From: Andrew Lorimer Date: Mon, 22 Apr 2019 09:03:14 +0000 (+1000) Subject: [chem] clarify yield, enthalpy and rates notes X-Git-Tag: yr12~161 X-Git-Url: https://git.lorimer.id.au/notes.git/diff_plain/9e3ba01eb06361f71766473eab929a0e12dd644d [chem] clarify yield, enthalpy and rates notes --- diff --git a/chem/graphics/catalyst-graph.png b/chem/graphics/catalyst-graph.png index 7675cbe..b4ecc2e 100644 Binary files a/chem/graphics/catalyst-graph.png and b/chem/graphics/catalyst-graph.png differ diff --git a/chem/graphics/ke-temperature.png b/chem/graphics/ke-temperature.png index acf9150..7ed5e20 100644 Binary files a/chem/graphics/ke-temperature.png and b/chem/graphics/ke-temperature.png differ diff --git a/chem/graphics/rxn-complete.png b/chem/graphics/rxn-complete.png index 89130f0..a359f4f 100644 Binary files a/chem/graphics/rxn-complete.png and b/chem/graphics/rxn-complete.png differ diff --git a/chem/graphics/rxn-incomplete.png b/chem/graphics/rxn-incomplete.png index 63c008b..b973f28 100644 Binary files a/chem/graphics/rxn-incomplete.png and b/chem/graphics/rxn-incomplete.png differ diff --git a/chem/reactions.md b/chem/reactions.md index 67f0a19..e1dc359 100644 --- a/chem/reactions.md +++ b/chem/reactions.md @@ -1,13 +1,26 @@ --- header-includes: - \usepackage{mhchem} + - \usepackage{tabularx} columns: 2 geometry: margin=2cm --- # Rates and Equilibria -## Energy profile diagrams +## Energy + +### Enthalphy + +$$\Delta H = H_{\text{products}} - H_{\text{reactants}}$$ + +**Endothermic** (products > reactants, $\Delta H > 0$) +**Exothermic** (reactants > products, $\Delta H < 0$) + +![](graphics/endothermic-profile.png){#id .class width=25%} +![](graphics/exothermic-profile.png){#id .class width=25%} + +### Activation energy $E_A$ $$E_A = E_{\text{max}} - E_{\text{initial}}$$ @@ -16,11 +29,14 @@ $$E_A = E_{\text{max}} - E_{\text{initial}}$$ - Most collisions are not fruitful - Energy must be greater than or equal to $E_A$ -**Endothermic** (products > reactants, $\Delta H > 0$) -**Exothermic** (reactants > products, $\Delta H < 0$) +### Kinetic energy -![](graphics/endothermic-profile.png) -![](graphics/exothermic-profile.png) +- **Temperature** - measure of _avg_ kinetic energy of particles. Over time each particle will eventually have enough energy to overcome $E_A$ +- Note same distribution indicates same temperature +- $\uparrow$ rate with $\uparrow T$ mainly caused by $\uparrow E_K \implies$ greater collision force +![](graphics/ke-temperature.png) + +## Rates **Ways to increase rate of reaction:** @@ -28,13 +44,7 @@ $$E_A = E_{\text{max}} - E_{\text{initial}}$$ 2. Increase concentration/pressure 3. Increase temperature -## Kinetic energy - -**Temperature** - measure of _avg_ kinetic energy of particles. Over time each particle will eventually have enough energy to overcome $E_A$. -Note same distribution indicates same temperature. -![](graphics/ke-temperature.png) - -## Catalysts +### Catalysts - alternate reaction pathway, with lower $E_A$ - increased rate of reaction @@ -47,8 +57,9 @@ Note same distribution indicates same temperature. **Hetrogenous** catalyst: different state, easily separated. Preferred for manufacturing. ![](graphics/catalyst-graph.png) -Many catalysts involve transition elements. -Haber process (ammonia producition) - enzymes are catalysts for one reaction each. Adsorption (bonding on surface) forms ammonia \ce{NH3} +- Many catalysts involve transition elements +- **Solid catalysts** - particles around catalyst with high surface energy *adsorb* gas molecules, lowering $E_A$ +- **Haber process** (ammonia producition) - enzymes are catalysts for one reaction each. Adsorption (bonding on surface) forms ammonia \ce{NH3}. ## Equilibrium systems @@ -56,22 +67,20 @@ Haber process (ammonia producition) - enzymes are catalysts for one reaction eac Reaction graphs - exponential/logarithmic curves for reaction rates with time (simultaneous curves forward/back) -![](graphics/rxn-complete.png){#id .class width=20%} -**Complete reaction** - all reactant becomes product - -![](graphics/rxn-incomplete.png){#id .class width=20%} -**Incomplete reaction** - goes both ways and reaches equilibrium +\begin{tabularx}{\columnwidth}{ | l | X |} + \hline + \parbox[c]{2.2cm}{\includegraphics[width=2cm]{graphics/rxn-complete.png} } & \textbf{Complete reaction} - all reactant becomes product \\ + \hline + \parbox[c]{2.2cm}{\includegraphics[width=2cm]{graphics/rxn-incomplete.png} } & \textbf{Incomplete reaction} - goes both ways and reaches equilibrium \\ + \hline +\end{tabularx} - All reactions are equilibrium reactions, but extent of backwards reaction may be negligible - Double arrow indicates equilibrium reaction - At equilibrium, rate of forward reaction = rate of back reaction. +- Approaching equilibrium, forward rate $>$ back rate -### States (not in course) - -- **Homogeneous** - all states are the same -- **Heterogeneous** - states are different - -## Equilibrium constant $K_c$ +### Equilibrium constant $K_c$ For \ce{$\alpha$A + $\beta$B + $\dots$ <=> $\chi$X + $\psi$Y + $\dots$}: @@ -88,6 +97,8 @@ If value is low ($< 10^4$), then [reactants] > [products] - **$K_c$ depends on direction that equation is written (L to R)** - If $K_c$ is small, equilibrium lies *to the left* - aka *equilibrium expression* +- For reverse reaction, use $K_c^\prime = {1 \over K_c}$ +- For coefficients, use $K_c^\prime = K_c^n$ ## Reaction constant (quotient) $Q$ @@ -122,6 +133,11 @@ Time-concentration graph: smooth change $$\text{yield \%} = {{\text{actual mass obtained} \over \text{theoretical maximum mass}} \times 100}$$ +- Yield may be lower than expected due to equilibrium reaction (incomplete) +- $\uparrow$ yield $\equiv$ forward rxn; $\downarrow$ yield $\equiv$ back rxn +- *Rate-yield conflict*: rxn is slower at eq. point further to RHS +- This is ameliorated by catalysts, high pressure and removal of product + ## Acid/base equilibria Strong acid: $\ce{HA -> H+ + A-}$ diff --git a/chem/reactions.pdf b/chem/reactions.pdf index 8419a6e..83780eb 100644 Binary files a/chem/reactions.pdf and b/chem/reactions.pdf differ