Physical Chemistry Flashcards
(30 cards)
What is the conservation of
energy principle?
Energy is conserved in chemical reactions. The amount of energy in the universe
at the end of a chemical reaction is the same as before the reaction takes place.
What is an exothermic
reaction? Give examples
A reaction where energy is transferred to the surroundings so that the
surroundings temperature increases – combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.
What is an endothermic
reaction? Give examples
A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases – thermal decomposition, reaction of citric acid and
sodium hydrogencarbonate. Positive sign of energy change.
What is activation energy?
Minimum amount of energy that particles need to react.
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and
products, as well as the activation energy of the reaction.
What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.
Energy is supplied to break bonds and energy is released when bonds are made; exothermic – energy released from forming bonds is greater than that needed to break the bonds; endothermic – energy needed to break bonds is greater than energy released making them.
What is the equation to find
enthalpy change in terms of
bond energies?
Energy of reaction = sum of bond energies of bonds broken – sum of bond energies of bonds made
How could you find the enthalpy change for a reaction involving aqueous solutions?
- a) Mix known quantities of the reagents (concentrations, volumes) in a polystyrene cup (a decent insulator, so as to minimise heat losses to surroundings).
- b) Measure the temperature change with a thermometer (ΔT)
- c) Assume the solution to have a density of H2O. Find the mass of the solution (m).
- d) Knowing 4.2 J are required to increase the temperature of 1 g of H2O by 1 degree,
calculate the heat change (c, specific heat capacity of water). - e) Enthalpy change (ΔH) = cmΔT. Convert to kJ/mol by including the stoichiometry of your
reagents - f) Include the sign of the enthalpy change, e.g. if the temperature decreased, the sign is
+ve (endothermic process).
How is rate of reaction
calculated?
Rate of reaction = amount (e.g. grams, cm3 ) of reactant used or product formed / time
Rate of reaction (mol/s) = Moles of reactant used or product formed / time
What are the various units for
rate of reaction?
E.g. g/s or cm3
/s or mol/s
Generally, mass/time, volume/time, moles/time
Name three common ways of
measuring rate of reaction
Loss in mass of reactants, volume of gas produced, time for a solution to become
opaque (precipitation).
Describe measuring the rate
by monitoring mass loss
Place the reaction flask on a balance. In these reactions (e.g. metal carbonate +
acid) a gas is given off, so record the decrease in mass in time intervals (note
hydrogen is too light). Plot a graph of mass vs time.
Describe measuring the rate by
monitoring the volume of a gas
Connect a gas syringe to a reaction flask and measure the volume of a gas
formed in time intervals. Plot a graph of volume vs time
Describe measuring the rate by
monitoring the disappearance
of a cross.
Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
How to find a rate of reaction at some time, t, from a graph of amount of reactant vs time?
Pick a point corresponding to the time t, and find the tangent to the curve at this
point.
The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values / change in x values.
State five factors affecting the
rate of a chemical reaction
- Concentration of reactants
- Pressure of gases (volume)
- Surface area
- Temperature
- Catalysts
What is the collision theory?
Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy).
Describe and explain the effect of increasing temperature on the rate of reaction
- T increases = faster reaction
- As T increases, kinetic energy of particles increases, i.e. more energetic collisions
- Also, they move faster, so they collide more frequently
- However, there is no straight line relationship between rate and temperature, i.e. they are not directly proportional to each other
Describe and explain the effect of increasing concentration on the rate of reaction
- Conc. increases = faster reaction,
- More reactants = more frequent collisions
Describe and explain the effect of increasing pressure of a gas on the rate of reaction
- Increasing the pressure of reacting gases, is the same as increasing concentration.
- It increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.
Note that volume and pressure are inversely proportional to each other. Increasing the volume slows down the reaction.
Describe and explain the
effect of increasing surface
area
- If solid reactants are in smaller pieces, they have a greater surface area.
- Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction, e.g. block of magnesium reacts slower with acid then magnesium powder.
What is a catalyst and how does it work?
How does it affect the reaction profile?
A catalyst changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy).
What is an enzyme?
An enzyme is a molecule that acts as a catalyst in a biological system.
What is a reversible reaction?
A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants.
