Chem: Energy + Enthalpy

What is calorimetry used for in chemistry experiments?

Calorimetry is used to measure the amount of energy transferred in a chemical reaction.

How can you reduce heat loss to the surroundings during a solution calorimetry experiment?

Place the polystyrene cup into a beaker surrounded by cotton wool for insulation, and put a lid on the cup to reduce energy lost through evaporation.

Describe the step-by-step method to measure enthalpy change during a neutralisation reaction between $HCl$ and $NaOH$.

1) Measure $25\text{ cm}^3$ of $HCl$ and $NaOH$ in separate beakers; 2) Place beakers in a $25\text{ }^\circ\text{C}$ water bath until both reach equal temperature; 3) Add $HCl$ then $NaOH$ to a polystyrene cup with a lid; 4) Record temperature every 30 seconds and note the highest temperature reached.

How can you minimise heat loss during a combustion calorimetry experiment?

Use a draught excluder/screen around the set-up to prevent draughts from blowing heat away from the copper can.

Why is a copper container used to hold water in combustion calorimetry?

Copper is an excellent thermal conductor, allowing heat to transfer efficiently from the flame to the water.

What equation is used to calculate the heat energy transferred ($Q$) to a liquid, and what does each symbol represent?

$Q = m \times c \times \Delta T$, where $Q$ is heat energy transferred (J), $m$ is mass of liquid heated (g), $c$ is specific heat capacity ($\text{J/g/}^\circ\text{C}$), and $\Delta T$ is change in temperature ($^\circ\text{C}$).

What is the specific heat capacity of water?

$4.2\text{ J/g/}^\circ\text{C}$.

How do you calculate molar enthalpy change ($\Delta H$) from heat energy transferred ($Q$)?

1) Convert heat energy $Q$ to kilojoules ($\text{kJ}$); 2) Calculate moles of reactant used ($\text{moles} = \text{mass} / M_r$); 3) Divide energy transferred in $\text{kJ}$ by moles ($\Delta H = Q / \text{moles}$); 4) Add a negative sign if the reaction is exothermic.

Why is experimental molar enthalpy change usually lower than theoretical data values?

Significant heat energy is lost to the surrounding air, container walls, or through incomplete combustion.

What is the difference between an exothermic and an endothermic reaction?

An exothermic reaction gives out energy to the surroundings (usually as heat), causing a rise in temperature, whereas an endothermic reaction takes in energy from the surroundings, causing a fall in temperature.

What is enthalpy change ($\Delta H$) and what are its units?

Enthalpy change is the overall change in energy during a reaction, measured in $\text{kJ/mol}$.

What do positive and negative $\Delta H$ values indicate about a reaction?

A negative $\Delta H$ indicates an exothermic reaction (energy given out), while a positive $\Delta H$ indicates an endothermic reaction (energy taken in).

What is activation energy ($E_a$)?

The minimum amount of energy that reactant particles need to collide with each other and react.

How do the relative energies of reactants and products differ on exothermic versus endothermic reaction profiles?

In exothermic reaction profiles, the products are at a lower energy than the reactants; in endothermic reaction profiles, the products are at a higher energy than the reactants.

Why is bond breaking endothermic and bond forming exothermic?

Energy must be supplied to break existing bonds (endothermic), whereas energy is released when new bonds are formed (exothermic).

How can you determine if an overall reaction is endothermic or exothermic using bond energies?

In endothermic reactions, the energy used to break bonds is greater than the energy released forming them; in exothermic reactions, the energy released forming bonds is greater than the energy used to break them.

What formula is used to calculate enthalpy change ($\Delta H$) using bond energies?

$\Delta H = \text{Total energy absorbed to break bonds} - \text{Total energy released in making bonds}$.