) + 6O2(g) ---> 6CO2(g) + 6H2O(l)

(g) calculate enthalpy changes from appropriate experimental results directly, including use of the relationship: energy change = mcT; 50cm3 of concentrated sulfuric acid, 20.0moldm-3, was added to 950cm3 of water at a temperatire of 19C. The solution was stirred gently until the maximum temperature was reached at 28c. What is the enthalpy change for this reaction?

To calculate enthalpy change, MCAT is used; M- mass C-specific heat capacity AT- change in temperature Step 1- Write down the equation MCAT Step 2- Write down the values for the symbols M, C and AT M= 50 + 950 = 1000cm3 C=4.2 AT= 28-19= 9C Step 3- do the calculation 1000 x 4.2 x 9 = 37800 37800/1000= 37.8KJ Because it is an exothermic reaction, it must be -37.8KJ Step 4- Calculate the number of moles of acid vol/1000 x conc = 50/1000 x 20 = 1 mole Step 5- divide the enthalpy change by the number of moles 37.8/1= 37.8KJ/mol-1

(j) state le Chatelier's principle;

When a system is in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise this change.